ZDB(8) FreeBSD System Manager's Manual ZDB(8)
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 --allocated-map [-mAFLPXY] [-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 -O [-K key] dataset object-id 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:
--allocated-map
Prints out a list of all the allocated regions in the pool.
Primarily intended for use with the zhack metaslab leak
subcommand.
-b, --block-stats
Display statistics regarding the number, size (logical, physical
and allocated) and deduplication of blocks.
--bin=(lsize|psize|asize)
When used with -bb, sort blocks into all three bins according to
the given size (instead of binning a block for each size
separately).
For instance, with --bin=lsize, a block with lsize of 16K and
psize of 4K will be added to the 16K bin in all three columns.
--class=(normal|special|dedup|other)[,<?>]
When used with -bb, only consider blocks from these allocation
classes.
-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 x 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. When used with -O, the path argument is interpreted as
an object identifier, not a path. 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 tunable to the provided value.
-o, --option=var<?>
Show the value of the given tunable.
-o, --option=show
Show all tunables and their values.
-o, --option=info=value<?>
Show info about a tunable, including their name, type and
description.
-o, --option=info
Show info about all tunables.
-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.
EXIT STATUS
The zdb utility exits 0 on success, 1 if a fatal error occurs, 2 if
invalid command line options were specified, or 3 if on-disk corruption
was detected, but was not fatal.
EXAMPLES
Example 1: Display the configuration of imported pool rrppooooll
# zdb -C rpool
MOS Configuration:
version: 28
name: 'rpool'
<?>
Example 2: Display basic dataset information about rrppooooll
# 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 rrppooooll//eexxppoorrtt//hhoommee
# 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 rrppooooll
# 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)
FreeBSD 15.1-STABLE-HBSD August 12, 2025 ZDB(8)