LVMVDO(7) LVMVDO(7)
NAME
lvmvdo -- Support for Virtual Data Optimizer in LVM
DESCRIPTION
VDO is software that provides inline block-level deduplication, com-
pression, and thin provisioning capabilities for primary storage.
Deduplication is a technique for reducing the consumption of storage
resources by eliminating multiple copies of duplicate blocks. Compres-
sion takes the individual unique blocks and shrinks them. These
reduced blocks are then efficiently packed together into physical
blocks. Thin provisioning manages the mapping from logical blocks pre-
sented by VDO to where the data has actually been physically stored,
and also eliminates any blocks of all zeroes.
With deduplication, instead of writing the same data more than once,
VDO detects and records each duplicate block as a reference to the
original block. VDO maintains a mapping from Logical Block Addresses
(LBA) (used by the storage layer above VDO) to physical block addresses
(used by the storage layer under VDO). After deduplication, multiple
logical block addresses may be mapped to the same physical block
address; these are called shared blocks and are reference-counted by
the software.
With compression, VDO compresses multiple blocks (or shared blocks)
with the fast LZ4 algorithm, and bins them together where possible so
that multiple compressed blocks fit within a 4 KB block on the underly-
ing storage. Mapping from LBA is to a physical block address and index
within it for the desired compressed data. All compressed blocks are
individually reference counted for correctness.
Block sharing and block compression are invisible to applications using
the storage, which read and write blocks as they would if VDO were not
present. When a shared block is overwritten, a new physical block is
allocated for storing the new block data to ensure that other logical
block addresses that are mapped to the shared physical block are not
modified.
To use VDO with lvm(8), you must install the standard VDO user-space
tools vdoformat(8) and the currently non-standard kernel VDO module
"kvdo".
The "kvdo" module implements fine-grained storage virtualization, thin
provisioning, block sharing, and compression. The "uds" module pro-
vides memory-efficient duplicate identification. The user-space tools
include vdostats(8) for extracting statistics from VDO volumes.
VDO TERMS
VDODataLV
VDO data LV
A large hidden LV with the _vdata suffix. It is created in a VG
used by the VDO kernel target to store all data and metadata
blocks.
VDOPoolLV
VDO pool LV
A pool for virtual VDOLV(s), which are the size of used VDO-
DataLV.
Only a single VDOLV is currently supported.
VDOLV
VDO LV
Created from VDOPoolLV.
Appears blank after creation.
VDO USAGE
The primary methods for using VDO with lvm2:
1. Create a VDOPoolLV and a VDOLV
Create a VDOPoolLV that will hold VDO data, and a virtual size VDOLV
that the user can use. If you do not specify the virtual size, then the
VDOLV is created with the maximum size that always fits into data vol-
ume even if no deduplication or compression can happen (i.e. it can
hold the incompressible content of /dev/urandom). If you do not spec-
ify the name of VDOPoolLV, it is taken from the sequence of vpool0,
vpool1 ...
Note: The performance of TRIM/Discard operations is slow for large vol-
umes of VDO type. Please try to avoid sending discard requests unless
necessary because it might take considerable amount of time to finish
the discard operation.
lvcreate --type vdo -n VDOLV -L DataSize -V LargeVirtualSize VG/VDOPoolLV
lvcreate --vdo -L DataSize VG
Example
# lvcreate --type vdo -n vdo0 -L 10G -V 100G vg/vdopool0
# mkfs.ext4 -E nodiscard /dev/vg/vdo0
2. Convert an existing LV into VDOPoolLV
Convert an already created or existing LV into a VDOPoolLV, which is a
volume that can hold data and metadata. You will be prompted to con-
firm such conversion because it IRREVERSIBLY DESTROYS the content of
such volume and the volume is immediately formatted by vdoformat(8) as
a VDO pool data volume. You can specify the virtual size of the VDOLV
associated with this VDOPoolLV. If you do not specify the virtual
size, it will be set to the maximum size that can keep 100% incompress-
ible data there.
lvconvert --type vdo-pool -n VDOLV -V VirtualSize VG/VDOPoolLV
lvconvert --vdopool VG/VDOPoolLV
Example
# lvconvert --type vdo-pool -n vdo0 -V10G vg/ExistingLV
3. Change the compression and deduplication of a VDOPoolLV
Disable or enable the compression and deduplication for VDOPoolLV (the
volume that maintains all VDO LV(s) associated with it).
lvchange --compression y|n --deduplication y|n VG/VDOPoolLV
Example
# lvchange --compression n vg/vdopool0
# lvchange --deduplication y vg/vdopool1
4. Change the default settings used for creating a VDOPoolLV
VDO allows to set a large variety of options. Lots of these settings
can be specified in lvm.conf or profile settings. You can prepare a
number of different profiles in the /etc/lvm/profile directory and just
specify the profile file name. Check the output of lvmconfig --type
default --withcomments for a detailed description of all individual VDO
settings.
