xfs_db(8) System Manager's Manual xfs_db(8)
NAME
xfs_db - debug an XFS filesystem
SYNOPSIS
xfs_db [ -c cmd ] ... [ -i|r|x|F ] [ -f ] [ -l logdev ] [ -p progname ]
device
xfs_db -V
DESCRIPTION
xfs_db is used to examine an XFS filesystem. Under rare circumstances
it can also be used to modify an XFS filesystem, but that task is nor-
mally left to xfs_repair(8) or to scripts such as xfs_admin(8) that run
xfs_db.
OPTIONS
-c cmd xfs_db commands may be run interactively (the default) or as
arguments on the command line. Multiple -c arguments may be
given. The commands are run in the sequence given, then the pro-
gram exits.
-f Specifies that the filesystem image to be processed is stored in
a regular file at device (see the mkfs.xfs(8) -d file option).
This might happen if an image copy of a filesystem has been made
into an ordinary file with xfs_copy(8).
-F Specifies that we want to continue even if the superblock magic
is not correct. For use in xfs_metadump.
-i Allows execution on a mounted filesystem, provided it is mounted
read-only. Useful for shell scripts which must only operate on
filesystems in a guaranteed consistent state (either unmounted
or mounted read-only). These semantics are slightly different to
that of the -r option.
-l logdev
Specifies the device where the filesystems external log resides.
Only for those filesystems which use an external log. See the
mkfs.xfs(8) -l option, and refer to xfs(5) for a detailed
description of the XFS log.
-p progname
Set the program name to progname for prompts and some error mes-
sages, the default value is xfs_db.
-r Open device or filename read-only. This option is required if
the filesystem is mounted. It is only necessary to omit this
flag if a command that changes data (write, blocktrash, crc) is
to be used.
-x Specifies expert mode. This enables the (write, blocktrash, crc
invalidate/revalidate) commands.
-V Prints the version number and exits.
CONCEPTS
xfs_db commands can be broken up into two classes. Most commands are
for the navigation and display of data structures in the filesystem.
Other commands are for scanning the filesystem in some way.
Commands which are used to navigate the filesystem structure take argu-
ments which reflect the names of filesystem structure fields. There
can be multiple field names separated by dots when the underlying
structures are nested, as in C. The field names can be indexed (as an
array index) if the underlying field is an array. The array indices
can be specified as a range, two numbers separated by a dash.
xfs_db maintains a current address in the filesystem. The granularity
of the address is a filesystem structure. This can be a filesystem
block, an inode or quota (smaller than a filesystem block), or a direc-
tory block (could be larger than a filesystem block). There are a
variety of commands to set the current address. Associated with the
current address is the current data type, which is the structural type
of this data. Commands which follow the structure of the filesystem
always set the type as well as the address. Commands which examine
pieces of an individual file (inode) need the current inode to be set,
this is done with the inode command.
The current address/type information is actually maintained in a stack
that can be explicitly manipulated with the push, pop, and stack com-
mands. This allows for easy examination of a nested filesystem struc-
ture. Also, the last several locations visited are stored in a ring
buffer which can be manipulated with the forward, back, and ring com-
mands.
XFS filesystems are divided into a small number of allocation groups.
xfs_db maintains a notion of the current allocation group which is
manipulated by some commands. The initial allocation group is 0.
COMMANDS
Many commands have extensive online help. Use the help command for more
details on any command.
a See the addr command.
ablock filoff
Set current address to the offset filoff (a filesystem block
number) in the attribute area of the current inode.
addr [field-expression]
Set current address to the value of the field-expression. This
is used to "follow" a reference in one structure to the object
being referred to. If no argument is given, the current address
is printed.
agf [agno]
Set current address to the AGF block for allocation group agno.
If no argument is given, use the current allocation group.
agfl [agno]
Set current address to the AGFL block for allocation group agno.
If no argument is given, use the current allocation group.
agi [agno]
Set current address to the AGI block for allocation group agno.
If no argument is given, use the current allocation group.
b See the back command.
back Move to the previous location in the position ring.
blockfree
Free block usage information collected by the last execution of
the blockget command. This must be done before another blockget
command can be given, presumably with different arguments than
the previous one.
blockget [-npvs] [-b bno] ... [-i ino] ...
Get block usage and check filesystem consistency. The informa-
tion is saved for use by a subsequent blockuse, ncheck, or
blocktrash command.
-b is used to specify filesystem block numbers about which
verbose information should be printed.
-i is used to specify inode numbers about which verbose
information should be printed.
-n is used to save pathnames for inodes visited, this is
used to support the xfs_ncheck(8) command. It also means
that pathnames will be printed for inodes that have prob-
lems. This option uses a lot of memory so is not enabled
by default.
-p causes error messages to be prefixed with the filesystem
name being processed. This is useful if several copies of
xfs_db are run in parallel.
-s restricts output to severe errors only. This is useful if
the output is too long otherwise.
-v enables verbose output. Messages will be printed for
every block and inode processed.
blocktrash [-z] [-o offset] [-n count] [-x min] [-y max] [-s seed]
[-0|1|2|3] [-t type] ...
