Re: zfs and device name changes

[Stephen Borrill <netbsd%precedence.co.uk@localhost>]

On Fri, 27 Mar 2026, Takashi YAMAMOTO wrote:
> hi,
>
> On Thu, Mar 26, 2026 at 12:41 PM Taylor R Campbell <riastradh%netbsd.org@localhost> wrote:
> > > Date: Tue, 24 Mar 2026 09:14:37 +0900
> > > From: Takashi YAMAMOTO <yamt9999%gmail.com@localhost>
> > >
> > > the attached patch is my attempt to make zfs a bit more robust against
> > > device name changes.
> > > the identical patch is available at github too:
> > > https://github.com/yamt/netbsd-src/commit/32283c2e362034301c3da218a05849c04ee20c2a
> > >
> > > while it seems working as far as i tested, i'd be happy if someone can review it
> > > as my knowledge of zfs (well, and recent netbsd in general) is weak.
> >
> > I don't understand why all this new code is needed.  Doesn't zfs
> > already have logic to scan all disks/partitions/wedges and find the
> > vdevs by guid?
>
> which code are you talking about?
> it's entirely possible i'm missing something as i'm new to the code base.
>
> > I am under the impression that /etc/zfs/zpool.cache may bypass the
> > scan so this doesn't work in some circumstances, but in my years of
> > using zfs on various machines with frequent device renumbering of cgd
> > volumes and dkN wedges, I have never encountered this type of trouble
> > myself, and I'm not sure what I'm doing differently.
>
> do you mean zfs finds vdevs after renumbering without zpool import?
> it doesn't match my experience.
> without this patch, i had to use zpool export/import after:
> - modify gpt in a way affecting dk numbering
> - swapping qemu disk images

Naively:

# zpool create tank mirror xbd2 xbd3 mirror xbd4 xbd5
# zpool status
  pool: tank
 state: ONLINE
  scan: none requested
config:

        NAME        STATE     READ WRITE CKSUM
        tank        ONLINE       0     0     0
          mirror-0  ONLINE       0     0     0
            xbd2    ONLINE       0     0     0
            xbd3    ONLINE       0     0     0
          mirror-1  ONLINE       0     0     0
            xbd4    ONLINE       0     0     0
            xbd5    ONLINE       0     0     0

errors: No known data errors
# halt -p

** Remove xbd1 to simulate failed/disconnected disk
** means xbd2 -> xbd1, xbd3 -> xbd2, etc.

After boot:

# zpool status
  pool: tank
 state: UNAVAIL
status: One or more devices could not be opened.  There are insufficient
        replicas for the pool to continue functioning.
action: Attach the missing device and online it using 'zpool online'.
   see: http://illumos.org/msg/ZFS-8000-3C
  scan: none requested
config:

        NAME                     STATE     READ WRITE CKSUM
        tank                     UNAVAIL      0     0     0
          mirror-0               UNAVAIL      0     0     0
            6289893268167966748  FAULTED      0     0     0  was /dev/xbd2
            4017376292647041077  FAULTED      0     0     0  was /dev/xbd3
          mirror-1               UNAVAIL      0     0     0
            4378765686596708079  FAULTED      0     0     0  was /dev/xbd4
            6863498524284650610  UNAVAIL      0     0     0  was /dev/xbd5

After yamt's patch:

dmesg shows:
ZFS WARNING: vdev guid mismatch for /dev/xbd2, actual 37c09674044d7835 
expected 574a309a2445c01c
ZFS: trying to find a vdev (/dev/xbd2) by guid 574a309a2445c01c
ZFS WARNING: vdev guid mismatch for /dev/xbd3, actual 3cc48031384426ef 
expected 37c09674044d7835
ZFS: trying to find a vdev (/dev/xbd3) by guid 37c09674044d7835
ZFS WARNING: vdev guid mismatch for /dev/xbd4, actual 5f400b8b20678472 
expected 3cc48031384426ef
ZFS: trying to find a vdev (/dev/xbd4) by guid 3cc48031384426ef
ZFS: trying to find a vdev (/dev/xbd5) by guid 5f400b8b20678472

# zpool status
  pool: tank
 state: UNAVAIL
status: One or more devices could not be opened.  There are insufficient
        replicas for the pool to continue functioning.
action: Attach the missing device and online it using 'zpool online'.
   see: http://illumos.org/msg/ZFS-8000-3C
  scan: none requested
config:

        NAME                     STATE     READ WRITE CKSUM
        tank                     UNAVAIL      0     0     0
          mirror-0               UNAVAIL      0     0     0
            6289893268167966748  UNAVAIL      0     0     0  was /dev/xbd2
            4017376292647041077  UNAVAIL      0     0     0  was /dev/xbd3
          mirror-1               UNAVAIL      0     0     0
            4378765686596708079  UNAVAIL      0     0     0  was /dev/xbd4
            6863498524284650610  UNAVAIL      0     0     0  was /dev/xbd5

--
Stephen