lvmraid(glossar.html) - phpMan

LVMRAID(7)                                                          LVMRAID(7)

NAME
       lvmraid -- LVM RAID

DESCRIPTION
       lvm(8) RAID is a way to create a Logical Volume (LV) that uses multiple
       physical devices to improve performance or  tolerate  device  failures.
       In  LVM,  the  physical  devices are Physical Volumes (PVs) in a single
       Volume Group (VG).
       How LV data blocks are placed onto PVs is determined by the RAID level.
       RAID  levels  are  commonly referred to as 'raid' followed by a number,
       e.g.  raid1, raid5 or raid6.  Selecting a RAID  level  involves  making
       tradeoffs  among:  physical  device  requirements, fault tolerance, and
       performance.  A description of the RAID levels can be found at
       www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf
       LVM RAID uses both Device Mapper (DM) and Multiple Device (MD)  drivers
       from  the  Linux  kernel.   DM is used to create and manage visible LVM
       devices, and MD is used to place data on physical devices.
       LVM creates hidden LVs (dm devices) layered between the visible LV  and
       physical  devices.   LVs  in the middle layers are called sub LVs.  For
       LVM raid, a sub LV pair to store data and metadata (raid superblock and
       write  intent  bitmap)  is  created per raid image/leg (see lvs command
       examples below).

Create a RAID LV
       To create a RAID LV, use lvcreate and specify an LV type.  The LV  type
       corresponds  to  a  RAID level.  The basic RAID levels that can be used
       are: raid0, raid1, raid4, raid5, raid6, raid10.
       lvcreate --type RaidLevel [OPTIONS] --name Name --size Size VG [PVs]
       To display the LV type of an existing LV, run:
       lvs -o name,segtype LV
       (The LV type is also referred to as "segment type" or "segtype".)
       LVs can be created with the following types:

   raid0

       Also called striping, raid0 spreads LV data across multiple devices  in
       units  of  stripe size.  This is used to increase performance.  LV data
       will be lost if any of the devices fail.
       lvcreate --type raid0 [--stripes Number --stripesize Size] VG [PVs]

       --stripes specifies the number of devices to spread the LV across.

       --stripesize specifies the size of each stripe in kilobytes.   This  is
              the  amount  of data that is written to one device before moving
              to the next.
       PVs specifies the devices to use.  If not specified,  lvm  will  choose
       Number  devices,  one for each stripe based on the number of PVs avail-
       able or supplied.

   raid1

       Also called mirroring, raid1 uses  multiple  devices  to  duplicate  LV
       data.   The  LV  data  remains  available if all but one of the devices
       fail.  The minimum number of devices (i.e. sub LV pairs) required is 2.
       lvcreate --type raid1 [--mirrors Number] VG [PVs]

       --mirrors specifies the number of mirror  images  in  addition  to  the
              original  LV  image, e.g. --mirrors 1 means there are two images
              of the data, the original and one mirror image.
       PVs specifies the devices to use.  If not specified,  lvm  will  choose
       Number devices, one for each image.

   raid4

       raid4  is a form of striping that uses an extra, first device dedicated
       to storing parity blocks.  The LV data remains available if one  device
       fails.  The parity is used to recalculate data that is lost from a sin-
       gle device.  The minimum number of devices required is 3.
       lvcreate --type raid4 [--stripes Number --stripesize Size] VG [PVs]

       --stripes specifies the number of devices to use  for  LV  data.   This
              does  not  include  the extra device lvm adds for storing parity
              blocks.  A  raid4  LV  with  Number  stripes  requires  Number+1
              devices.  Number must be 2 or more.

       --stripesize  specifies  the size of each stripe in kilobytes.  This is
              the amount of data that is written to one device  before  moving
              to the next.
       PVs  specifies  the  devices to use.  If not specified, lvm will choose
       Number+1 separate devices.
       raid4 is called non-rotating  parity  because  the  parity  blocks  are
       always stored on the same device.

   raid5

       raid5  is a form of striping that uses an extra device for storing par-
       ity blocks.  LV data and parity blocks are stored on each device, typi-
       cally  in  a  rotating  pattern  for  performance reasons.  The LV data
       remains available if one device fails.  The parity is used to  recalcu-
       late  data  that  is  lost from a single device.  The minimum number of
       devices required is 3 (unless converting from 2 legged raid1 to reshape
       to more stripes; see reshaping).
       lvcreate --type raid5 [--stripes Number --stripesize Size] VG [PVs]

       --stripes  specifies  the  number  of devices to use for LV data.  This
              does not include the extra device lvm adds  for  storing  parity
              blocks.   A  raid5  LV  with  Number  stripes  requires Number+1
              devices.  Number must be 2 or more.

       --stripesize specifies the size of each stripe in kilobytes.   This  is
              the  amount  of data that is written to one device before moving
              to the next.
       PVs specifies the devices to use.  If not specified,  lvm  will  choose
       Number+1 separate devices.
       raid5 is called rotating parity because the parity blocks are placed on
       different devices in a round-robin sequence.  There are  variations  of
       raid5  with  different  algorithms  for placing the parity blocks.  The
       default variant is raid5_ls (raid5 left symmetric, which is a  rotating
       parity 0 with data restart.)  See RAID5 variants below.

   raid6

       raid6  is a form of striping like raid5, but uses two extra devices for
       parity blocks.  LV data and parity blocks are stored  on  each  device,
       typically  in  a rotating pattern for perfomramce reasons.  The LV data
       remains available if up to two devices fail.  The  parity  is  used  to
       recalculate  data  that  is  lost from one or two devices.  The minimum
       number of devices required is 5.
       lvcreate --type raid6 [--stripes Number --stripesize Size] VG [PVs]

       --stripes specifies the number of devices to use  for  LV  data.   This
              does not include the extra two devices lvm adds for storing par-
              ity blocks.  A raid6 LV with Number  stripes  requires  Number+2
              devices.  Number must be 3 or more.

       --stripesize  specifies  the size of each stripe in kilobytes.  This is
              the amount of data that is written to one device  before  moving
              to the next.
       PVs  specifies  the  devices to use.  If not specified, lvm will choose
       Number+2 separate devices.
       Like raid5, there are variations of raid6 with different algorithms for
       placing the parity blocks.  The default variant is raid6_zr (raid6 zero
       restart, aka left symmetric, which is a rotating  parity  0  with  data
       restart.)  See RAID6 variants below.

   raid10

       raid10  is  a combination of raid1 and raid0, striping data across mir-
       rored devices.  LV data  remains  available  if  one  or  more  devices
       remains  in each mirror set.  The minimum number of devices required is
       4.
       lvcreate --type raid10
              [--mirrors NumberMirrors]
              [--stripes NumberStripes --stripesize Size]
              VG [PVs]

       --mirrors specifies the number of mirror  images  within  each  stripe.
              e.g.   --mirrors  1  means there are two images of the data, the
              original and one mirror image.

       --stripes specifies the total number of devices to  use  in  all  raid1
              images (not the number of raid1 devices to spread the LV across,
              even though that  is  the  effective  result).   The  number  of
              devices  in  each raid1 mirror will be NumberStripes/(NumberMir-
              rors+1), e.g. mirrors 1 and stripes 4 will  stripe  data  across
              two raid1 mirrors, where each mirror is devices.

       --stripesize  specifies  the size of each stripe in kilobytes.  This is
              the amount of data that is written to one device  before  moving
              to the next.
       PVs  specifies  the  devices to use.  If not specified, lvm will choose
       the necessary devices.  Devices are used to create mirrors in the order
       listed,  e.g. for mirrors 1, stripes 2, listing PV1 PV2 PV3 PV4 results
       in mirrors PV1/PV2 and PV3/PV4.
       RAID10 is not mirroring on top of stripes, which would be RAID01, which
       is less tolerant of device failures.