Trash randomly selected filesystem metadata blocks. Trashing
occurs to randomly selected bits in the chosen blocks. This
command is available only in debugging versions of xfs_db. It
is useful for testing xfs_repair(8).
-0 | -1 | -2 | -3
These are used to set the operating mode for blocktrash.
Only one can be used: -0 changed bits are cleared; -1
changed bits are set; -2 changed bits are inverted; -3
changed bits are randomized.
-n supplies the count of block-trashings to perform (default
1).
-o supplies the bit offset at which to start trashing the
block. If the value is preceded by a '+', the trashing
will start at a randomly chosen offset that is larger
than the value supplied. The default is to randomly
choose an offset anywhere in the block.
-s supplies a seed to the random processing.
-t gives a type of blocks to be selected for trashing. Mul-
tiple -t options may be given. If no -t options are given
then all metadata types can be trashed.
-x sets the minimum size of bit range to be trashed. The
default value is 1.
-y sets the maximum size of bit range to be trashed. The
default value is 1024.
-z trashes the block at the top of the stack. It is not
necessary to run blockget if this option is supplied.
blockuse [-n] [-c count]
Print usage for current filesystem block(s). For each block,
the type and (if any) inode are printed.
-c specifies a count of blocks to process. The default value
is 1 (the current block only).
-n specifies that file names should be printed. The prior
blockget command must have also specified the -n option.
bmap [-a] [-d] [block [len]]
Show the block map for the current inode. The map display can
be restricted to an area of the file with the block and len
arguments. If block is given and len is omitted then 1 is
assumed for len.
The -a and -d options are used to select the attribute or data
area of the inode, if neither option is given then both areas
are shown.
btdump [-a] [-i]
If the cursor points to a btree node, dump the btree from that
block downward. If instead the cursor points to an inode, dump
the data fork block mapping btree if there is one. If the cur-
sor points to a directory or extended attribute btree node, dump
that. By default, only records stored in the btree are dumped.
-a If the cursor points at an inode, dump the extended
attribute block mapping btree, if present.
-i Dump all keys and pointers in intermediate btree nodes,
and all records in leaf btree nodes.
check See the blockget command.
convert type number [type number] ... type
Convert from one address form to another. The known types, with
alternate names, are:
agblock or agbno (filesystem block within an allocation
group)
agino or aginode (inode number within an allocation group)
agnumber or agno (allocation group number)
bboff or daddroff (byte offset in a daddr)
blkoff or fsboff or agboff (byte offset in a agblock or
fsblock)
byte or fsbyte (byte address in filesystem)
daddr or bb (disk address, 512-byte blocks)
fsblock or fsb or fsbno (filesystem block, see the fsblock
command)
ino or inode (inode number)
inoidx or offset (index of inode in filesystem block)
inooff or inodeoff (byte offset in inode)
Only conversions that "make sense" are allowed. The compound
form (with more than three arguments) is useful for conversions
such as convert agno ag agbno agb fsblock.
crc [-i|-r|-v]
Invalidates, revalidates, or validates the CRC (checksum) field
of the current structure, if it has one. This command is avail-
able only on CRC-enabled filesystems. With no argument, valida-
tion is performed. Each command will display the resulting CRC
value and state.
-i Invalidate the structure's CRC value (incrementing it by
one), and write it to disk.
-r Recalculate the current structure's correct CRC value,
and write it to disk.
-v Validate and display the current value and state of the
structure's CRC.
daddr [d]
Set current address to the daddr (512 byte block) given by d.
If no value for d is given, the current address is printed,
expressed as a daddr. The type is set to data (uninterpreted).
dblock filoff
Set current address to the offset filoff (a filesystem block
number) in the data area of the current inode.
debug [flagbits]
Set debug option bits. These are used for debugging xfs_db. If
no value is given for flagbits, print the current debug option
bits. These are for the use of the implementor.
dquot [-g|-p|-u] id
Set current address to a group, project or user quota block for
the given ID. Defaults to user quota.
echo [arg] ...
Echo the arguments to the output.
f See the forward command.
forward
Move forward to the next entry in the position ring.
frag [-adflqRrv]
Get file fragmentation data. This prints information about frag-
mentation of file data in the filesystem (as opposed to fragmen-
tation of freespace, for which see the freesp command). Every
file in the filesystem is examined to see how far from ideal its
extent mappings are. A summary is printed giving the totals.
-v sets verbosity, every inode has information printed for
it. The remaining options select which inodes and
extents are examined. If no options are given then all
are assumed set, otherwise just those given are enabled.
-a enables processing of attribute data.
-d enables processing of directory data.
-f enables processing of regular file data.
-l enables processing of symbolic link data.
-q enables processing of quota file data.
-R enables processing of realtime control file data.
-r enables processing of realtime file data.
freesp [-bcds] [-A alignment] [-a ag] ... [-e i] [-h h1] ... [-m m]
Summarize free space for the filesystem. The free blocks are
examined and totalled, and displayed in the form of a histogram,
with a count of extents in each range of free extent sizes.
-A reports only free extents with starting blocks aligned to
alignment blocks.
-a adds ag to the list of allocation groups to be processed.
If no -a options are given then all allocation groups are
processed.
-b specifies that the histogram buckets are binary-sized,
with the starting sizes being the powers of 2.
