[zfs-discuss] Recovering data from a mirror

Stephan Stachurski ses1984 at gmail.com
Fri Dec 15 09:20:36 EST 2017

I'm not quite certain how to import just a single drive, everything I've
tried results in "cannot import: one or more devices is currently

On Tue, Dec 12, 2017 at 3:49 AM, Sam Van den Eynde <svde.tech at gmail.com>

> First thing I would try is to put only the last original 6TB drive, import
> read-only. That should still be consistent.
> On Dec 12, 2017 7:03 AM, "Stephan Stachurski via zfs-discuss" <
> zfs-discuss at list.zfsonlinux.org> wrote:
> Hi
> Long story short is I tried to grow a 2x6TB mirror to 2x8TB drives. I saw
> a lot of checksum errors so I tried to roll back, and ended up with
> corrupted data. I'm trying to figure out how to recover as much as
> possible, because restoring from backup is pretty inconvenient.
> I'm using ECC memory, and CPU and memtests are OK. I think cables and
> other associated bits of hardware are OK because I've been swapping slots
> of disks in this bad array with disks another good array and the other good
> array is running smoothly, zero checksum errors.
> I started with 2x6TB drives in a mirror configuration. I wanted to replace
> these drives with 8TB drives so I first attached a 8TB disk to the 2x6TB
> mirror. It resilvered overnight and I saw 150 checksum errors.
> I didn't think much of it. I thought maybe the drive was bad, or there was
> some hiccup in the system. I shut down the system so I could pull 1x 6TB
> drive and add the second 8TB drive. I added it and it also started to
> resilver.
> I think my first mistake was adding the 2nd drive to a potentially
> unhealthy mirror using read/write mode instead of read only. But we can get
> back to this later.
> After the 2nd 8TB drive resilvered, the two 8TB drives now have 2k
> checksum errors each.
> I shutdown and pulled both 8TB drives and added the pulled 6TB drive back
> to the system. I boot, zpool import, and I see bad things. Half my data is
> gone. When I ls, I can see the file names there, but the attrs are question
> marks, and when I try to stat those files, I see "Invalid exchange"
> I shutdown and pulled both 6TB drives. This time I add the 8TB drives and
> zpool import readonly, and things look better, but still ~15% of my data is
> messed up.
> I'm not sure what i did wrong, but I did not expect this kind of
> corruption just from adding and removing disks from a mirror. I know I had
> checksum errors, but I thought zfs would have me covered. I did not think
> there was even a slim chance that I could have corrupted data at rest from
> just adding or removing drives from a mirror. I know I had a potentially
> degraded mirror that I did not mount read only, but I did not think doing
> so would cause old data to go bad, because zfs checksums and redundancy
> would stop that from happening.
> So, if nothing else, let this be a lesson. ZFS doesn't magically save you
> from being stupid and/or bad hardware.
> Anyway, I was wondering if there was a process I could follow to attempt
> to recover as much as possible from the array. I still have a hard time
> believing that data I wrote to the 2x6TB disks a year ago somehow went bad.
> I think between the original 2x6TB disks, and the new 2x8TB disks that
> resilvered successfully, I should probably be able to recover most if not
> all of the data. Maybe I'm being naively optimistic. It would be a pain the
> butt to restore a couple TB from optical media.
> Thanks,
> Steve
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Stephan E Stachurski
ses1984 at gmail.com
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