Synchronization
       Synchronization  is the process that makes all the devices in a RAID LV
       consistent with each other.
       In a RAID1 LV, all mirror images should have the same data.  When a new
       mirror  image  is added, or a mirror image is missing data, then images
       need to be synchronized.  Data blocks are copied from an existing image
       to a new or outdated image to make them match.
       In a RAID 4/5/6 LV, parity blocks and data blocks should match based on
       the parity calculation.  When the devices in a RAID LV change, the data
       and  parity blocks can become inconsistent and need to be synchronized.
       Correct blocks are read, parity is calculated, and recalculated  blocks
       are written.
       The  RAID  implementation  keeps  track of which parts of a RAID LV are
       synchronized.  When a RAID LV is first created and activated the  first
       synchronization is called initialization.  A pointer stored in the raid
       metadata keeps track of the initialization process thus allowing it  to
       be restarted after a deactivation of the RaidLV or a crash.  Any writes
       to the RaidLV dirties the respective region of the write intent  bitmap
       which  allow  for  fast recovery of the regions after a crash.  Without
       this, the entire LV would need to be synchronized  every  time  it  was
       activated.
       Automatic  synchronization  happens when a RAID LV is activated, but it
       is usually partial because  the  bitmaps  reduce  the  areas  that  are
       checked.  A full sync becomes necessary when devices in the RAID LV are
       replaced.
       The synchronization status of a RAID LV is reported  by  the  following
       command, where "Cpy%Sync" = "100%" means sync is complete:
       lvs -a -o name,sync_percent

   Scrubbing
       Scrubbing is a full scan of the RAID LV requested by a user.  Scrubbing
       can find problems that are missed by partial synchronization.
       Scrubbing assumes that RAID metadata and bitmaps may be inaccurate,  so
       it  verifies  all RAID metadata, LV data, and parity blocks.  Scrubbing
       can find inconsistencies caused  by  hardware  errors  or  degradation.
       These  kinds of problems may be undetected by automatic synchronization
       which excludes areas outside of the RAID write-intent bitmap.
       The command to scrub a RAID LV can operate in two different modes:
       lvchange --syncaction check|repair LV

       check Check mode is read-only and only detects  inconsistent  areas  in
              the RAID LV, it does not correct them.

       repair  Repair  mode  checks and writes corrected blocks to synchronize
              any inconsistent areas.

       Scrubbing can consume a lot of bandwidth and slow down application  I/O
       on the RAID LV.  To control the I/O rate used for scrubbing, use:

       --maxrecoveryrate Size[k|UNIT]
              Sets the maximum recovery rate for a RAID LV.  Size is specified
              as an amount per second for each device in  the  array.   If  no
              suffix  is  given,  then  KiB/sec/device  is  used.  Setting the
              recovery rate to 0 means it will be unbounded.

       --minrecoveryrate Size[k|UNIT]
              Sets the minimum recovery rate for a RAID LV.  Size is specified
              as  an  amount  per  second for each device in the array.  If no
              suffix is given,  then  KiB/sec/device  is  used.   Setting  the
              recovery rate to 0 means it will be unbounded.

       To  display  the  current scrubbing in progress on an LV, including the
       syncaction mode and percent complete, run:
       lvs -a -o name,raid_sync_action,sync_percent
       After scrubbing is complete, to  display  the  number  of  inconsistent
       blocks found, run:
       lvs -o name,raid_mismatch_count
       Also,  if  mismatches  were  found, the lvs attr field will display the
       letter "m" (mismatch) in the 9th position, e.g.
       # lvs -o name,vgname,segtype,attr vg/lv
         LV VG   Type  Attr
         lv vg   raid1 Rwi-a-r-m-

   Scrubbing Limitations
       The check mode can only report the number of  inconsistent  blocks,  it
       cannot  report which blocks are inconsistent.  This makes it impossible
       to know which device has errors, or if the errors  affect  file  system
       data, metadata or nothing at all.
       The  repair  mode can make the RAID LV data consistent, but it does not
       know which data is correct.  The result may be consistent but incorrect
       data.   When  two  different blocks of data must be made consistent, it
       chooses the block from the  device  that  would  be  used  during  RAID
       intialization.   However,  if  the  PV  holding  corrupt data is known,
       lvchange --rebuild can be used in place of scrubbing to reconstruct the
       data on the bad device.
       Future developments might include:
       Allowing a user to choose the correct version of data during repair.
       Using a majority of devices to determine the correct version of data to
       use in a 3-way RAID1 or RAID6 LV.
       Using a checksumming device  to  pin-point  when  and  where  an  error
       occurs, allowing it to be rewritten.

SubLVs
       An  LV  is  often a combination of other hidden LVs called SubLVs.  The
       SubLVs either use physical devices, or  are  built  from  other  SubLVs
       themselves.   SubLVs  hold LV data blocks, RAID parity blocks, and RAID
       metadata.  SubLVs are  generally  hidden,  so  the  lvs  -a  option  is
       required to display them:
       lvs -a -o name,segtype,devices
       SubLV  names begin with the visible LV name, and have an automatic suf-
       fix indicating its role:

       o  SubLVs holding LV data or parity blocks have the  suffix  _rimage_#.
          These SubLVs are sometimes referred to as DataLVs.

       o  SubLVs  holding  RAID metadata have the suffix _rmeta_#.  RAID meta-
          data includes superblock information, RAID type, bitmap, and  device
          health  information.  These SubLVs are sometimes referred to as Met-
          aLVs.

       SubLVs are an internal implementation detail of LVM.  The way they  are
       used, constructed and named may change.
       The following examples show the SubLV arrangement for each of the basic
       RAID LV types, using the fewest number of devices allowed for each.