-c specifies that freesp will search the by-size (cnt) space
Btree instead of the default by-block (bno) space Btree.
-d specifies that every free extent will be displayed.
-e specifies that the histogram buckets are equal-sized,
with the size specified as i.
-h specifies a starting block number for a histogram bucket
as h1. Multiple -h's are given to specify the complete
set of buckets.
-m specifies that the histogram starting block numbers are
powers of m. This is the general case of -b.
-s specifies that a final summary of total free extents,
free blocks, and the average free extent size is printed.
fsb See the fsblock command.
fsblock [fsb]
Set current address to the fsblock value given by fsb. If no
value for fsb is given the current address is printed, expressed
as an fsb. The type is set to data (uninterpreted). XFS
filesystem block numbers are computed ((agno << agshift) |
agblock) where agshift depends on the size of an allocation
group. Use the convert command to convert to and from this form.
Block numbers given for file blocks (for instance from the bmap
command) are in this form.
fsmap [ start ] [ end ]
Prints the mapping of disk blocks used by an XFS filesystem.
The map lists each extent used by files, allocation group meta-
data, journalling logs, and static filesystem metadata, as well
as any regions that are unused. All blocks, offsets, and
lengths are specified in units of 512-byte blocks, no matter
what the filesystem's block size is. The optional start and end
arguments can be used to constrain the output to a particular
range of disk blocks.
fuzz [-c] [-d] field action
Write garbage into a specific structure field on disk. Expert
mode must be enabled to use this command. The operation happens
immediately; there is no buffering.
The fuzz command can take the following actions against a field:
zeroes
Clears all bits in the field.
ones
Sets all bits in the field.
firstbit
Flips the first bit in the field. For a scalar value,
this is the highest bit.
middlebit
Flips the middle bit in the field.
lastbit
Flips the last bit in the field. For a scalar value,
this is the lowest bit.
add Adds a small value to a scalar field.
sub Subtracts a small value from a scalar field.
random
Randomizes the contents of the field.
The following switches affect the write behavior:
-c Skip write verifiers and CRC recalculation; allows
invalid data to be written to disk.
-d Skip write verifiers but perform CRC recalculation;
allows invalid data to be written to disk to test detec-
tion of invalid data.
hash string
Prints the hash value of string using the hash function of the
XFS directory and attribute implementation.
help [command]
Print help for one or all commands.
info Displays selected geometry information about the filesystem.
The output will have the same format that mkfs.xfs(8) prints
when creating a filesystem or xfs_info(8) prints when querying a
filesystem.
inode [inode#]
Set the current inode number. If no inode# is given, print the
current inode number.
label [label]
Set the filesystem label. The filesystem label can be used by
mount(8) instead of using a device special file. The maximum
length of an XFS label is 12 characters - use of a longer label
will result in truncation and a warning will be issued. If no
label is given, the current filesystem label is printed.
log [stop | start filename]
Start logging output to filename, stop logging, or print the
current logging status.
logres Print transaction reservation size information for each transac-
tion type. This makes it easier to find discrepancies in the
reservation calculations between xfsprogs and the kernel, which
will help when diagnosing minimum log size calculation errors.
metadump [-egow] filename
Dumps metadata to a file. See xfs_metadump(8) for more informa-
tion.
ncheck [-s] [-i ino] ...
Print name-inode pairs. A blockget -n command must be run first
to gather the information.
-i specifies an inode number to be printed. If no -i options
are given then all inodes are printed.
-s specifies that only setuid and setgid files are printed.
p See the print command.
pop Pop location from the stack.
print [field-expression] ...
Print field values. If no argument is given, print all fields
in the current structure.
push [command]
Push location to the stack. If command is supplied, set the cur-
rent location to the results of command after pushing the old
location.
q See the quit command.
quit Exit xfs_db.
ring [index]
Show position ring (if no index argument is given), or move to a
specific entry in the position ring given by index.
sb [agno]
Set current address to SB header in allocation group agno. If
no agno is given, use the current allocation group number.
source source-file
Process commands from source-file. source commands can be
nested.
stack View the location stack.
type [type]
Set the current data type to type. If no argument is given,
show the current data type. The possible data types are: agf,
agfl, agi, attr, bmapbta, bmapbtd, bnobt, cntbt, data, dir,
dir2, dqblk, inobt, inode, log, refcntbt, rmapbt, rtbitmap,
rtsummary, sb, symlink and text. See the TYPES section below
for more information on these data types.
timelimit [OPTIONS]
Print the minimum and maximum supported values for inode time-
stamps, quota expiration timers, and quota grace periods sup-
ported by this filesystem. Options include:
--bigtime
Print the time limits of an XFS filesystem with the big-
time feature enabled.
--classic
Print the time limits of a classic XFS filesystem.
--compact
Print all limits as raw values on a single line.
--pretty
Print the timestamps in the current locale's date and
time format instead of raw seconds since the Unix epoch.
uuid [uuid | generate | rewrite | restore]
Set the filesystem universally unique identifier (UUID). The
filesystem UUID can be used by mount(8) instead of using a
device special file. The uuid can be set directly to the
desired UUID, or it can be automatically generated using the
generate option. These options will both write the UUID into
every copy of the superblock in the filesystem. On a CRC-
enabled filesystem, this will set an incompatible superblock
flag, and the filesystem will not be mountable with older ker-
nels. This can be reverted with the restore option, which will
copy the original UUID back into place and clear the incompati-
ble flag as needed. rewrite copies the current UUID from the
primary superblock to all secondary copies of the superblock.
