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Man Pages
ZFSD(8) FreeBSD System Manager's Manual ZFSD(8)

zfsd
ZFS fault management daemon

zfsd [
-d
]

zfsd attempts to resolve ZFS faults that the kernel can't resolve by itself. It listens to devctl(4) events, which are how the kernel notifies userland of events such as I/O errors and disk removals. zfsd attempts to resolve these faults by activating or deactivating hot spares and onlining offline vdevs.
The following options are available:
Run in the foreground instead of daemonizing.
System administrators never interact with zfsd directly. Instead, they control its behavior indirectly through zpool configuration. There are two ways to influence zfsd: assigning hotspares and setting pool properties. Currently, only the autoreplace property has any effect. See zpool(8) for details.
zfsd will attempt to resolve the following types of fault:
device removal
When a leaf vdev disappears, zfsd will activate any available hotspare.
device arrival
When a new GEOM device appears, zfsd will attempt to read its ZFS label, if any. If it matches a previously removed vdev on an active pool, zfsd will online it. Once resilvering completes, any active hotspare will detach automatically.
If the new device has no ZFS label but its physical path matches the physical path of a previously removed vdev on an active pool, and that pool has the autoreplace property set, then zfsd will replace the missing vdev with the newly arrived device. Once resilvering completes, any active hotspare will detach automatically.
vdev degrade or fault events
If a vdev becomes degraded or faulted, zfsd will activate any available hotspare.
I/O errors
If a leaf vdev generates more than 50 I/O errors in a 60 second period, then zfsd will mark that vdev as FAULTED. zfs(4) will no longer issue any I/Os to it. zfsd will activate a hotspare if one is available.
Checksum errors
If a leaf vdev generates more than 50 checksum errors in a 60 second period, then zfsd will mark that vdev as DEGRADED. zfs(4) will still use it, but zfsd will activate a spare anyway.
Spare addition
If the system administrator adds a hotspare to a pool that is already degraded, zfsd will activate the spare.
Resilver complete
zfsd will detach any hotspare once a permanent replacement finishes resilvering.
Physical path change
If the physical path of an existing disk changes, zfsd will attempt to replace any missing disk with the same physical path, if its pool's autoreplace property is set.
zfsd will log interesting events and its actions to syslog with facility daemon and identity [
zfsd
].

/var/db/zfsd/cases
When zfsd exits, it serializes any unresolved casefiles here, then reads them back in when next it starts up.

devctl(4), zfs(4), zpool(8)

zfsd first appeared in FreeBSD 11.0.

zfsd was originally written by Justin Gibbs <gibbs@FreeBSD.org> and
Alan Somers <asomers@FreeBSD.org>

In the future, zfsd should be able to resume a pool that became suspended due to device removals, if enough missing devices have returned.
May 26, 2016 FreeBSD 12.0-RELEASE

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