   Examples
       raid0
       Each rimage SubLV holds a portion of LV data.  No parity is  used.   No
       RAID metadata is used.
       # lvcreate --type raid0 --stripes 2 --name lvr0 ...
       # lvs -a -o name,segtype,devices
         lvr0            raid0  lvr0_rimage_0(0),lvr0_rimage_1(0)
         [lvr0_rimage_0] linear /dev/sda(...)
         [lvr0_rimage_1] linear /dev/sdb(...)
       raid1
       Each rimage SubLV holds a complete copy of LV data.  No parity is used.
       Each rmeta SubLV holds RAID metadata.
       # lvcreate --type raid1 --mirrors 1 --name lvr1 ...
       # lvs -a -o name,segtype,devices
         lvr1            raid1  lvr1_rimage_0(0),lvr1_rimage_1(0)
         [lvr1_rimage_0] linear /dev/sda(...)
         [lvr1_rimage_1] linear /dev/sdb(...)
         [lvr1_rmeta_0]  linear /dev/sda(...)
         [lvr1_rmeta_1]  linear /dev/sdb(...)
       raid4
       At least three rimage SubLVs each hold a portion of  LV  data  and  one
       rimage SubLV holds parity.  Each rmeta SubLV holds RAID metadata.
       # lvcreate --type raid4 --stripes 2 --name lvr4 ...
       # lvs -a -o name,segtype,devices
         lvr4            raid4  lvr4_rimage_0(0),\
                                lvr4_rimage_1(0),\
                                lvr4_rimage_2(0)
         [lvr4_rimage_0] linear /dev/sda(...)
         [lvr4_rimage_1] linear /dev/sdb(...)
         [lvr4_rimage_2] linear /dev/sdc(...)
         [lvr4_rmeta_0]  linear /dev/sda(...)
         [lvr4_rmeta_1]  linear /dev/sdb(...)
         [lvr4_rmeta_2]  linear /dev/sdc(...)
       raid5
       At  least  three rimage SubLVs each typcially hold a portion of LV data
       and parity (see section on raid5) Each rmeta SubLV holds RAID metadata.
       # lvcreate --type raid5 --stripes 2 --name lvr5 ...
       # lvs -a -o name,segtype,devices
         lvr5            raid5  lvr5_rimage_0(0),\
                                lvr5_rimage_1(0),\
                                lvr5_rimage_2(0)
         [lvr5_rimage_0] linear /dev/sda(...)
         [lvr5_rimage_1] linear /dev/sdb(...)
         [lvr5_rimage_2] linear /dev/sdc(...)
         [lvr5_rmeta_0]  linear /dev/sda(...)
         [lvr5_rmeta_1]  linear /dev/sdb(...)
         [lvr5_rmeta_2]  linear /dev/sdc(...)
       raid6
       At least five rimage SubLVs each typically hold a portion  of  LV  data
       and  parity.   (see section on raid6) Each rmeta SubLV holds RAID meta-
       data.
       # lvcreate --type raid6 --stripes 3 --name lvr6
       # lvs -a -o name,segtype,devices
         lvr6            raid6  lvr6_rimage_0(0),\
                                lvr6_rimage_1(0),\
                                lvr6_rimage_2(0),\
                                lvr6_rimage_3(0),\
                                lvr6_rimage_4(0),\
                                lvr6_rimage_5(0)
         [lvr6_rimage_0] linear /dev/sda(...)
         [lvr6_rimage_1] linear /dev/sdb(...)
         [lvr6_rimage_2] linear /dev/sdc(...)
         [lvr6_rimage_3] linear /dev/sdd(...)
         [lvr6_rimage_4] linear /dev/sde(...)
         [lvr6_rimage_5] linear /dev/sdf(...)
         [lvr6_rmeta_0]  linear /dev/sda(...)
         [lvr6_rmeta_1]  linear /dev/sdb(...)
         [lvr6_rmeta_2]  linear /dev/sdc(...)
         [lvr6_rmeta_3]  linear /dev/sdd(...)
         [lvr6_rmeta_4]  linear /dev/sde(...)
         [lvr6_rmeta_5]  linear /dev/sdf(...)
       raid10
       At least four rimage SubLVs each hold a portion of LV data.  No  parity
       is used.  Each rmeta SubLV holds RAID metadata.
       # lvcreate --type raid10 --stripes 2 --mirrors 1 --name lvr10
       # lvs -a -o name,segtype,devices
         lvr10            raid10 lvr10_rimage_0(0),\
                                 lvr10_rimage_1(0),\
                                 lvr10_rimage_2(0),\
                                 lvr10_rimage_3(0)
         [lvr10_rimage_0] linear /dev/sda(...)
         [lvr10_rimage_1] linear /dev/sdb(...)
         [lvr10_rimage_2] linear /dev/sdc(...)
         [lvr10_rimage_3] linear /dev/sdd(...)
         [lvr10_rmeta_0]  linear /dev/sda(...)
         [lvr10_rmeta_1]  linear /dev/sdb(...)
         [lvr10_rmeta_2]  linear /dev/sdc(...)
         [lvr10_rmeta_3]  linear /dev/sdd(...)

Device Failure
       Physical devices in a RAID LV can fail or be lost for multiple reasons.
       A device could be disconnected, permanently failed, or temporarily dis-
       connected.   The  purpose  of RAID LVs (levels 1 and higher) is to con-
       tinue operating in a degraded mode, without losing LV data, even  after
       a  device  fails.  The number of devices that can fail without the loss
       of LV data depends on the RAID level:

       o  RAID0 (striped) LVs cannot tolerate losing  any  devices.   LV  data
          will be lost if any devices fail.

       o  RAID1  LVs  can  tolerate  losing all but one device without LV data
          loss.

       o  RAID4 and RAID5 LVs can tolerate losing one device without  LV  data
          loss.

       o  RAID6 LVs can tolerate losing two devices without LV data loss.

       o  RAID10  is  variable,  and  depends  on  which devices are lost.  It
          stripes across multiple mirror groups with raid1 layout thus it  can
          tolerate  losing  all but one device in each of these groups without
          LV data loss.

       If a RAID LV is missing devices, or has other device-related  problems,
       lvs reports this in the health_status (and attr) fields:
       lvs -o name,lv_health_status
       partial
       Devices  are missing from the LV.  This is also indicated by the letter
       "p" (partial) in the 9th position of the lvs attr field.
       refresh needed
       A device was temporarily missing but has returned.  The LV needs to  be
       refreshed  to  use the device again (which will usually require partial
       synchronization).  This is also indicated by the  letter  "r"  (refresh
       needed)  in  the 9th position of the lvs attr field.  See Refreshing an
       LV.  This could also indicate a problem with the device, in which  case
       it should be be replaced, see Replacing Devices.
       mismatches exist
       See Scrubbing.
       Most commands will also print a warning if a device is missing, e.g.
       WARNING: Device for PV uItL3Z-wBME-DQy0-... not found or rejected ...
       This  warning will go away if the device returns or is removed from the
       VG (see vgreduce --removemissing).

   Activating an LV with missing devices
       A RAID LV that is missing devices may be activated or not, depending on
       the "activation mode" used in lvchange:
       lvchange -ay --activationmode complete|degraded|partial LV
       complete
       The LV is only activated if all devices are present.
       degraded
       The LV is activated with missing devices if the RAID level can tolerate
       the number of missing devices without LV data loss.
       partial
       The LV is always activated, even if portions of the LV data are missing
       because  of the missing device(s).  This should only be used to perform
       extreme recovery or repair operations.
       lvm.conf(5) activation/activation_mode
       controls the activation mode when not specified by the command.
       The default value is printed by:
       lvmconfig --type default activation/activation_mode

   Replacing Devices
       Devices in a RAID LV can be replaced by other devices in the VG.   When
       replacing  devices that are no longer visible on the system, use lvcon-
       vert --repair.  When replacing devices  that  are  still  visible,  use
       lvconvert  --replace.   The  repair command will attempt to restore the
       same number of data LVs that were previously in the  LV.   The  replace
       option  can  be  repeated to replace multiple PVs.  Replacement devices
       can be optionally listed with either option.
       lvconvert --repair LV [NewPVs]
       lvconvert --replace OldPV LV [NewPV]
       lvconvert --replace OldPV1 --replace OldPV2 LV [NewPVs]
       New devices require synchronization with existing devices, see Synchro-
       nization.