If no argument is given, the current filesystem UUID is printed.
version [feature | versionnum features2]
Enable selected features for a filesystem (certain features can
be enabled on an unmounted filesystem, after mkfs.xfs(8) has
created the filesystem). Support for unwritten extents can be
enabled using the extflg option. Support for version 2 log for-
mat can be enabled using the log2 option. Support for extended
attributes can be enabled using the attr1 or attr2 option. Once
enabled, extended attributes cannot be disabled, but the user
may toggle between attr1 and attr2 at will (older kernels may
not support the newer version).
If no argument is given, the current version and feature bits
are printed. With one argument, this command will write the
updated version number into every copy of the superblock in the
filesystem. If two arguments are given, they will be used as
numeric values for the versionnum and features2 bits respec-
tively, and their string equivalent reported (but no modifica-
tions are made).
write [-c|-d] [field value] ...
Write a value to disk. Specific fields can be set in structures
(struct mode), or a block can be set to data values (data mode),
or a block can be set to string values (string mode, for symlink
blocks). The operation happens immediately: there is no buffer-
ing.
Struct mode is in effect when the current type is structural,
i.e. not data. For struct mode, the syntax is "write field
value".
Data mode is in effect when the current type is data. In this
case the contents of the block can be shifted or rotated left or
right, or filled with a sequence, a constant value, or a random
value. In this mode write with no arguments gives more informa-
tion on the allowed commands.
-c Skip write verifiers and CRC recalculation; allows
invalid data to be written to disk.
-d Skip write verifiers but perform CRC recalculation. This
allows invalid data to be written to disk to test detec-
tion of invalid data. (This is not possible for some
types.)
TYPES
This section gives the fields in each structure type and their mean-
ings. Note that some types of block cover multiple actual structures,
for instance directory blocks.
agf The AGF block is the header for block allocation information;
it is in the second 512-byte block of each allocation group.
The following fields are defined:
magicnum AGF block magic number, 0x58414746 ('XAGF').
versionnum version number, currently 1.
seqno sequence number starting from 0.
length size in filesystem blocks of the allocation
group. All allocation groups except the last
one of the filesystem have the superblock's
agblocks value here.
bnoroot block number of the root of the Btree holding
free space information sorted by block num-
ber.
cntroot block number of the root of the Btree holding
free space information sorted by block count.
bnolevel number of levels in the by-block-number
Btree.
cntlevel number of levels in the by-block-count Btree.
flfirst index into the AGFL block of the first active
entry.
fllast index into the AGFL block of the last active
entry.
flcount count of active entries in the AGFL block.
freeblks count of blocks represented in the freespace
Btrees.
longest longest free space represented in the
freespace Btrees.
btreeblks number of blocks held in the AGF Btrees.
agfl The AGFL block contains block numbers for use of the block
allocator; it is in the fourth 512-byte block of each alloca-
tion group. Each entry in the active list is a block number
within the allocation group that can be used for any purpose
if space runs low. The AGF block fields flfirst, fllast, and
flcount designate which entries are currently active. Entry
space is allocated in a circular manner within the AGFL
block. Fields defined:
bno array of all block numbers. Even those which
are not active are printed.
agi The AGI block is the header for inode allocation information;
it is in the third 512-byte block of each allocation group.
Fields defined:
magicnum AGI block magic number, 0x58414749 ('XAGI').
versionnum version number, currently 1.
seqno sequence number starting from 0.
length size in filesystem blocks of the allocation
group.
count count of inodes allocated.
root block number of the root of the Btree holding
inode allocation information.
level number of levels in the inode allocation
Btree.
freecount count of allocated inodes that are not in
use.
newino last inode number allocated.
dirino unused.
unlinked an array of inode numbers within the alloca-
tion group. The entries in the AGI block are
the heads of lists which run through the
inode next_unlinked field. These inodes are
to be unlinked the next time the filesystem
is mounted.
attr An attribute fork is organized as a Btree with the actual
data embedded in the leaf blocks. The root of the Btree is
found in block 0 of the fork. The index (sort order) of the
Btree is the hash value of the attribute name. All the
blocks contain a blkinfo structure at the beginning, see type
dir for a description. Nonleaf blocks are identical in format
to those for version 1 and version 2 directories, see type
dir for a description. Leaf blocks can refer to "local" or
"remote" attribute values. Local values are stored directly
in the leaf block. Leaf blocks contain the following fields:
hdr header containing a blkinfo structure info
(magic number 0xfbee), a count of active
entries, usedbytes total bytes of names and
values, the firstused byte in the name area,
holes set if the block needs compaction, and
array freemap as for dir leaf blocks.
entries array of structures containing a hashval,
nameidx (index into the block of the name),
and flags incomplete, root, and local.
nvlist array of structures describing the attribute
names and values. Fields always present: val-
uelen (length of value in bytes), namelen,
and name. Fields present for local values:
value (value string). Fields present for
remote values: valueblk (fork block number of
containing the value).
