---
name: "zhack"
section: "1"
platform: "linux"
package: "zfsutils-linux"
version: "2.3.9-0+deb13u1"
release: "Debian 13 (Trixie)"
platforms: ["linux", "freebsd"]
url: "https://new.carta.tech/zhack"
---

# zhack(1)

libzpool debugging tool

## 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)](/zfs/8.md) 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)](/zpool-online/8.md) 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/...
```

## SEE ALSO

[ztest(1)](/ztest/1.md), [zpool-features(7)](/zpool-features/7.md), [zpoolprops(7)](/zpoolprops/7.md), [zfs(8)](/zfs/8.md), [zpool-import(8)](/zpool-import/8.md), [zpool-online(8)](/zpool-online/8.md)