   Refreshing an LV
       Refreshing  a  RAID LV clears any transient device failures (device was
       temporarily disconnected) and returns the LV  to  its  fully  redundant
       mode.   Restoring  a  device will usually require at least partial syn-
       chronization (see Synchronization).  Failure to clear a transient fail-
       ure results in the RAID LV operating in degraded mode until it is reac-
       tivated.  Use the lvchange command to refresh an LV:
       lvchange --refresh LV
       # lvs -o name,vgname,segtype,attr,size vg
         LV VG   Type  Attr       LSize
         lv vg   raid1 Rwi-a-r-r- 100.00g
       # lvchange --refresh vg/lv
       # lvs -o name,vgname,segtype,attr,size vg
         LV VG   Type  Attr       LSize
         lv vg   raid1 Rwi-a-r--- 100.00g

   Automatic repair
       If a device in a RAID LV fails, device-mapper in  the  kernel  notifies
       the  dmeventd(8)  monitoring process (see Monitoring).  dmeventd can be
       configured to automatically respond using:
       lvm.conf(5) activation/raid_fault_policy
       Possible settings are:
       warn
       A warning is added to the system  log  indicating  that  a  device  has
       failed  in  the RAID LV.  It is left to the user to repair the LV, e.g.
       replace failed devices.
       allocate
       dmeventd automatically attempts to repair the LV using spare devices in
       the  VG.   Note that even a transient failure is treated as a permanent
       failure under this setting.  A new device is allocated  and  full  syn-
       chronization is started.
       The specific command run by dmeventd to warn or repair is:
       lvconvert --repair --use-policies LV

   Corrupted Data
       Data  on  a  device can be corrupted due to hardware errors without the
       device ever being disconnected or there being any fault  in  the  soft-
       ware.  This should be rare, and can be detected (see Scrubbing).

   Rebuild specific PVs
       If  specific  PVs in a RAID LV are known to have corrupt data, the data
       on those PVs can be reconstructed with:
       lvchange --rebuild PV LV
       The rebuild option can be repeated with different PVs  to  replace  the
       data on multiple PVs.

Monitoring
       When a RAID LV is activated the dmeventd(8) process is started to moni-
       tor the health of the LV.  Various events detected in  the  kernel  can
       cause  a  notification  to be sent from device-mapper to the monitoring
       process, including device failures and synchronization completion (e.g.
       for initialization or scrubbing).
       The  LVM  configuration file contains options that affect how the moni-
       toring process will respond to failure events (e.g. raid_fault_policy).
       It  is  possible to turn on and off monitoring with lvchange, but it is
       not recommended to turn this off unless you have a  thorough  knowledge
       of the consequences.

Configuration Options
       There are a number of options in the LVM configuration file that affect
       the behavior of RAID LVs.  The tunable options  are  listed  below.   A
       detailed description of each can be found in the LVM configuration file
       itself.
               mirror_segtype_default
               raid10_segtype_default
               raid_region_size
               raid_fault_policy
               activation_mode

RAID1 Tuning
       A RAID1 LV can be tuned so that certain devices are avoided for reading
       while all devices are still written to.
       lvchange --[raid]writemostly PV[:y|n|t] LV
       The specified device will be marked as "write mostly", which means that
       reading from this device will be avoided, and  other  devices  will  be
       preferred  for  reading  (unless no other devices are available.)  This
       minimizes the I/O to the specified device.
       If the PV name has no suffix, the write mostly attribute  is  set.   If
       the  PV  name has the suffix :n, the write mostly attribute is cleared,
       and the suffix :t toggles the current setting.
       The write mostly option can be repeated on the command line  to  change
       multiple devices at once.
       To  report  the  current  write mostly setting, the lvs attr field will
       show the letter "w" in the 9th position when write mostly is set:
       lvs -a -o name,attr
       When a device is marked write mostly, the maximum number of outstanding
       writes  to that device can be configured.  Once the maximum is reached,
       further writes become synchronous.  When synchronous, a write to the LV
       will not complete until writes to all the mirror images are complete.
       lvchange --[raid]writebehind Number LV
       To report the current write behind setting, run:
       lvs -o name,raid_write_behind
       When  write  behind  is  not configured, or set to 0, all LV writes are
       synchronous.

RAID Takeover
       RAID takeover is converting a RAID LV from one RAID level  to  another,
       e.g.   raid5  to  raid6.   Changing  the  RAID level is usually done to
       increase or decrease resilience to device failures or to restripe  LVs.
       This  is  done using lvconvert and specifying the new RAID level as the
       LV type:
       lvconvert --type RaidLevel LV [PVs]
       The most common and recommended RAID takeover conversions are:

       linear to raid1
              Linear is a single image of LV data, and converting it to  raid1
              adds  a mirror image which is a direct copy of the original lin-
              ear image.

       striped/raid0 to raid4/5/6
              Adding parity devices to a striped volume results in raid4/5/6.

       Unnatural conversions  that  are  not  recommended  include  converting
       between  striped  and  non-striped types.  This is because file systems
       often optimize I/O patterns based on device striping values.  If  those
       values change, it can decrease performance.
       Converting  to  a  higher  RAID level requires allocating new SubLVs to
       hold RAID metadata, and new SubLVs to hold parity blocks for  LV  data.
       Converting  to a lower RAID level removes the SubLVs that are no longer
       needed.
       Conversion often requires full synchronization of the RAID LV (see Syn-
       chronization).  Converting to RAID1 requires copying all LV data blocks
       to N new images on new devices.  Converting  to  a  parity  RAID  level
       requires  reading  all  LV data blocks, calculating parity, and writing
       the new parity blocks.  Synchronization can take a long time  depending
       on  the throughpout of the devices used and the size of the RaidLV.  It
       can degrade performance (rate controls also apply  to  conversion;  see
       --minrecoveryrate and --maxrecoveryrate.)
       Warning:  though  it  is possible to create striped LVs  with up to 128
       stripes, a maximum of 64 stripes can  be  converted  to  raid0,  63  to
       raid4/5  and 62 to raid6 because of the added parity SubLVs.  A striped
       LV with a maximum of 32 stripes can be converted to raid10.

       The following takeover conversions are currently possible:
       o  between striped and raid0.
       o  between linear and raid1.
       o  between mirror and raid1.
       o  between raid1 with two images and raid4/5.
       o  between striped/raid0 and raid4.
       o  between striped/raid0 and raid5.
       o  between striped/raid0 and raid6.
       o  between raid4 and raid5.
       o  between raid4/raid5 and raid6.
       o  between striped/raid0 and raid10.
       o  between striped and raid4.

   Indirect conversions
       Converting from one raid level to another may require  multiple  steps,
       converting first to intermediate raid levels.
       linear to raid6
       To convert an LV from linear to raid6:
       1. convert to raid1 with two images
       2. convert to raid5 (internally raid5_ls) with two images
       3. convert to raid5 with three or more stripes (reshape)
       4. convert to raid6 (internally raid6_ls_6)
       5. convert to raid6 (internally raid6_zr, reshape)
       The commands to perform the steps above are:
       1. lvconvert --type raid1 --mirrors 1 LV
       2. lvconvert --type raid5 LV
       3. lvconvert --stripes 3 LV
       4. lvconvert --type raid6 LV
       5. lvconvert --type raid6 LV
       The  final  conversion from raid6_ls_6 to raid6_zr is done to avoid the
       potential write/recovery performance reduction in raid6_ls_6 because of
       the  dedicated  parity device.  raid6_zr rotates data and parity blocks
       to avoid this.
       linear to striped
       To convert an LV from linear to striped:
       1. convert to raid1 with two images
       2. convert to raid5_n
       3. convert to raid5_n with five 128k stripes (reshape)
       4. convert raid5_n to striped
       The commands to perform the steps above are:
       1. lvconvert --type raid1 --mirrors 1 LV
       2. lvconvert --type raid5_n LV
       3. lvconvert --stripes 5 --stripesize 128k LV
       4. lvconvert --type striped LV
       The raid5_n type in step 2 is used because it has dedicated parity Sub-
       LVs  at  the end, and can be converted to striped directly.  The stripe
       size is increased in step 3 to  add  extra  space  for  the  conversion
       process.   This step grows the LV size by a factor of five.  After con-
       version, this extra space can be reduced (or used to grow the file sys-
       tem using the LV).
       Reversing these steps will convert a striped LV to linear.
       raid6 to striped
       To convert an LV from raid6_nr to striped:
       1. convert to raid6_n_6
       2. convert to striped
       The commands to perform the steps above are:
       1. lvconvert --type raid6_n_6 LV
       2. lvconvert --type striped LV

