Provided by: zfsutils-linux_2.3.4-1ubuntu2_amd64 
NAME
zdb — display ZFS storage pool debugging and consistency information
SYNOPSIS
zdb [-AbcdDFGhikLMNPsTvXYy] [-e [-V] [-p path]…] [-I inflight-I/O-ops] [-o var=value]… [-t txg]
[-U cache] [-x dumpdir] [-K key] [poolname[/dataset|objset-ID]] [object|range…]
zdb [-AdiPv] [-e [-V] [-p path]…] [-U cache] [-K key] poolname[/dataset|objset-ID] [object|range…]
zdb -B [-e [-V] [-p path]…] [-U cache] [-K key] poolname/objset-ID [backup-flags]
zdb -C [-A] [-U cache] [poolname]
zdb -E [-A] word0:word1:…:word15
zdb -l [-Aqu] device
zdb -m [-AFLPXY] [-e [-V] [-p path]…] [-t txg] [-U cache] poolname [vdev [metaslab]…]
zdb -O [-K key] dataset path
zdb -r [-K key] dataset path destination
zdb -R [-A] [-e [-V] [-p path]…] [-U cache] poolname vdev:offset:[lsize/]psize[:flags]
zdb -S [-AP] [-e [-V] [-p path]…] [-U cache] poolname
DESCRIPTION
The zdb utility displays information about a ZFS pool useful for debugging and performs some amount of
consistency checking. It is a not a general purpose tool and options (and facilities) may change. It is
not a fsck(8) utility.
The output of this command in general reflects the on-disk structure of a ZFS pool, and is inherently
unstable. The precise output of most invocations is not documented, a knowledge of ZFS internals is
assumed.
If the dataset argument does not contain any "/" or "@" characters, it is interpreted as a pool name.
The root dataset can be specified as "pool/".
zdb is an "offline" tool; it accesses the block devices underneath the pools directly from userspace and
does not care if the pool is imported or datasets are mounted (or even if the system understands ZFS at
all). When operating on an imported and active pool it is possible, though unlikely, that zdb may
interpret inconsistent pool data and behave erratically.
OPTIONS
Display options:
-b, --block-stats
Display statistics regarding the number, size (logical, physical and allocated) and deduplication
of blocks.
-B, --backup
Generate a backup stream, similar to zfs send, but for the numeric objset ID, and without opening
the dataset. This can be useful in recovery scenarios if dataset metadata has become corrupted
but the dataset itself is readable. The optional flags argument is a string of one or more of
the letters e, L, c, and w, which correspond to the same flags in zfs-send(8).
-c, --checksum
Verify the checksum of all metadata blocks while printing block statistics (see -b).
If specified multiple times, verify the checksums of all blocks.
-C, --config
Display information about the configuration. If specified with no other options, instead display
information about the cache file (/etc/zfs/zpool.cache). To specify the cache file to display,
see -U.
If specified multiple times, and a pool name is also specified display both the cached
configuration and the on-disk configuration. If specified multiple times with -e also display
the configuration that would be used were the pool to be imported.
-d, --datasets
Display information about datasets. Specified once, displays basic dataset information: ID,
create transaction, size, and object count. See -N for determining if
poolname[/dataset|objset-ID] is to use the specified dataset|objset-ID as a string (dataset name)
or a number (objset ID) when datasets have numeric names.
If specified multiple times provides greater and greater verbosity.
If object IDs or object ID ranges are specified, display information about those specific objects
or ranges only.
An object ID range is specified in terms of a colon-separated tuple of the form
⟨start⟩:⟨end⟩[:⟨flags⟩]. The fields start and end are integer object identifiers that denote the
upper and lower bounds of the range. An end value of -1 specifies a range with no upper bound.
The flags field optionally specifies a set of flags, described below, that control which object
types are dumped. By default, all object types are dumped. A minus sign (-) negates the effect
of the flag that follows it and has no effect unless preceded by the A flag. For example, the
range 0:-1:A-d will dump all object types except for directories.
A Dump all objects (this is the default)
d Dump ZFS directory objects
f Dump ZFS plain file objects
m Dump SPA space map objects
z Dump ZAP objects
- Negate the effect of next flag
-D, --dedup-stats
Display deduplication statistics, including the deduplication ratio (dedup), compression ratio
(compress), inflation due to the zfs copies property (copies), and an overall effective ratio
(dedup × compress / copies).
-DD Display a histogram of deduplication statistics, showing the allocated (physically present on
disk) and referenced (logically referenced in the pool) block counts and sizes by reference
count.