Remote values are stored in an independent block in the
attribute fork. Prior to v5, value blocks had no structure,
but in v5 they acquired a header structure with the following
fields:
magic attr3 remote block magic number, 0x5841524d
('XARM').
offset Byte offset of this data block within the
overall attribute value.
bytes Number of bytes stored in this block.
crc Checksum of the attribute block contents.
uuid Filesystem UUID.
owner Inode that owns this attribute value.
bno Block offset of this block within the inode's
attribute fork.
lsn Log serial number of the last time this block
was logged.
data The attribute value data.
bmapbt Files with many extents in their data or attribute fork will
have the extents described by the contents of a Btree for
that fork, instead of being stored directly in the inode.
Each bmap Btree starts with a root block contained within the
inode. The other levels of the Btree are stored in filesys-
tem blocks. The blocks are linked to sibling left and right
blocks at each level, as well as by pointers from parent to
child blocks. Each block contains the following fields:
magic bmap Btree block magic number, 0x424d4150
('BMAP').
level level of this block above the leaf level.
numrecs number of records or keys in the block.
leftsib left (logically lower) sibling block, 0 if
none.
rightsib right (logically higher) sibling block, 0 if
none.
recs [leaf blocks only] array of extent records.
Each record contains startoff, startblock,
blockcount, and extentflag (1 if the extent
is unwritten).
keys [non-leaf blocks only] array of key records.
These are the first key value of each block
in the level below this one. Each record con-
tains startoff.
ptrs [non-leaf blocks only] array of child block
pointers. Each pointer is a filesystem block
number to the next level in the Btree.
bnobt There is one set of filesystem blocks forming the by-block-
number allocation Btree for each allocation group. The root
block of this Btree is designated by the bnoroot field in the
corresponding AGF block. The blocks are linked to sibling
left and right blocks at each level, as well as by pointers
from parent to child blocks. Each block has the following
fields:
magic BNOBT block magic number, 0x41425442
('ABTB').
level level number of this block, 0 is a leaf.
numrecs number of data entries in the block.
leftsib left (logically lower) sibling block, 0 if
none.
rightsib right (logically higher) sibling block, 0 if
none.
recs [leaf blocks only] array of freespace
records. Each record contains startblock and
blockcount.
keys [non-leaf blocks only] array of key records.
These are the first value of each block in
the level below this one. Each record con-
tains startblock and blockcount.
ptrs [non-leaf blocks only] array of child block
pointers. Each pointer is a block number
within the allocation group to the next level
in the Btree.
cntbt There is one set of filesystem blocks forming the by-block-
count allocation Btree for each allocation group. The root
block of this Btree is designated by the cntroot field in the
corresponding AGF block. The blocks are linked to sibling
left and right blocks at each level, as well as by pointers
from parent to child blocks. Each block has the following
fields:
magic CNTBT block magic number, 0x41425443
('ABTC').
level level number of this block, 0 is a leaf.
numrecs number of data entries in the block.
leftsib left (logically lower) sibling block, 0 if
none.
rightsib right (logically higher) sibling block, 0 if
none.
recs [leaf blocks only] array of freespace
records. Each record contains startblock and
blockcount.
keys [non-leaf blocks only] array of key records.
These are the first value of each block in
the level below this one. Each record con-
tains blockcount and startblock.
ptrs [non-leaf blocks only] array of child block
pointers. Each pointer is a block number
within the allocation group to the next level
in the Btree.
data User file blocks, and other blocks whose type is unknown,
have this type for display purposes in xfs_db. The block
data is displayed in hexadecimal format.
dir A version 1 directory is organized as a Btree with the direc-
tory data embedded in the leaf blocks. The root of the Btree
is found in block 0 of the file. The index (sort order) of
the Btree is the hash value of the entry name. All the blocks
contain a blkinfo structure at the beginning with the follow-
ing fields:
forw next sibling block.
back previous sibling block.
magic magic number for this block type.
The non-leaf (node) blocks have the following fields:
hdr header containing a blkinfo structure info
(magic number 0xfebe), the count of active
entries, and the level of this block above
the leaves.
btree array of entries containing hashval and
before fields. The before value is a block
number within the directory file to the child
block, the hashval is the last hash value in
that block.
The leaf blocks have the following fields:
hdr header containing a blkinfo structure info
(magic number 0xfeeb), the count of active
entries, namebytes (total name string bytes),
holes flag (block needs compaction), and
freemap (array of base, size entries for free
regions).
entries array of structures containing hashval,
nameidx (byte index into the block of the
name string), and namelen.
namelist array of structures containing inumber and
name.
dir2 A version 2 directory has four kinds of blocks. Data blocks
start at offset 0 in the file. There are two kinds of data
blocks: single-block directories have the leaf information
embedded at the end of the block, data blocks in multi-block
directories do not. Node and leaf blocks start at offset
32GiB (with either a single leaf block or the root node
block). Freespace blocks start at offset 64GiB. The node
and leaf blocks form a Btree, with references to the data in
the data blocks. The freespace blocks form an index of long-
est free spaces within the data blocks.