   Examples
       Converting an LV from linear to raid1.
       # lvs -a -o name,segtype,size vg
         LV   Type   LSize
         lv   linear 300.00g
       # lvconvert --type raid1 --mirrors 1 vg/lv
       # lvs -a -o name,segtype,size vg
         LV            Type   LSize
         lv            raid1  300.00g
         [lv_rimage_0] linear 300.00g
         [lv_rimage_1] linear 300.00g
         [lv_rmeta_0]  linear   3.00m
         [lv_rmeta_1]  linear   3.00m
       Converting an LV from mirror to raid1.
       # lvs -a -o name,segtype,size vg
         LV            Type   LSize
         lv            mirror 100.00g
         [lv_mimage_0] linear 100.00g
         [lv_mimage_1] linear 100.00g
         [lv_mlog]     linear   3.00m
       # lvconvert --type raid1 vg/lv
       # lvs -a -o name,segtype,size vg
         LV            Type   LSize
         lv            raid1  100.00g
         [lv_rimage_0] linear 100.00g
         [lv_rimage_1] linear 100.00g
         [lv_rmeta_0]  linear   3.00m
         [lv_rmeta_1]  linear   3.00m
       Converting an LV from linear to raid1 (with 3 images).
       # lvconvert --type raid1 --mirrors 2 vg/lv
       Converting an LV from striped (with 4 stripes) to raid6_n_6.
       # lvcreate --stripes 4 -L64M -n lv vg
       # lvconvert --type raid6 vg/lv
       # lvs -a -o lv_name,segtype,sync_percent,data_copies
         LV            Type      Cpy%Sync #Cpy
         lv            raid6_n_6 100.00      3
         [lv_rimage_0] linear
         [lv_rimage_1] linear
         [lv_rimage_2] linear
         [lv_rimage_3] linear
         [lv_rimage_4] linear
         [lv_rimage_5] linear
         [lv_rmeta_0]  linear
         [lv_rmeta_1]  linear
         [lv_rmeta_2]  linear
         [lv_rmeta_3]  linear
         [lv_rmeta_4]  linear
         [lv_rmeta_5]  linear
       This  convert  begins  by  allocating MetaLVs (rmeta_#) for each of the
       existing stripe devices.  It then creates  2  additional  MetaLV/DataLV
       pairs (rmeta_#/rimage_#) for dedicated raid6 parity.
       If  rotating data/parity is required, such as with raid6_nr, it must be
       done by reshaping (see below).

RAID Reshaping
       RAID reshaping is changing attributes of a RAID LV  while  keeping  the
       same  RAID  level.  This includes changing RAID layout, stripe size, or
       number of stripes.
       When changing the RAID layout or stripe size, no new SubLVs (MetaLVs or
       DataLVs)  need  to  be  allocated,  but DataLVs are extended by a small
       amount (typically 1 extent).  The extra space allows blocks in a stripe
       to  be  updated  safely, and not be corrupted in case of a crash.  If a
       crash occurs, reshaping can just be restarted.
       (If blocks in a stripe were updated in place, a crash could leave  them
       partially  updated  and corrupted.  Instead, an existing stripe is qui-
       esced, read, changed in layout, and the  new  stripe  written  to  free
       space.  Once that is done, the new stripe is unquiesced and used.)