-DDD Display the statistics independently for each deduplication table.
-DDDD Dump the contents of the deduplication tables describing duplicate blocks.
-DDDDD Also dump the contents of the deduplication tables describing unique blocks.
-E, --embedded-block-pointer=word0:word1:…:word15
Decode and display block from an embedded block pointer specified by the word arguments.
-h, --history
Display pool history similar to zpool history, but include internal changes, transaction, and
dataset information.
-i, --intent-logs
Display information about intent log (ZIL) entries relating to each dataset. If specified
multiple times, display counts of each intent log transaction type.
-k, --checkpointed-state
Examine the checkpointed state of the pool. Note, the on disk format of the pool is not reverted
to the checkpointed state.
-l, --label=device
Read the vdev labels and L2ARC header from the specified device. zdb -l will return 0 if valid
label was found, 1 if error occurred, and 2 if no valid labels were found. The presence of L2ARC
header is indicated by a specific sequence (L2ARC_DEV_HDR_MAGIC). If there is an accounting
error in the size or the number of L2ARC log blocks zdb -l will return 1. Each unique
configuration is displayed only once.
-ll device
In addition display label space usage stats. If a valid L2ARC header was found also display the
properties of log blocks used for restoring L2ARC contents (persistent L2ARC).
-lll device
Display every configuration, unique or not. If a valid L2ARC header was found also display the
properties of log entries in log blocks used for restoring L2ARC contents (persistent L2ARC).
If the -q option is also specified, don't print the labels or the L2ARC header.
If the -u option is also specified, also display the uberblocks on this device. Specify multiple
times to increase verbosity.
-L, --disable-leak-tracking
Disable leak detection and the loading of space maps. By default, zdb verifies that all non-free
blocks are referenced, which can be very expensive.
-m, --metaslabs
Display the offset, spacemap, free space of each metaslab, all the log spacemaps and their
obsolete entry statistics.
-mm Also display information about the on-disk free space histogram associated with each metaslab.
-mmm Display the maximum contiguous free space, the in-core free space histogram, and the percentage
of free space in each space map.
-mmmm Display every spacemap record.
-M, --metaslab-groups
Display all "normal" vdev metaslab group information - per-vdev metaslab count, fragmentation,
and free space histogram, as well as overall pool fragmentation and histogram.
-MM "Special" vdevs are added to -M's normal output. Also display information about the maximum
contiguous free space and the percentage of free space in each space map.
-MMM Display every spacemap record.
-N Same as -d but force zdb to interpret the [dataset|objset-ID] in [poolname[/dataset|objset-ID]]
as a numeric objset ID.
-O, --object-lookups=dataset path
Look up the specified path inside of the dataset and display its metadata and indirect blocks.
Specified path must be relative to the root of dataset. This option can be combined with -v for
increasing verbosity.
-r, --copy-object=dataset path destination
Copy the specified path inside of the dataset to the specified destination. Specified path must
be relative to the root of dataset. This option can be combined with -v for increasing
verbosity.
-R, --read-block=poolname vdev:offset:[lsize/]psize[:flags]
Read and display a block from the specified device. By default the block is displayed as a hex
dump, but see the description of the r flag, below.
The block is specified in terms of a colon-separated tuple vdev (an integer vdev identifier)
offset (the offset within the vdev) size (the physical size, or logical size / physical size) of
the block to read and, optionally, flags (a set of flags, described below).
b offset Print block pointer at hex offset
c Calculate and display checksums
d Decompress the block. Set environment variable ZDB_NO_ZLE to skip zle when guessing.
e Byte swap the block
g Dump gang block header
i Dump indirect block
r Dump raw uninterpreted block data
v Verbose output for guessing compression algorithm
-s, --io-stats
Report statistics on zdb I/O. Display operation counts, bandwidth, and error counts of I/O to
the pool from zdb.
-S, --simulate-dedup
Simulate the effects of deduplication, constructing a DDT and then display that DDT as with -DD.
-T, --brt-stats
Display block reference table (BRT) statistics, including the size of uniques blocks cloned, the
space saving as a result of cloning, and the saving ratio.
-TT Display the per-vdev BRT statistics, including total references.
-TTT Display histograms of per-vdev BRT refcounts.
-TTTT Dump the contents of the block reference tables.
-u, --uberblock
Display the current uberblock.
Other options:
-A, --ignore-assertions
Do not abort should any assertion fail.
-AA Enable panic recovery, certain errors which would otherwise be fatal are demoted to warnings.
