zhack.1

ZHACK(1)

General Commands Manual

ZHACK(1)

NAME

zhack — libzpool debugging tool

DESCRIPTION

This utility pokes configuration changes directly into a ZFS pool, which is dangerous and can cause data corruption.

SYNOPSIS

zhack feature stat pool

List feature flags.

zhack feature enable [-d description] [-r] pool guid

Add a new feature to pool that is uniquely identified by guid, which is specified in the same form as a zfs(8) user property.

The description is a short human readable explanation of the new feature.

The -r flag indicates that pool can be safely opened in read-only mode by a system that does not understand the guid feature.

zhack feature ref [-d|-m] pool guid

Increment the reference count of the guid feature in pool.

The -d flag decrements the reference count of the guid feature in pool instead.

The -m flag indicates that the guid feature is now required to read the pool MOS.

zhack label repair [-cu] device

Repair labels of a specified device according to options.

Flags may be combined to do their functions simultaneously.

The -c flag repairs corrupted label checksums

The -u flag restores the label on a detached device

Example:

zhack label repair -cu device Fix checksums and undetach a device

zhack mmp reclaim pool

Import a pool whose uberblock claim cannot reach every mirror leg the configuration still expects to be present, then mark the unreachable leaves offline so that ordinary imports succeed, and export it again.

This exists for one situation: a host has failed together with the mirror legs attached to it, so the surviving labels still describe those legs as healthy, and every later import demands writes to them that nobody can make.

This does not make the operation safe on its own. A live host whose legs are all invisible from here cannot be detected by any write and read scheme, and this command will take the pool from it. Fence the failed host, or otherwise prove it is powered off, before running this.

The activity check itself is unchanged: every write the reachable legs allow, the wait, and the re-read all still happen, so a competing host which shares any leg with this one is still detected and the import is refused. Only the number of writes the claim demands is lowered, and only for mirror legs this host cannot open, each of which is then marked offline.

The pool is left exported, and imports degraded with those leaves offline. Bring each one back with zpool-online(8) once the hardware returns. A leaf holding the only copy of some data cannot be offlined: it is reported and left online, and imports from other hosts stay refused. No leaf is marked offline on a pool which does not have multihost set, since no claim runs for it and an ordinary import already succeeds. Leaves of log, cache, and spare vdevs are not touched, as the claim does not write to them.

zhack metaslab leak [-f] pool

Apply a fragmentation profile generated by zdb to the specified pool.

The -f flag forces the profile to apply even if the vdevs in the pool don't have the same number of metaslabs as the fragmentation profile.

GLOBAL OPTIONS

The following can be passed to all zhack invocations before any subcommand:

-c cachefile: Read pool configuration from the cachefile, which is /etc/zfs/zpool.cache by default.
-d dir: Search for pool members in dir. Can be specified more than once.
-o var=value: Set the given tunable to the provided value.

EXAMPLES

# zhack feature stat tank
for_read_obj:
	org.illumos:lz4_compress = 0
for_write_obj:
	com.delphix:async_destroy = 0
	com.delphix:empty_bpobj = 0
descriptions_obj:
	com.delphix:async_destroy = Destroy filesystems asynchronously.
	com.delphix:empty_bpobj = Snapshots use less space.
	org.illumos:lz4_compress = LZ4 compression algorithm support.

# zhack feature enable -d 'Predict future disk failures.' tank com.example:clairvoyance
# zhack feature ref tank com.example:clairvoyance

Recover a pool whose peer host died together with its mirror legs, after confirming that host is down:

# zpool import -f tank
cannot import 'tank': the multihost claim could not be written to a device
the pool configuration expects to be present.
If the device is permanently gone, recover with 'zhack mmp reclaim'.
# zhack mmp reclaim tank
tank: marked absent leaf 5646371977210937898 offline
# zpool import tank
# zpool online tank /dev/disk/by-id/...