   Examples
       (Command output shown in examples may change.)
       Converting raid6_n_6 to raid6_nr with rotating data/parity.
       This   conversion   naturally   follows   a  previous  conversion  from
       striped/raid0 to raid6_n_6 (shown above).  It completes the  transition
       to a more traditional RAID6.
       # lvs -o lv_name,segtype,sync_percent,data_copies
         LV            Type      Cpy%Sync #Cpy
         lv            raid6_n_6 100.00      3
         [lv_rimage_0] linear
         [lv_rimage_1] linear
         [lv_rimage_2] linear
         [lv_rimage_3] linear
         [lv_rimage_4] linear
         [lv_rimage_5] linear
         [lv_rmeta_0]  linear
         [lv_rmeta_1]  linear
         [lv_rmeta_2]  linear
         [lv_rmeta_3]  linear
         [lv_rmeta_4]  linear
         [lv_rmeta_5]  linear
       # lvconvert --type raid6_nr vg/lv
       # lvs -a -o lv_name,segtype,sync_percent,data_copies
         LV            Type     Cpy%Sync #Cpy
         lv            raid6_nr 100.00      3
         [lv_rimage_0] linear
         [lv_rimage_0] linear
         [lv_rimage_1] linear
         [lv_rimage_1] linear
         [lv_rimage_2] linear
         [lv_rimage_2] linear
         [lv_rimage_3] linear
         [lv_rimage_3] linear
         [lv_rimage_4] linear
         [lv_rimage_5] linear
         [lv_rmeta_0]  linear
         [lv_rmeta_1]  linear
         [lv_rmeta_2]  linear
         [lv_rmeta_3]  linear
         [lv_rmeta_4]  linear
         [lv_rmeta_5]  linear
       The  DataLVs  are  larger  (additional  segment in each) which provides
       space for out-of-place reshaping.  The result is:
       # lvs -a -o lv_name,segtype,seg_pe_ranges,dataoffset
         LV            Type     PE Ranges          DOff
         lv            raid6_nr lv_rimage_0:0-32 \
                                lv_rimage_1:0-32 \
                                lv_rimage_2:0-32 \
                                lv_rimage_3:0-32
         [lv_rimage_0] linear   /dev/sda:0-31      2048
         [lv_rimage_0] linear   /dev/sda:33-33
         [lv_rimage_1] linear   /dev/sdaa:0-31     2048
         [lv_rimage_1] linear   /dev/sdaa:33-33
         [lv_rimage_2] linear   /dev/sdab:1-33     2048
         [lv_rimage_3] linear   /dev/sdac:1-33     2048
         [lv_rmeta_0]  linear   /dev/sda:32-32
         [lv_rmeta_1]  linear   /dev/sdaa:32-32
         [lv_rmeta_2]  linear   /dev/sdab:0-0
         [lv_rmeta_3]  linear   /dev/sdac:0-0
       All segments with PE ranges '33-33' provide  the  out-of-place  reshape
       space.   The  dataoffset column shows that the data was moved from ini-
       tial offset 0 to 2048 sectors on each component DataLV.
       For performance reasons the raid6_nr RaidLV can be restriped.   Convert
       it from 3-way striped to 5-way-striped.
       # lvconvert --stripes 5 vg/lv
         Using default stripesize 64.00 KiB.
         WARNING: Adding stripes to active logical volume vg/lv will \
         grow it from 99 to 165 extents!
         Run "lvresize -l99 vg/lv" to shrink it or use the additional \
         capacity.
         Logical volume vg/lv successfully converted.
       # lvs vg/lv
         LV   VG     Attr       LSize   Cpy%Sync
         lv   vg     rwi-a-r-s- 652.00m 52.94
       # lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV            Attr       Type     PE Ranges          DOff
         lv            rwi-a-r--- raid6_nr lv_rimage_0:0-33 \
                                           lv_rimage_1:0-33 \
                                           lv_rimage_2:0-33 ... \
                                           lv_rimage_5:0-33 \
                                           lv_rimage_6:0-33   0
         [lv_rimage_0] iwi-aor--- linear   /dev/sda:0-32      0
         [lv_rimage_0] iwi-aor--- linear   /dev/sda:34-34
         [lv_rimage_1] iwi-aor--- linear   /dev/sdaa:0-32     0
         [lv_rimage_1] iwi-aor--- linear   /dev/sdaa:34-34
         [lv_rimage_2] iwi-aor--- linear   /dev/sdab:0-32     0
         [lv_rimage_2] iwi-aor--- linear   /dev/sdab:34-34
         [lv_rimage_3] iwi-aor--- linear   /dev/sdac:1-34     0
         [lv_rimage_4] iwi-aor--- linear   /dev/sdad:1-34     0
         [lv_rimage_5] iwi-aor--- linear   /dev/sdae:1-34     0
         [lv_rimage_6] iwi-aor--- linear   /dev/sdaf:1-34     0
         [lv_rmeta_0]  ewi-aor--- linear   /dev/sda:33-33
         [lv_rmeta_1]  ewi-aor--- linear   /dev/sdaa:33-33
         [lv_rmeta_2]  ewi-aor--- linear   /dev/sdab:33-33
         [lv_rmeta_3]  ewi-aor--- linear   /dev/sdac:0-0
         [lv_rmeta_4]  ewi-aor--- linear   /dev/sdad:0-0
         [lv_rmeta_5]  ewi-aor--- linear   /dev/sdae:0-0
         [lv_rmeta_6]  ewi-aor--- linear   /dev/sdaf:0-0
       Stripes  also  can  be  removed  from  raid5  and 6.  Convert the 5-way
       striped raid6_nr LV to 4-way-striped.  The force  option  needs  to  be
       used,  because  removing stripes (i.e. image SubLVs) from a RaidLV will
       shrink its size.
       # lvconvert --stripes 4 vg/lv
         Using default stripesize 64.00 KiB.
         WARNING: Removing stripes from active logical volume vg/lv will \
         shrink it from 660.00 MiB to 528.00 MiB!
         THIS MAY DESTROY (PARTS OF) YOUR DATA!
         If that leaves the logical volume larger than 206 extents due \
         to stripe rounding,
         you may want to grow the content afterwards (filesystem etc.)
         WARNING: to remove freed stripes after the conversion has finished,\
         you have to run "lvconvert --stripes 4 vg/lv"
         Logical volume vg/lv successfully converted.
       # lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV            Attr       Type     PE Ranges          DOff
         lv            rwi-a-r-s- raid6_nr lv_rimage_0:0-33 \
                                           lv_rimage_1:0-33 \
                                           lv_rimage_2:0-33 ... \
                                           lv_rimage_5:0-33 \
                                           lv_rimage_6:0-33   0
         [lv_rimage_0] Iwi-aor--- linear   /dev/sda:0-32      0
         [lv_rimage_0] Iwi-aor--- linear   /dev/sda:34-34
         [lv_rimage_1] Iwi-aor--- linear   /dev/sdaa:0-32     0
         [lv_rimage_1] Iwi-aor--- linear   /dev/sdaa:34-34
         [lv_rimage_2] Iwi-aor--- linear   /dev/sdab:0-32     0
         [lv_rimage_2] Iwi-aor--- linear   /dev/sdab:34-34
         [lv_rimage_3] Iwi-aor--- linear   /dev/sdac:1-34     0
         [lv_rimage_4] Iwi-aor--- linear   /dev/sdad:1-34     0
         [lv_rimage_5] Iwi-aor--- linear   /dev/sdae:1-34     0
         [lv_rimage_6] Iwi-aor-R- linear   /dev/sdaf:1-34     0
         [lv_rmeta_0]  ewi-aor--- linear   /dev/sda:33-33
         [lv_rmeta_1]  ewi-aor--- linear   /dev/sdaa:33-33
         [lv_rmeta_2]  ewi-aor--- linear   /dev/sdab:33-33
         [lv_rmeta_3]  ewi-aor--- linear   /dev/sdac:0-0
         [lv_rmeta_4]  ewi-aor--- linear   /dev/sdad:0-0
         [lv_rmeta_5]  ewi-aor--- linear   /dev/sdae:0-0
         [lv_rmeta_6]  ewi-aor-R- linear   /dev/sdaf:0-0
       The 's' in column 9 of the attribute field shows the  RaidLV  is  still
       reshaping.  The 'R' in the same column of the attribute field shows the
       freed image Sub LVs which will need removing once  the  reshaping  fin-
       ished.
       # lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV   Attr       Type     PE Ranges          DOff
         lv   rwi-a-r-R- raid6_nr lv_rimage_0:0-33 \
                                  lv_rimage_1:0-33 \
                                  lv_rimage_2:0-33 ... \
                                  lv_rimage_5:0-33 \
                                  lv_rimage_6:0-33   8192
       Now  that  the reshape is finished the 'R' atribute on the RaidLV shows
       images can be removed.
       # lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV   Attr       Type     PE Ranges          DOff
         lv   rwi-a-r-R- raid6_nr lv_rimage_0:0-33 \
                                  lv_rimage_1:0-33 \
                                  lv_rimage_2:0-33 ... \
                                  lv_rimage_5:0-33 \
                                  lv_rimage_6:0-33   8192
       This is achieved by  repeating  the  command  ("lvconvert  --stripes  4
       vg/lv" would be sufficient).
       # lvconvert --stripes 4 vg/lv
         Using default stripesize 64.00 KiB.
         Logical volume vg/lv successfully converted.
       # lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV            Attr       Type     PE Ranges          DOff
         lv            rwi-a-r--- raid6_nr lv_rimage_0:0-33 \
                                           lv_rimage_1:0-33 \
                                           lv_rimage_2:0-33 ... \
                                           lv_rimage_5:0-33   8192
         [lv_rimage_0] iwi-aor--- linear   /dev/sda:0-32      8192
         [lv_rimage_0] iwi-aor--- linear   /dev/sda:34-34
         [lv_rimage_1] iwi-aor--- linear   /dev/sdaa:0-32     8192
         [lv_rimage_1] iwi-aor--- linear   /dev/sdaa:34-34
         [lv_rimage_2] iwi-aor--- linear   /dev/sdab:0-32     8192
         [lv_rimage_2] iwi-aor--- linear   /dev/sdab:34-34
         [lv_rimage_3] iwi-aor--- linear   /dev/sdac:1-34     8192
         [lv_rimage_4] iwi-aor--- linear   /dev/sdad:1-34     8192
         [lv_rimage_5] iwi-aor--- linear   /dev/sdae:1-34     8192
         [lv_rmeta_0]  ewi-aor--- linear   /dev/sda:33-33
         [lv_rmeta_1]  ewi-aor--- linear   /dev/sdaa:33-33
         [lv_rmeta_2]  ewi-aor--- linear   /dev/sdab:33-33
         [lv_rmeta_3]  ewi-aor--- linear   /dev/sdac:0-0
         [lv_rmeta_4]  ewi-aor--- linear   /dev/sdad:0-0
         [lv_rmeta_5]  ewi-aor--- linear   /dev/sdae:0-0
       # lvs -a -o lv_name,attr,segtype,reshapelen vg
         LV            Attr       Type     RSize
         lv            rwi-a-r--- raid6_nr 24.00m
         [lv_rimage_0] iwi-aor--- linear    4.00m
         [lv_rimage_0] iwi-aor--- linear
         [lv_rimage_1] iwi-aor--- linear    4.00m
         [lv_rimage_1] iwi-aor--- linear
         [lv_rimage_2] iwi-aor--- linear    4.00m
         [lv_rimage_2] iwi-aor--- linear
         [lv_rimage_3] iwi-aor--- linear    4.00m
         [lv_rimage_4] iwi-aor--- linear    4.00m
         [lv_rimage_5] iwi-aor--- linear    4.00m
         [lv_rmeta_0]  ewi-aor--- linear
         [lv_rmeta_1]  ewi-aor--- linear
         [lv_rmeta_2]  ewi-aor--- linear
         [lv_rmeta_3]  ewi-aor--- linear
         [lv_rmeta_4]  ewi-aor--- linear
         [lv_rmeta_5]  ewi-aor--- linear
       Future  developments  might  include  automatic  removal  of  the freed
       images.
       If the reshape space shall be removed any lvconvert command not  chang-
       ing the layout can be used:
       # lvconvert --stripes 4 vg/lv
         Using default stripesize 64.00 KiB.
         No change in RAID LV vg/lv layout, freeing reshape space.
         Logical volume vg/lv successfully converted.
       # lvs -a -o lv_name,attr,segtype,reshapelen vg
         LV            Attr       Type     RSize
         lv            rwi-a-r--- raid6_nr    0
         [lv_rimage_0] iwi-aor--- linear      0
         [lv_rimage_0] iwi-aor--- linear
         [lv_rimage_1] iwi-aor--- linear      0
         [lv_rimage_1] iwi-aor--- linear
         [lv_rimage_2] iwi-aor--- linear      0
         [lv_rimage_2] iwi-aor--- linear
         [lv_rimage_3] iwi-aor--- linear      0
         [lv_rimage_4] iwi-aor--- linear      0
         [lv_rimage_5] iwi-aor--- linear      0
         [lv_rmeta_0]  ewi-aor--- linear
         [lv_rmeta_1]  ewi-aor--- linear
         [lv_rmeta_2]  ewi-aor--- linear
         [lv_rmeta_3]  ewi-aor--- linear
         [lv_rmeta_4]  ewi-aor--- linear
         [lv_rmeta_5]  ewi-aor--- linear
       In case the RaidLV should be converted to striped:
       # lvconvert --type striped vg/lv
         Unable to convert LV vg/lv from raid6_nr to striped.
         Converting vg/lv from raid6_nr is directly possible to the \
         following layouts:
           raid6_nc
           raid6_zr
           raid6_la_6
           raid6_ls_6
           raid6_ra_6
           raid6_rs_6
           raid6_n_6
       A  direct conversion isn't possible thus the command informed about the
       possible ones.  raid6_n_6 is suitable to convert to striped so  convert
       to  it first (this is a reshape changing the raid6 layout from raid6_nr
       to raid6_n_6).
       # lvconvert --type raid6_n_6
         Using default stripesize 64.00 KiB.
         Converting raid6_nr LV vg/lv to raid6_n_6.
       Are you sure you want to convert raid6_nr LV vg/lv? [y/n]: y
         Logical volume vg/lv successfully converted.
       Wait for the reshape to finish.
       # lvconvert --type striped vg/lv
         Logical volume vg/lv successfully converted.
       # lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV   Attr       Type    PE Ranges  DOff
         lv   -wi-a----- striped /dev/sda:2-32 \
                                 /dev/sdaa:2-32 \
                                 /dev/sdab:2-32 \
                                 /dev/sdac:3-33
         lv   -wi-a----- striped /dev/sda:34-35 \
                                 /dev/sdaa:34-35 \
                                 /dev/sdab:34-35 \
                                 /dev/sdac:34-35
       From striped we can convert to raid10
       # lvconvert --type raid10 vg/lv
         Using default stripesize 64.00 KiB.
         Logical volume vg/lv successfully converted.
       # lvs -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         LV   Attr       Type   PE Ranges          DOff
         lv   rwi-a-r--- raid10 lv_rimage_0:0-32 \
                                lv_rimage_4:0-32 \
                                lv_rimage_1:0-32 ... \
                                lv_rimage_3:0-32 \
                                lv_rimage_7:0-32   0
       # lvs -a -o lv_name,attr,segtype,seg_pe_ranges,dataoffset vg
         WARNING: Cannot find matching striped segment for vg/lv_rimage_3.
         LV            Attr       Type   PE Ranges          DOff
         lv            rwi-a-r--- raid10 lv_rimage_0:0-32 \
                                         lv_rimage_4:0-32 \
                                         lv_rimage_1:0-32 ... \
                                         lv_rimage_3:0-32 \
                                         lv_rimage_7:0-32   0
         [lv_rimage_0] iwi-aor--- linear /dev/sda:2-32      0
         [lv_rimage_0] iwi-aor--- linear /dev/sda:34-35
         [lv_rimage_1] iwi-aor--- linear /dev/sdaa:2-32     0
         [lv_rimage_1] iwi-aor--- linear /dev/sdaa:34-35
         [lv_rimage_2] iwi-aor--- linear /dev/sdab:2-32     0
         [lv_rimage_2] iwi-aor--- linear /dev/sdab:34-35
         [lv_rimage_3] iwi-XXr--- linear /dev/sdac:3-35     0
         [lv_rimage_4] iwi-aor--- linear /dev/sdad:1-33     0
         [lv_rimage_5] iwi-aor--- linear /dev/sdae:1-33     0
         [lv_rimage_6] iwi-aor--- linear /dev/sdaf:1-33     0
         [lv_rimage_7] iwi-aor--- linear /dev/sdag:1-33     0
         [lv_rmeta_0]  ewi-aor--- linear /dev/sda:0-0
         [lv_rmeta_1]  ewi-aor--- linear /dev/sdaa:0-0
         [lv_rmeta_2]  ewi-aor--- linear /dev/sdab:0-0
         [lv_rmeta_3]  ewi-aor--- linear /dev/sdac:0-0
         [lv_rmeta_4]  ewi-aor--- linear /dev/sdad:0-0
         [lv_rmeta_5]  ewi-aor--- linear /dev/sdae:0-0
         [lv_rmeta_6]  ewi-aor--- linear /dev/sdaf:0-0
         [lv_rmeta_7]  ewi-aor--- linear /dev/sdag:0-0
       raid10 allows to add stripes but can't remove them.