Example
# cat <<EOF > /etc/lvm/profile/vdo_create.profile
allocation {
vdo_use_compression=1
vdo_use_deduplication=1
vdo_use_metadata_hints=1
vdo_minimum_io_size=4096
vdo_block_map_cache_size_mb=128
vdo_block_map_period=16380
vdo_use_sparse_index=0
vdo_index_memory_size_mb=256
vdo_slab_size_mb=2048
vdo_ack_threads=1
vdo_bio_threads=1
vdo_bio_rotation=64
vdo_cpu_threads=2
vdo_hash_zone_threads=1
vdo_logical_threads=1
vdo_physical_threads=1
vdo_write_policy="auto"
vdo_max_discard=1
}
EOF
# lvcreate --vdo -L10G --metadataprofile vdo_create vg/vdopool0
# lvcreate --vdo -L10G --config 'allocation/vdo_cpu_threads=4' vg/vdopool1
5. Set or change VDO settings with option --vdosettings
Use the form 'option=value' or 'option1=value option2=value', or repeat
--vdosettings for each option being set. Options are listed in the
Example section above, for the full description see lvm.conf(5).
Options can omit 'vdo_' and 'vdo_use_' prefixes and all its under-
scores. So i.e. vdo_use_metadata_hints=1 and metadatahints=1 are
equivalent. To change the option for an already existing VDOPoolLV use
lvchange(8) command. However not all option can be changed. Only com-
pression and deduplication options can be also changed for an active
VDO LV. Lowest priority options are specified with configuration file,
then with --vdosettings and highest are expliction option --compression
and --deduplication.
Example
# lvcreate --vdo -L10G --vdosettings 'ack_threads=1 hash_zone_threads=2' vg/vdopool0
# lvchange --vdosettings 'bio_threads=2 deduplication=1' vg/vdopool0
6. Checking the usage of VDOPoolLV
To quickly check how much data on a VDOPoolLV is already consumed, use
lvs(8). The Data% field reports how much data is occupied in the con-
tent of the virtual data for the VDOLV and how much space is already
consumed with all the data and metadata blocks in the VDOPoolLV. For a
detailed description, use the vdostats(8) command.
Note: vdostats(8) currently understands only /dev/mapper device names.
Example
# lvcreate --type vdo -L10G -V20G -n vdo0 vg/vdopool0
# mkfs.ext4 -E nodiscard /dev/vg/vdo0
# lvs -a vg
LV VG Attr LSize Pool Origin Data%
vdo0 vg vwi-a-v--- 20.00g vdopool0 0.01
vdopool0 vg dwi-ao---- 10.00g 30.16
[vdopool0_vdata] vg Dwi-ao---- 10.00g
# vdostats --all /dev/mapper/vg-vdopool0-vpool
/dev/mapper/vg-vdopool0 :
version : 30
release version : 133524
data blocks used : 79
...
7. Extending the VDOPoolLV size
You can add more space to hold VDO data and metadata by extending the
VDODataLV using the commands lvresize(8) and lvextend(8). The exten-
sion needs to add at least one new VDO slab. You can configure the slab
size with the allocation/vdo_slab_size_mb setting.
You can also enable automatic size extension of a monitored VDOPoolLV
with the activation/vdo_pool_autoextend_percent and activation/
vdo_pool_autoextend_threshold settings.
Note: You cannot reduce the size of a VDOPoolLV.
lvextend -L+AddingSize VG/VDOPoolLV
Example
# lvextend -L+50G vg/vdopool0
# lvresize -L300G vg/vdopool1
8. Extending or reducing the VDOLV size
You can extend or reduce a virtual VDO LV as a standard LV with the
lvresize(8), lvextend(8), and lvreduce(8) commands.
Note: The reduction needs to process TRIM for reduced disk area to
unmap used data blocks from the VDOPoolLV, which might take a long
time.
lvextend -L+AddingSize VG/VDOLV
lvreduce -L-ReducingSize VG/VDOLV
Example
# lvextend -L+50G vg/vdo0
# lvreduce -L-50G vg/vdo1
# lvresize -L200G vg/vdo2
9. Component activation of a VDODataLV
You can activate a VDODataLV separately as a component LV for examina-
tion purposes. The activation of the VDODataLV activates the data LV in
read-only mode, and the data LV cannot be modified. If the VDODataLV
is active as a component, any upper LV using this volume CANNOT be
activated. You have to deactivate the VDODataLV first to continue to
use the VDOPoolLV.
