Ubuntu Manpage: xfs_db - debug an XFS filesystem

Provided by: xfsprogs_5.3.0-1ubuntu2_amd64

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 normally 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 program 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 messages, 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 arguments 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 directory 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 commands. This allows for easy
examination of a nested filesystem structure. Also, the last several locations visited
are stored in a ring buffer which can be manipulated with the forward, back, and ring
commands.

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 information 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 problems. 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. Multiple -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 cursor 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 available only on CRC-enabled
filesystems. With no argument, validation 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 fragmentation of file
data in the filesystem (as opposed to fragmentation 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 metadata, 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 detection 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 transaction 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 information.

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 current 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.

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
kernels. This can be reverted with the restore option, which will copy the
original UUID back into place and clear the incompatible 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
format 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 respectively, and their string
equivalent reported (but no modifications 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 buffering.

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
information 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 detection of invalid data. (This is not
possible for some types.)

TYPES

This section gives the fields in each structure type and their meanings. 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 number.
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 allocation 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 allocation 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: valuelen (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 filesystem
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
contains 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
contains 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
contains 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 directory 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 following 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 longest 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 represent 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
number 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 allocation Btree for
each allocation group. The root block of this Btree is designated by the root
field in the corresponding AGI 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 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 bitmap, 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
contains 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 structure
to describe attribute data, that is different in size, format, and location
depending on the presence and representation 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" format.
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 converted 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 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 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
contains 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 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 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 contains 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-dimensional 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 necessary 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 contain.
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: version 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 accounting 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 meaningful 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 allocation 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 allocation 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 allocation 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 number, 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