Re: zfs and device name changes

[Takashi YAMAMOTO <yamt9999%gmail.com@localhost>]

On Fri, Mar 27, 2026 at 9:17 PM Stephen Borrill <netbsd%precedence.co.uk@localhost> wrote:
>
> 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

my patch doesn't scan xbd disks as it wasn't included in the list.
(see device_is_eligible_for_vdev)
if you add it, it should be found.

>
> # 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