A single-block directory block contains the following fields:
bhdr header containing magic number 0x58443242
('XD2B') and an array bestfree of the longest
3 free spaces in the block (offset, length).
bu array of union structures. Each element is
either an entry or a freespace. For entries,
there are the following fields: inumber,
namelen, name, and tag. For freespace, there
are the following fields: freetag (0xffff),
length, and tag. The tag value is the byte
offset in the block of the start of the entry
it is contained in.
bleaf array of leaf entries containing hashval and
address. The address is a 64-bit word offset
into the file.
btail tail structure containing the total count of
leaf entries and stale count of unused leaf
entries.
A data block contains the following fields:
dhdr header containing magic number 0x58443244
('XD2D') and an array bestfree of the longest
3 free spaces in the block (offset, length).
du array of union structures as for bu.
Leaf blocks have two possible forms. If the Btree consists of
a single leaf then the freespace information is in the leaf
block, otherwise it is in separate blocks and the root of the
Btree is a node block. A leaf block contains the following
fields:
lhdr header containing a blkinfo structure info
(magic number 0xd2f1 for the single leaf
case, 0xd2ff for the true Btree case), the
total count of leaf entries, and stale count
of unused leaf entries.
lents leaf entries, as for bleaf.
lbests [single leaf only] array of values which rep-
resent the longest freespace in each data
block in the directory.
ltail [single leaf only] tail structure containing
bestcount count of lbests.
A node block is identical to that for types attr and dir.
A freespace block contains the following fields:
fhdr header containing magic number 0x58443246
('XD2F'), firstdb first data block number
covered by this freespace block, nvalid num-
ber of valid entries, and nused number of
entries representing real data blocks.
fbests array of values as for lbests.
dqblk The quota information is stored in files referred to by the
superblock uquotino and pquotino fields. Each filesystem
block in a quota file contains a constant number of quota
entries. The quota entry size is currently 136 bytes, so with
a 4KiB filesystem block size there are 30 quota entries per
block. The dquot command is used to locate these entries in
the filesystem. The file entries are indexed by the user or
project identifier to determine the block and offset. Each
quota entry has the following fields:
magic magic number, 0x4451 ('DQ').
version version number, currently 1.
flags flags, values include 0x01 for user quota,
0x02 for project quota.
id user or project identifier.
blk_hardlimit absolute limit on blocks in use.
blk_softlimit preferred limit on blocks in use.
ino_hardlimit absolute limit on inodes in use.
ino_softlimit preferred limit on inodes in use.
bcount blocks actually in use.
icount inodes actually in use.
itimer time when service will be refused if soft
limit is violated for inodes.
btimer time when service will be refused if soft
limit is violated for blocks.
iwarns number of warnings issued about inode
limit violations.
bwarns number of warnings issued about block
limit violations.
rtb_hardlimit absolute limit on realtime blocks in use.
rtb_softlimit preferred limit on realtime blocks in use.
rtbcount realtime blocks actually in use.
rtbtimer time when service will be refused if soft
limit is violated for realtime blocks.
rtbwarns number of warnings issued about realtime
block limit violations.
inobt There is one set of filesystem blocks forming the inode allo-
cation Btree for each allocation group. The root block of
this Btree is designated by the root field in the correspond-
ing AGI block. The blocks are linked to sibling left and
right blocks at each level, as well as by pointers from par-
ent to child blocks. Each block has the following fields:
magic INOBT block magic number, 0x49414254
('IABT').
level level number of this block, 0 is a leaf.
numrecs number of data entries in the block.
leftsib left (logically lower) sibling block, 0 if
none.
rightsib right (logically higher) sibling block, 0 if
none.
recs [leaf blocks only] array of inode records.
Each record contains startino allocation-
group relative inode number, freecount count
of free inodes in this chunk, and free bit-
map, LSB corresponds to inode 0.
keys [non-leaf blocks only] array of key records.
These are the first value of each block in
the level below this one. Each record con-
tains startino.
ptrs [non-leaf blocks only] array of child block
pointers. Each pointer is a block number
within the allocation group to the next level
in the Btree.
inode Inodes are allocated in "chunks" of 64 inodes each. Usually a
chunk is multiple filesystem blocks, although there are cases
with large filesystem blocks where a chunk is less than one
block. The inode Btree (see inobt above) refers to the inode
numbers per allocation group. The inode numbers directly
reflect the location of the inode block on disk. Use the
inode command to point xfs_db to a specific inode. Each inode
contains four regions: core, next_unlinked, u, and a. core
contains the fixed information. next_unlinked is separated
from the core due to journaling considerations, see type agi
field unlinked. u is a union structure that is different in
size and format depending on the type and representation of
the file data ("data fork"). a is an optional union struc-
ture to describe attribute data, that is different in size,
format, and location depending on the presence and represen-
tation of attribute data, and the size of the u data
("attribute fork"). xfs_db automatically selects the proper
union members based on information in the inode.