       A more elaborate example to convert from linear to striped with interim
       conversions to raid1 then raid5 followed by restripe (4 steps).
       We start with the linear LV.
       # lvs -a -o name,size,segtype,syncpercent,datastripes,\
                   stripesize,reshapelenle,devices vg
         LV   LSize   Type   Cpy%Sync #DStr Stripe RSize Devices
         lv   128.00m linear              1     0        /dev/sda(0)
       Then convert it to a 2-way raid1.
       # lvconvert --mirrors 1 vg/lv
         Logical volume vg/lv successfully converted.
       # lvs -a -o name,size,segtype,datastripes,\
                   stripesize,reshapelenle,devices vg
         LV            LSize   Type   #DStr Stripe RSize Devices
         lv            128.00m raid1      2     0        lv_rimage_0(0),\
                                                         lv_rimage_1(0)
         [lv_rimage_0] 128.00m linear     1     0        /dev/sda(0)
         [lv_rimage_1] 128.00m linear     1     0        /dev/sdhx(1)
         [lv_rmeta_0]    4.00m linear     1     0        /dev/sda(32)
         [lv_rmeta_1]    4.00m linear     1     0        /dev/sdhx(0)
       Once  the raid1 LV is fully synchronized we convert it to raid5_n (only
       2-way raid1 LVs can be converted to raid5).   We  select  raid5_n  here
       because  it has dedicated parity SubLVs at the end and can be converted
       to striped directly without any additional conversion.
       # lvconvert --type raid5_n vg/lv
         Using default stripesize 64.00 KiB.
         Logical volume vg/lv successfully converted.
       # lvs -a -o name,size,segtype,syncpercent,datastripes,\
                   stripesize,reshapelenle,devices vg
         LV            LSize   Type    #DStr Stripe RSize Devices
         lv            128.00m raid5_n     1 64.00k     0 lv_rimage_0(0),\
                                                          lv_rimage_1(0)
         [lv_rimage_0] 128.00m linear      1     0      0 /dev/sda(0)
         [lv_rimage_1] 128.00m linear      1     0      0 /dev/sdhx(1)
         [lv_rmeta_0]    4.00m linear      1     0        /dev/sda(32)
         [lv_rmeta_1]    4.00m linear      1     0        /dev/sdhx(0)
       Now we'll change the number of data stripes from 1  to  5  and  request
       128K  stripe size in one command.  This will grow the size of the LV by
       a factor of 5 (we add 4 data stripes to the one given).  That additonal
       space  can  be  used by e.g. growing any contained filesystem or the LV
       can be reduced in size after the reshaping conversion has finished.
       # lvconvert --stripesize 128k --stripes 5 vg/lv
         Converting stripesize 64.00 KiB of raid5_n LV vg/lv to 128.00 KiB.
         WARNING: Adding stripes to active logical volume vg/lv will grow \
         it from 32 to 160 extents!
         Run "lvresize -l32 vg/lv" to shrink it or use the additional capacity.
         Logical volume vg/lv successfully converted.
       # lvs -a -o name,size,segtype,datastripes,\
                   stripesize,reshapelenle,devices
         LV            LSize   Type    #DStr Stripe  RSize Devices
         lv            640.00m raid5_n     5 128.00k     6 lv_rimage_0(0),\
                                                           lv_rimage_1(0),\
                                                           lv_rimage_2(0),\
                                                           lv_rimage_3(0),\
                                                           lv_rimage_4(0),\
                                                           lv_rimage_5(0)
         [lv_rimage_0] 132.00m linear      1      0      1 /dev/sda(33)
         [lv_rimage_0] 132.00m linear      1      0        /dev/sda(0)
         [lv_rimage_1] 132.00m linear      1      0      1 /dev/sdhx(33)
         [lv_rimage_1] 132.00m linear      1      0        /dev/sdhx(1)
         [lv_rimage_2] 132.00m linear      1      0      1 /dev/sdhw(33)
         [lv_rimage_2] 132.00m linear      1      0        /dev/sdhw(1)
         [lv_rimage_3] 132.00m linear      1      0      1 /dev/sdhv(33)
         [lv_rimage_3] 132.00m linear      1      0        /dev/sdhv(1)
         [lv_rimage_4] 132.00m linear      1      0      1 /dev/sdhu(33)
         [lv_rimage_4] 132.00m linear      1      0        /dev/sdhu(1)
         [lv_rimage_5] 132.00m linear      1      0      1 /dev/sdht(33)
         [lv_rimage_5] 132.00m linear      1      0        /dev/sdht(1)
         [lv_rmeta_0]    4.00m linear      1      0        /dev/sda(32)
         [lv_rmeta_1]    4.00m linear      1      0        /dev/sdhx(0)
         [lv_rmeta_2]    4.00m linear      1      0        /dev/sdhw(0)
         [lv_rmeta_3]    4.00m linear      1      0        /dev/sdhv(0)
         [lv_rmeta_4]    4.00m linear      1      0        /dev/sdhu(0)
         [lv_rmeta_5]    4.00m linear      1      0        /dev/sdht(0)
       Once the conversion has finished we can can convert to striped.
       # lvconvert --type striped vg/lv
         Logical volume vg/lv successfully converted.
       # lvs -a -o name,size,segtype,datastripes,\
                   stripesize,reshapelenle,devices vg
         LV   LSize   Type    #DStr Stripe  RSize Devices
         lv   640.00m striped     5 128.00k       /dev/sda(33),\
                                                  /dev/sdhx(33),\
                                                  /dev/sdhw(33),\
                                                  /dev/sdhv(33),\
                                                  /dev/sdhu(33)
         lv   640.00m striped     5 128.00k       /dev/sda(0),\
                                                  /dev/sdhx(1),\
                                                  /dev/sdhw(1),\
                                                  /dev/sdhv(1),\
                                                  /dev/sdhu(1)
       Reversing these steps will convert a given striped LV to linear.
       Mind the facts that stripes are removed thus the capacity of the RaidLV
       will shrink and that changing the RaidLV layout will influence its per-
       formance.
       "lvconvert --stripes 1 vg/lv" for converting to 1  stripe  will  inform
       upfront  about  the  reduced  size to allow for resizing the content or
       growing the RaidLV before actually converting to 1 stripe.  The --force
       option  is  needed to allow stripe removing conversions to prevent data
       loss.
       Of course any interim step can be the intended last one (e.g. striped->
       raid1).