-AAA Do not abort if asserts fail and also enable panic recovery.
-e, --exported=[-p path]…
Operate on an exported pool, not present in /etc/zfs/zpool.cache. The -p flag specifies the path
under which devices are to be searched.
-x, --dump-blocks=dumpdir
All blocks accessed will be copied to files in the specified directory. The blocks will be
placed in sparse files whose name is the same as that of the file or device read. zdb can be
then run on the generated files. Note that the -bbc flags are sufficient to access (and thus
copy) all metadata on the pool.
-F, --automatic-rewind
Attempt to make an unreadable pool readable by trying progressively older transactions.
-G, --dump-debug-msg
Dump the contents of the zfs_dbgmsg buffer before exiting zdb. zfs_dbgmsg is a buffer used by
ZFS to dump advanced debug information.
-I, --inflight=inflight-I/O-ops
Limit the number of outstanding checksum I/O operations to the specified value. The default
value is 200. This option affects the performance of the -c option.
-K, --key=key
Decryption key needed to access an encrypted dataset. This will cause zdb to attempt to unlock
the dataset using the encryption root, key format and other encryption parameters on the given
dataset. zdb can still inspect pool and dataset structures on encrypted datasets without
unlocking them, but will not be able to access file names and attributes and object contents.
WARNING: The raw decryption key and any decrypted data will be in user memory while zdb is
running. Other user programs may be able to extract it by inspecting zdb as it runs. Exercise
extreme caution when using this option in shared or uncontrolled environments.
-o, --option=var=value…
Set the given global libzpool variable to the provided value. The value must be an unsigned
32-bit integer. Currently only little-endian systems are supported to avoid accidentally setting
the high 32 bits of 64-bit variables.
-P, --parseable
Print numbers in an unscaled form more amenable to parsing, e.g. 1000000 rather than 1M.
-t, --txg=transaction
Specify the highest transaction to use when searching for uberblocks. See also the -u and -l
options for a means to see the available uberblocks and their associated transaction numbers.
-U, --cachefile=cachefile
Use a cache file other than /etc/zfs/zpool.cache.
-v, --verbose
Enable verbosity. Specify multiple times for increased verbosity.
-V, --verbatim
Attempt verbatim import. This mimics the behavior of the kernel when loading a pool from a
cachefile. Only usable with -e.
-X, --extreme-rewind
Attempt "extreme" transaction rewind, that is attempt the same recovery as -F but read
transactions otherwise deemed too old.
-Y, --all-reconstruction
Attempt all possible combinations when reconstructing indirect split blocks. This flag disables
the individual I/O deadman timer in order to allow as much time as required for the attempted
reconstruction.
-y, --livelist
Perform validation for livelists that are being deleted. Scans through the livelist and
metaslabs, checking for duplicate entries and compares the two, checking for potential double
frees. If it encounters issues, warnings will be printed, but the command will not necessarily
fail.
Specifying a display option more than once enables verbosity for only that option, with more occurrences
enabling more verbosity.
If no options are specified, all information about the named pool will be displayed at default verbosity.
EXAMPLES
Example 1: Display the configuration of imported pool rpool
# zdb -C rpool
MOS Configuration:
version: 28
name: 'rpool'
…
Example 2: Display basic dataset information about rpool
# zdb -d rpool
Dataset mos [META], ID 0, cr_txg 4, 26.9M, 1051 objects
Dataset rpool/swap [ZVOL], ID 59, cr_txg 356, 486M, 2 objects
…
Example 3: Display basic information about object 0 in rpool/export/home
# zdb -d rpool/export/home 0
Dataset rpool/export/home [ZPL], ID 137, cr_txg 1546, 32K, 8 objects
Object lvl iblk dblk dsize lsize %full type
0 7 16K 16K 15.0K 16K 25.00 DMU dnode
Example 4: Display the predicted effect of enabling deduplication on rpool
# zdb -S rpool
Simulated DDT histogram:
bucket allocated referenced
______ ______________________________ ______________________________
refcnt blocks LSIZE PSIZE DSIZE blocks LSIZE PSIZE DSIZE
------ ------ ----- ----- ----- ------ ----- ----- -----
1 694K 27.1G 15.0G 15.0G 694K 27.1G 15.0G 15.0G
2 35.0K 1.33G 699M 699M 74.7K 2.79G 1.45G 1.45G
…
dedup = 1.11, compress = 1.80, copies = 1.00, dedup * compress / copies = 2.00
SEE ALSO
zfs(8), zpool(8)
OpenZFS October 27, 2024 ZDB(8)