.\"-
.\" Copyright (c) 2016 Allan Jude
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.Dd February 20, 2024
.Dt ZFSD 8
.Os
.Sh NAME
.Nm zfsd
.Nd ZFS fault management daemon
.Sh SYNOPSIS
.Nm
.Op Fl d
.Sh DESCRIPTION
.Nm
attempts to resolve ZFS faults that the kernel can't resolve by itself.
It listens to
.Xr devctl 4
events, which are how the kernel notifies userland of events such as I/O
errors and disk removals.
.Nm
attempts to resolve these faults by activating or deactivating hot spares
and onlining offline vdevs.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl d
Run in the foreground instead of daemonizing.
.El
.Pp
System administrators never interact with
.Nm
directly.
Instead, they control its behavior indirectly through zpool configuration.
There are two ways to influence
.Nm :
assigning hot spares and setting pool properties.
Currently, only the
.Em autoreplace
property has any effect.
See
.Xr zpool 8
for details.
.Pp
.Nm
will attempt to resolve the following types of fault:
.Bl -tag -width a
.It device removal
When a leaf vdev disappears,
.Nm
will activate any available hot spare.
.It device arrival
When a new GEOM device appears,
.Nm
will attempt to read its ZFS label, if any.
If it matches a previously removed vdev on an active pool,
.Nm
will online it.
Once resilvering completes, any active hot spare will detach automatically.
.Pp
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
.Nm
will replace the missing vdev with the newly arrived device.
Once resilvering completes, any active hot spare will detach automatically.
.It vdev degrade or fault events
If a vdev becomes degraded or faulted,
.Nm
will activate any available hot spare.
.It I/O errors
By default, if a leaf vdev generates more than 50 I/O errors in a 60 second
period, then
.Nm
will mark that vdev as
.Em FAULTED .
ZFS will no longer issue any I/Os to it.
.Nm
will activate a hot spare if one is available. The defaults can be changed by
setting the
.Em io_n
and/or
.Em io_t
vdev properties. See
.Xr vdevprops 7
for details.
.It I/O delays
By default, if a leaf vdev generates more than delayed 8 I/O events in a 60
second period, then
.Nm
will mark that vdev as
.Em FAULTED .
ZFS will no longer issue any I/Os to it.
.Nm
will activate a hot spare if one is available. The defaults can be changed by
setting the
.Em slow_io_n
and/or
.Em slow_io_t
vdev properties. See
.Xr vdevprops 7
for details.
.It Checksum errors
By default, if a leaf vdev generates more than 50 checksum errors in a 60
second period, then
.Nm
will mark that vdev as
.Em DEGRADED .
ZFS will still use it, but zfsd will also activate a hot spare if one is
available. The defaults can be changed by setting the
.Em checksum_n
and/or
.Em checksum_t
vdev properties. See
.Xr vdevprops 7
for details.
.It Spare addition
If the system administrator adds a hot spare to a pool that is already degraded,
.Nm
will activate the spare.
.It Resilver complete
.Nm
will detach any hot spare once a permanent replacement finishes resilvering.
.It Physical path change
If the physical path of an existing disk changes,
.Nm
will attempt to replace any missing disk with the same physical path,
if its pool's autoreplace property is set.
.El
.Pp
.Nm
will log interesting events and its actions to syslog with facility
.Em daemon
and identity
.Op zfsd .
.El
.Sh FILES
.Bl -tag -width a -compact
.It Pa /var/db/zfsd/cases
When
.Nm
exits, it serializes any unresolved casefiles here,
then reads them back in when next it starts up.
.El
.Sh SEE ALSO
.Xr devctl 4 ,
.Xr vdevprops 7 ,
.Xr zpool 8
.Sh HISTORY
.Nm
first appeared in
.Fx 11.0 .
.Sh AUTHORS
.Nm
was originally written by
.An Justin Gibbs Aq Mt gibbs@FreeBSD.org
and
.An Alan Somers Aq Mt asomers@FreeBSD.org
.Sh TODO
In the future,
.Nm
should be able to resume a pool that became suspended due to device
removals, if enough missing devices have returned.