Example
# lvchange -ay vg/vpool0_vdata
# lvchange -an vg/vpool0_vdata
VDO TOPICS
1. Stacking VDO
You can convert or stack a VDOPooLV with these currently supported vol-
ume types: linear, stripe, raid, and cache with cachepool.
2. VDOPoolLV on top of raid
Using a raid type LV for a VDODataLV.
Example
# lvcreate --type raid1 -L 5G -n vdopool vg
# lvconvert --type vdo-pool -V 10G vg/vdopool
3. Caching a VDOPoolLV
VDOPoolLV (accepts also VDODataLV volume name) caching provides a mech-
anism to accelerate reads and writes of already compressed and dedupli-
cated data blocks together with VDO metadata.
Example
# lvcreate --type vdo -L 5G -V 10G -n vdo1 vg/vdopool
# lvcreate --type cache-pool -L 1G -n cachepool vg
# lvconvert --cache --cachepool vg/cachepool vg/vdopool
# lvconvert --uncache vg/vdopool
4. Caching a VDOLV
VDO LV cache allow you to 'cache' a device for better performance
before it hits the processing of the VDO Pool LV layer.
Example
# lvcreate --type vdo -L 5G -V 10G -n vdo1 vg/vdopool
# lvcreate --type cache-pool -L 1G -n cachepool vg
# lvconvert --cache --cachepool vg/cachepool vg/vdo1
# lvconvert --uncache vg/vdo1
5. Usage of Discard/TRIM with a VDOLV
You can discard data on a VDO LV and reduce used blocks on a VDOPoolLV.
However, the current performance of discard operations is still not
optimal and takes a considerable amount of time and CPU. Unless you
really need it, you should avoid using discard.
When a block device is going to be rewritten, its blocks will be auto-
matically reused for new data. Discard is useful in situations when
user knows that the given portion of a VDO LV is not going to be used
and the discarded space can be used for block provisioning in other
regions of the VDO LV. For the same reason, you should avoid using
mkfs with discard for a freshly created VDO LV to save a lot of time
that this operation would take otherwise as device is already expected
to be empty.
6. Memory usage
The VDO target requires 38 MiB of RAM and several variable amounts:
o 1.15 MiB of RAM for each 1 MiB of configured block map cache size.
The block map cache requires a minimum of 150 MiB RAM.
o 1.6 MiB of RAM for each 1 TiB of logical space.
o 268 MiB of RAM for each 1 TiB of physical storage managed by the vol-
ume.
UDS requires a minimum of 250 MiB of RAM, which is also the default
amount that deduplication uses.
The memory required for the UDS index is determined by the index type
and the required size of the deduplication window and is controlled by
the allocation/vdo_use_sparse_index setting.
With enabled UDS sparse indexing, it relies on the temporal locality of
data and attempts to retain only the most relevant index entries in
memory and can maintain a deduplication window that is ten times larger
than with dense while using the same amount of memory.
Although the sparse index provides the greatest coverage, the dense
index provides more deduplication advice. For most workloads, given
the same amount of memory, the difference in deduplication rates
between dense and sparse indexes is negligible.
A dense index with 1 GiB of RAM maintains a 1 TiB deduplication window,
while a sparse index with 1 GiB of RAM maintains a 10 TiB deduplication
window. In general, 1 GiB is sufficient for 4 TiB of physical space
with a dense index and 40 TiB with a sparse index.
7. Storage space requirements
You can configure a VDOPoolLV to use up to 256 TiB of physical storage.
Only a certain part of the physical storage is usable to store data.
This section provides the calculations to determine the usable size of
a VDO-managed volume.
The VDO target requires storage for two types of VDO metadata and for
the UDS index:
o The first type of VDO metadata uses approximately 1 MiB for each 4
GiB of physical storage plus an additional 1 MiB per slab.
o The second type of VDO metadata consumes approximately 1.25 MiB for
each 1 GiB of logical storage, rounded up to the nearest slab.
o The amount of storage required for the UDS index depends on the type
of index and the amount of RAM allocated to the index. For each 1 GiB
of RAM, a dense UDS index uses 17 GiB of storage and a sparse UDS
index will use 170 GiB of storage.
SEE ALSO
lvm(8), lvm.conf(5), lvmconfig(8), lvcreate(8), lvconvert(8),
lvchange(8), lvextend(8), lvreduce(8), lvresize(8), lvremove(8),
lvs(8),
vdo(8), vdoformat(8), vdostats(8),
mkfs(8)
Red Hat, Inc LVM TOOLS 2.03.14(2)-RHEL8 (2021-10-20) LVMVDO(7)
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