RAID5 Variants
       raid5_ls
       o RAID5 left symmetric
       o Rotating parity N with data restart
       raid5_la
       o RAID5 left symmetric
       o Rotating parity N with data continuation
       raid5_rs
       o RAID5 right symmetric
       o Rotating parity 0 with data restart
       raid5_ra
       o RAID5 right asymmetric
       o Rotating parity 0 with data continuation
       raid5_n
       o RAID5 parity n
       o Dedicated parity device n used for striped/raid0 conversions
       o Used for RAID Takeover

RAID6 Variants
       raid6
       o RAID6 zero restart (aka left symmetric)
       o Rotating parity 0 with data restart
       o Same as raid6_zr
       raid6_zr
       o RAID6 zero restart (aka left symmetric)
       o Rotating parity 0 with data restart
       raid6_nr
       o RAID6 N restart (aka right symmetric)
       o Rotating parity N with data restart
       raid6_nc
       o RAID6 N continue
       o Rotating parity N with data continuation
       raid6_n_6
       o RAID6 last parity devices
       o Fixed dedicated last devices (P-Syndrome N-1 and Q-Syndrome N)
         with striped data used for striped/raid0 conversions
       o Used for RAID Takeover
       raid6_{ls,rs,la,ra}_6
       o RAID6 last parity device
       o Dedicated last parity device used for conversions from/to
         raid5_{ls,rs,la,ra}
       raid6_ls_6
       o RAID6 N continue
       o Same as raid5_ls for N-1 devices with fixed Q-Syndrome N
       o Used for RAID Takeover
       raid6_la_6
       o RAID6 N continue
       o Same as raid5_la for N-1 devices with fixed Q-Syndrome N
       o Used forRAID Takeover
       raid6_rs_6
       o RAID6 N continue
       o Same as raid5_rs for N-1 devices with fixed Q-Syndrome N
       o Used for RAID Takeover
       raid6_ra_6
       o RAID6 N continue
       o Same as raid5_ra for N-1 devices with fixed Q-Syndrome N
       o Used for RAID Takeover


History
       The  2.6.38-rc1  version of the Linux kernel introduced a device-mapper
       target to interface with the software RAID  (MD)  personalities.   This
       provided device-mapper with RAID 4/5/6 capabilities and a larger devel-
       opment community.  Later, support for RAID1, RAID10, and  RAID1E  (RAID
       10 variants) were added.  Support for these new kernel RAID targets was
       added to LVM version 2.02.87.  The capabilities of the LVM  raid1  type
       have  surpassed  the old mirror type.  raid1 is now recommended instead
       of mirror.  raid1 became the  default  for  mirroring  in  LVM  version
       2.02.100.


Red Hat, Inc       LVM TOOLS 2.02.187(2)-RHEL7 (2020-03-24)         LVMRAID(7)