ZDB(1M) Maintenance Commands ZDB(1M)


zdb - Display zpool debugging and consistency information


zdb [-CmdibcsDvhLMXFPAG] [-e [-p path...]] [-t txg]
[-U cache] [-I inflight I/Os] [-x dumpdir]
[-o var=value] ... [poolname [object ...]]

zdb [-divPA] [-e [-p path...]] [-U cache]
dataset [object ...]

zdb -m [-MLXFPA] [-t txg] [-e [-p path...]] [-U cache]
poolname [vdev [metaslab ...]]

zdb -R [-A] [-e [-p path...]] [-U cache] poolname

zdb -S [-AP] [-e [-p path...]] [-U cache] poolname

zdb -l [-Aqu] device

zdb -C [-A] [-U cache]


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(1M) nor an fsdb(1M) 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:


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


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

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


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.


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

If specified multiple times provides greater and greater verbosity.

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


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).

If specified twice, 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.

If specified a third time, display the statistics independently for
each deduplication table.

If specified a fourth time, dump the contents of the deduplication
tables describing duplicate blocks.

If specified a fifth time, also dump the contents of the
deduplication tables describing unique blocks.


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


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

-l device

Read the vdev labels from the specified device. zdb -l will return 0
if valid label was found, 1 if error occured, and 2 if no valid
labels were found.

If the -u option is also specified, also display the uberblocks on
this device.

If the -q option is also specified, don't print the labels.


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


Display the offset, spacemap, and free space of each metaslab. When
specified twice, also display information about the on-disk free
space histogram associated with each metaslab. When specified three
time, display the maximum contiguous free space, the in-core free
space histogram, and the percentage of free space in each space map.
When specified four times display every spacemap record.


Display the offset, spacemap, and free space of each metaslab. When
specified twice, also display information about the maximum
contiguous free space and the percentage of free space in each space
map. When specified three times display every spacemap record.

-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


Decompress the block


Byte swap the block


Dump gang block header


Dump indirect block


Dump raw uninterpreted block data


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


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


Display the current uberblock.

Other options:


Do not abort should any assertion fail.


Enable panic recovery, certain errors which would otherwise be fatal
are demoted to warnings.


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 searched.

-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.


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


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.


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.


Enable verbosity. Specify multiple times for increased verbosity.


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


# 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(1M), zpool(1M)

April 9, 2016 ZDB(1M)