ZDB(8)

NAME
zdb - Display zpool debugging and consistency information

SYNOPSIS
zdb [-CumdibcsDvhLMXFPA] [-e [-p path...]] [-t txg]
    [-U cache] [-I inflight I/Os] [-x dumpdir]
    [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
    vdev:offset:size[:flags]

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

zdb -l [-uA] device

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

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. 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 erratically.

OPTIONS
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 -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

    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.

-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).

    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.

-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.

-l device

    Display the vdev labels from the specified device. If the -u option is also specified, also display the uberblocks on this device.

-L

    Disable leak tracing 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, 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.

-M

    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

    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's 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 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.

-F

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

-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.

-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.

-X

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

-V

    Attempt a verbatim import. This mimics the behavior of the kernel when loading a pool from a cachefile.

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

ENVIRONMENT VARIABLES

SPA_CONFIG_PATH
    Override the default spa_config_path (/etc/zfs/zpool.cache) setting. If -U flag is specified it will override this environment variable settings once again.

SEE ALSO
zfs(8), zpool(8)