The following are fields in the inode core:
magic inode magic number, 0x494e ('IN').
mode mode and type of file, as described in
chmod(2), mknod(2), and stat(2).
version inode version, 1 or 2.
format format of u union data (0: xfs_dev_t, 1:
local file - in-inode directory or symlink,
2: extent list, 3: Btree root, 4: unique id
[unused]).
nlinkv1 number of links to the file in a version 1
inode.
nlinkv2 number of links to the file in a version 2
inode.
projid_lo owner's project id (low word; version 2 inode
only). projid_hi owner's project id (high
word; version 2 inode only).
uid owner's user id.
gid owner's group id.
atime time last accessed (seconds and nanoseconds).
mtime time last modified.
ctime time created or inode last modified.
size number of bytes in the file.
nblocks total number of blocks in the file including
indirect and attribute.
extsize basic/minimum extent size for the file.
nextents number of extents in the data fork.
naextents number of extents in the attribute fork.
forkoff attribute fork offset in the inode, in 64-bit
words from the start of u.
aformat format of a data (1: local attribute data, 2:
extent list, 3: Btree root).
dmevmask DMAPI event mask.
dmstate DMAPI state information.
newrtbm file is the realtime bitmap and is "new" for-
mat.
prealloc file has preallocated data space after EOF.
realtime file data is in the realtime subvolume.
gen inode generation number.
The following fields are in the u data fork union:
bmbt bmap Btree root. This looks like a bmapbtd
block with redundant information removed.
bmx array of extent descriptors.
dev dev_t for the block or character device.
sfdir shortform (in-inode) version 1 directory.
This consists of a hdr containing the parent
inode number and a count of active entries in
the directory, followed by an array list of
hdr.count entries. Each such entry contains
inumber, namelen, and name string.
sfdir2 shortform (in-inode) version 2 directory.
This consists of a hdr containing a count of
active entries in the directory, an i8count
of entries with inumbers that don't fit in a
32-bit value, and the parent inode number,
followed by an array list of hdr.count
entries. Each such entry contains namelen, a
saved offset used when the directory is con-
verted to a larger form, a name string, and
the inumber.
symlink symbolic link string value.
The following fields are in the a attribute fork union if it
exists:
bmbt bmap Btree root, as above.
bmx array of extent descriptors.
sfattr shortform (in-inode) attribute values. This
consists of a hdr containing a totsize (total
size in bytes) and a count of active entries,
followed by an array list of hdr.count
entries. Each such entry contains namelen,
valuelen, root flag, name, and value.
log Log blocks contain the journal entries for XFS. It's not
useful to examine these with xfs_db, use xfs_logprint(8)
instead.
refcntbt There is one set of filesystem blocks forming the reference
count Btree for each allocation group. The root block of this
Btree is designated by the refcntroot field in the corre-
sponding AGF block. The blocks are linked to sibling left
and right blocks at each level, as well as by pointers from
parent to child blocks. Each block has the following fields:
magic REFC block magic number, 0x52334643 ('R3FC').
level level number of this block, 0 is a leaf.
numrecs number of data entries in the block.
leftsib left (logically lower) sibling block, 0 if
none.
rightsib right (logically higher) sibling block, 0 if
none.
recs [leaf blocks only] array of reference count
records. Each record contains startblock,
blockcount, and refcount.
keys [non-leaf blocks only] array of key records.
These are the first value of each block in
the level below this one. Each record con-
tains startblock.
ptrs [non-leaf blocks only] array of child block
pointers. Each pointer is a block number
within the allocation group to the next level
in the Btree.
rmapbt There is one set of filesystem blocks forming the reverse
mapping Btree for each allocation group. The root block of
this Btree is designated by the rmaproot field in the corre-
sponding AGF block. The blocks are linked to sibling left
and right blocks at each level, as well as by pointers from
parent to child blocks. Each block has the following fields:
magic RMAP block magic number, 0x524d4233 ('RMB3').
level level number of this block, 0 is a leaf.
numrecs number of data entries in the block.
leftsib left (logically lower) sibling block, 0 if
none.
rightsib right (logically higher) sibling block, 0 if
none.
recs [leaf blocks only] array of reference count
records. Each record contains startblock,
blockcount, owner, offset, attr_fork,
bmbt_block, and unwritten.
keys [non-leaf blocks only] array of double-key
records. The first ("low") key contains the
first value of each block in the level below
this one. The second ("high") key contains
the largest key that can be used to identify
any record in the subtree. Each record con-
tains startblock, owner, offset, attr_fork,
and bmbt_block.
ptrs [non-leaf blocks only] array of child block
pointers. Each pointer is a block number
within the allocation group to the next level
in the Btree.
rtbitmap If the filesystem has a realtime subvolume, then the rbmino
field in the superblock refers to a file that contains the
realtime bitmap. Each bit in the bitmap file controls the
allocation of a single realtime extent (set == free). The
bitmap is processed in 32-bit words, the LSB of a word is
used for the first extent controlled by that bitmap word. The
atime field of the realtime bitmap inode contains a counter
that is used to control where the next new realtime file will
start.
rtsummary If the filesystem has a realtime subvolume, then the rsumino
field in the superblock refers to a file that contains the
realtime summary data. The summary file contains a two-dimen-
sional array of 16-bit values. Each value counts the number
of free extent runs (consecutive free realtime extents) of a
given range of sizes that starts in a given bitmap block.
