ZDB(8) Maintenance Procedures ZDB(8)


zdb - display zpool debugging and consistency information


zdb [-AbcdDFGhikLMPsvX] [-e [-V] [-p path ...]] [-I inflight I/Os]
[-o var=value]... [-t txg] [-U cache] [-x dumpdir]
[poolname [object ...]]
zdb [-AdiPv] [-e [-V] [-p path ...]] [-U cache] dataset [object ...]
zdb -C [-A] [-U cache]
zdb -E [-A] word0:word1:...:word15
zdb -l [-Aqu] device
zdb -m [-AFLPX] [-e [-V] [-p path ...]] [-t txg] [-U cache]
poolname [vdev [metaslab ...]]
zdb -O dataset path
zdb -R [-A] [-e [-V] [-p path ...]] [-U cache]
poolname vdev:offset:size[:flags]
zdb -S [-AP] [-e [-V] [-p path ...]] [-U cache] poolname


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. This is neither a
fsck(8) nor an fsdb(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/
(pool name followed by a slash).

When operating on an imported and active pool it is possible, though
unlikely, that zdb may interpret inconsistent pool data and behave


Display options:

-b Display statistics regarding the number, size (logical, physical
and allocated) and deduplication of blocks.

-c Verify the checksum of all metadata blocks while printing block
statistics (see -b).

If specified multiple times, verify the checksums of all blocks.

-C 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

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 Display information about datasets. Specified once, displays basic
dataset information: ID, create transaction, size, and object

If specified multiple times provides greater and greater verbosity.

If object IDs are specified, display information about those
specific objects only.

-D 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

-DDDDD Also dump the contents of the deduplication tables describing
unique blocks.

-E word0:word1:...:word15
Decode and display block from an embedded block pointer specified
by the word arguments.

-h Display pool history similar to zpool history, but include internal
changes, transaction, and dataset information.

-i Display information about intent log (ZIL) entries relating to each
dataset. If specified multiple times, display counts of each
intent log transaction type.

-k Examine the checkpointed state of the pool. Note, the on disk
format of the pool is not reverted to the checkpointed state.

-l 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 detection and the loading of space maps. By default,
zdb verifies that all non-free blocks are referenced, which can be
very expensive.

-m 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 Display the offset, spacemap, and free space of each metaslab.

-MM Also display information about the maximum contiguous free space
and the percentage of free space in each space map.

-MMM Display every spacemap record.

-O 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 poolname vdev:offset:size[: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 size of the block to read) and, optionally, flags (a set of
flags, described below).

b offset Print block pointer
d Decompress the block
e Byte swap the block
g Dump gang block header
i Dump indirect block
r Dump raw uninterpreted block data

-s Report statistics on zdb I/O. Display operation counts, bandwidth,
and error counts of I/O to the pool from zdb.

-S Simulate the effects of deduplication, constructing a DDT and then
display that DDT as with -DD.

-u Display the current uberblock.

Other options:

-A 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 [-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

-x 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 Attempt to make an unreadable pool readable by trying progressively
older transactions.

-G Dump the contents of the zfs_dbgmsg buffer before exiting zdb.
zfs_dbgmsg is a buffer used by ZFS to dump advanced debug

-I inflight I/Os
Limit the number of outstanding checksum I/Os to the specified
value. The default value is 200. This option affects the
performance of the -c option.

-o 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 Print numbers in an unscaled form more amenable to parsing, eg.
1000000 rather than 1M.

-t 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
Use a cache file other than /etc/zfs/zpool.cache.

-v Enable verbosity. Specify multiple times for increased verbosity.

-V Attempt verbatim import. This mimics the behavior of the kernel
when loading a pool from a cachefile. Only usable with -e.

-X Attempt "extreme" transaction rewind, that is attempt the same
recovery as -F but read transactions otherwise deemed too old.

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.


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
______ ______________________________ ______________________________
------ ------ ----- ----- ----- ------ ----- ----- -----
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


zfs(8), zpool(8)

OmniOS April 14, 2017 OmniOS