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.\" Copyright (c) 2012, 2018 by Delphix. All rights reserved.
.\" Copyright (c) 2012 Cyril Plisko. All Rights Reserved.
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.Dd June 28, 2023
.Dt ZPOOL-ATTACH 8
.Os
.
.Sh NAME
.Nm zpool-attach
.Nd attach new device to existing ZFS vdev
.Sh SYNOPSIS
.Nm zpool
.Cm attach
.Op Fl fsw
.Oo Fl o Ar property Ns = Ns Ar value Oc
.Ar pool device new_device
.
.Sh DESCRIPTION
Attaches
.Ar new_device
to the existing
.Ar device .
The behavior differs depending on if the existing
.Ar device
is a RAID-Z device, or a mirror/plain device.
.Pp
If the existing device is a mirror or plain device
.Pq e.g. specified as Qo Li sda Qc or Qq Li mirror-7 ,
the new device will be mirrored with the existing device, a resilver will be
initiated, and the new device will contribute to additional redundancy once the
resilver completes.
If
.Ar device
is not currently part of a mirrored configuration,
.Ar device
automatically transforms into a two-way mirror of
.Ar device
and
.Ar new_device .
If
.Ar device
is part of a two-way mirror, attaching
.Ar new_device
creates a three-way mirror, and so on.
In either case,
.Ar new_device
begins to resilver immediately and any running scrub is cancelled.
.Pp
If the existing device is a RAID-Z device
.Pq e.g. specified as Qq Ar raidz2-0 ,
the new device will become part of that RAID-Z group.
A "raidz expansion" will be initiated, and once the expansion completes,
the new device will contribute additional space to the RAID-Z group.
The expansion entails reading all allocated space from existing disks in the
RAID-Z group, and rewriting it to the new disks in the RAID-Z group (including
the newly added
.Ar device ) .
Its progress can be monitored with
.Nm zpool Cm status .
.Pp
Data redundancy is maintained during and after the expansion.
If a disk fails while the expansion is in progress, the expansion pauses until
the health of the RAID-Z vdev is restored (e.g. by replacing the failed disk
and waiting for reconstruction to complete).
Expansion does not change the number of failures that can be tolerated
without data loss (e.g. a RAID-Z2 is still a RAID-Z2 even after expansion).
A RAID-Z vdev can be expanded multiple times.
.Pp
After the expansion completes, old blocks retain their old data-to-parity
ratio
.Pq e.g. 5-wide RAID-Z2 has 3 data and 2 parity
but distributed among the larger set of disks.
New blocks will be written with the new data-to-parity ratio (e.g. a 5-wide
RAID-Z2 which has been expanded once to 6-wide, has 4 data and 2 parity).
However, the vdev's assumed parity ratio does not change, so slightly less
space than is expected may be reported for newly-written blocks, according to
.Nm zfs Cm list ,
.Nm df ,
.Nm ls Fl s ,
and similar tools.
.Pp
A pool-wide scrub is initiated at the end of the expansion in order to verify
the checksums of all blocks which have been copied during the expansion.
.Bl -tag -width Ds
.It Fl f
Forces use of
.Ar new_device ,
even if it appears to be in use.
Not all devices can be overridden in this manner.
.It Fl o Ar property Ns = Ns Ar value
Sets the given pool properties.
See the
.Xr zpoolprops 7
manual page for a list of valid properties that can be set.
The only property supported at the moment is
.Sy ashift .
.It Fl s
When attaching to a mirror or plain device, the
.Ar new_device
is reconstructed sequentially to restore redundancy as quickly as possible.
Checksums are not verified during sequential reconstruction so a scrub is
started when the resilver completes.
.It Fl w
Waits until
.Ar new_device
has finished resilvering or expanding before returning.
.El
.
.Sh SEE ALSO
.Xr zpool-add 8 ,
.Xr zpool-detach 8 ,
.Xr zpool-import 8 ,
.Xr zpool-initialize 8 ,
.Xr zpool-online 8 ,
.Xr zpool-replace 8 ,
.Xr zpool-resilver 8