The size ranges are binary buckets (low size in the bucket is
a power of 2). There are as many size ranges as are neces-
sary given the size of the realtime subvolume. The first
dimension is the size range, the second dimension is the
starting bitmap block number (adjacent entries are for the
same size, adjacent bitmap blocks).
sb There is one sb (superblock) structure per allocation group.
It is the first disk block in the allocation group. Only the
first one (block 0 of the filesystem) is actually used; the
other blocks are redundant information for xfs_repair(8) to
use if the first superblock is damaged. Fields defined:
magicnum superblock magic number, 0x58465342 ('XFSB').
blocksize filesystem block size in bytes.
dblocks number of filesystem blocks present in the
data subvolume.
rblocks number of filesystem blocks present in the
realtime subvolume.
rextents number of realtime extents that rblocks con-
tain.
uuid unique identifier of the filesystem.
logstart starting filesystem block number of the log
(journal). If this value is 0 the log is
"external".
rootino root inode number.
rbmino realtime bitmap inode number.
rsumino realtime summary data inode number.
rextsize realtime extent size in filesystem blocks.
agblocks size of an allocation group in filesystem
blocks.
agcount number of allocation groups.
rbmblocks number of realtime bitmap blocks.
logblocks number of log blocks (filesystem blocks).
versionnum filesystem version information. This value
is currently 1, 2, 3, or 4 in the low 4 bits.
If the low bits are 4 then the other bits
have additional meanings. 1 is the original
value. 2 means that attributes were used. 3
means that version 2 inodes (large link
counts) were used. 4 is the bitmask version
of the version number. In this case, the
other bits are used as flags (0x0010:
attributes were used, 0x0020: version 2
inodes were used, 0x0040: quotas were used,
0x0080: inode cluster alignment is in force,
0x0100: data stripe alignment is in force,
0x0200: the shared_vn field is used, 0x1000:
unwritten extent tracking is on, 0x2000: ver-
sion 2 directories are in use).
sectsize sector size in bytes, currently always 512.
This is the size of the superblock and the
other header blocks.
inodesize inode size in bytes.
inopblock number of inodes per filesystem block.
fname obsolete, filesystem name.
fpack obsolete, filesystem pack name.
blocklog log2 of blocksize.
sectlog log2 of sectsize.
inodelog log2 of inodesize.
inopblog log2 of inopblock.
agblklog log2 of agblocks (rounded up).
rextslog log2 of rextents.
inprogress mkfs.xfs(8) or xfs_copy(8) aborted before
completing this filesystem.
imax_pct maximum percentage of filesystem space used
for inode blocks.
icount number of allocated inodes.
ifree number of allocated inodes that are not in
use.
fdblocks number of free data blocks.
frextents number of free realtime extents.
uquotino user quota inode number.
pquotino project quota inode number; this is currently
unused.
qflags quota status flags (0x01: user quota account-
ing is on, 0x02: user quota limits are
enforced, 0x04: quotacheck has been run on
user quotas, 0x08: project quota accounting
is on, 0x10: project quota limits are
enforced, 0x20: quotacheck has been run on
project quotas).
flags random flags. 0x01: only read-only mounts are
allowed.
shared_vn shared version number (shared readonly
filesystems).
inoalignmt inode chunk alignment in filesystem blocks.
unit stripe or RAID unit.
width stripe or RAID width.
dirblklog log2 of directory block size (filesystem
blocks).
symlink Symbolic link blocks are used only when the symbolic link
value does not fit inside the inode. The block content is
just the string value. Bytes past the logical end of the
symbolic link value have arbitrary values.
text User file blocks, and other blocks whose type is unknown,
have this type for display purposes in xfs_db. The block
data is displayed in two columns: Hexadecimal format and
printable ASCII chars.
DIAGNOSTICS
Many messages can come from the check (blockget) command. If the
filesystem is completely corrupt, a core dump might be produced instead
of the message
device is not a valid filesystem
If the filesystem is very large (has many files) then check might run
out of memory. In this case the message
out of memory
is printed.
The following is a description of the most likely problems and the
associated messages. Most of the diagnostics produced are only mean-
ingful with an understanding of the structure of the filesystem.
agf_freeblks n, counted m in ag a
The freeblocks count in the allocation group header for alloca-
tion group a doesn't match the number of blocks counted free.
agf_longest n, counted m in ag a
The longest free extent in the allocation group header for allo-
cation group a doesn't match the longest free extent found in
the allocation group.
agi_count n, counted m in ag a
The allocated inode count in the allocation group header for
allocation group a doesn't match the number of inodes counted in
the allocation group.
agi_freecount n, counted m in ag a
The free inode count in the allocation group header for alloca-
tion group a doesn't match the number of inodes counted free in
the allocation group.
block a/b expected inum 0 got i
The block number is specified as a pair (allocation group num-
ber, block in the allocation group). The block is used multiple
times (shared), between multiple inodes. This message usually
follows a message of the next type.
block a/b expected type unknown got y
The block is used multiple times (shared).
block a/b type unknown not expected
SEE ALSO
mkfs.xfs(8), xfs_admin(8), xfs_copy(8), xfs_logprint(8), xfs_metad-
ump(8), xfs_ncheck(8), xfs_repair(8), mount(8), chmod(2), mknod(2),
stat(2), xfs(5).
xfs_db(8)
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