Device missing with RAID1 on boot - observations

Device missing with RAID1 on boot - observations

[Steven Davies]

Kernel: 5.11.8 vanilla, btrfs-progs 5.11.1

I booted a box with a root btrfs raid1 across two devices, /dev/nvme0n1p2 (devid 2) and 
/dev/sda2 (devid 3). For whatever reason during the initrd stage, btrfs device scan was unable 
to see the NVMe device and mounted the rootfs degraded after multiple retries as I had designed 
in the init script.

Once booted apparently the kernel was able to see nvme0n1p2 again (with no intervention from me) 
and btrfs device usage / btrfs filesystem show did not report any missing devices. btrfs scrub 
reported that devid 2 was unwriteable but the scrub completed successfully on devid 3 with no 
errors. New block groups for data and metadata were being created as single on devid 3.

I balanced with -dconvert=single -mconvert=dup which moved all block groups to devid 3 and 
completed successfully; there was nothing remaining on devid 2 so I removed the device from the 
filesystem and re-added it as devid 4. Once I'd balanced the filesystem back to -dconvert=raid1 
-mconvert=raid1 everything was back to normal.

My main observation was that it was very hard to notice that there was an issue. Yes, I'd 
purposefully mounted as degraded, but there was no indication from the btrfs tools as to why new 
block groups were only being created as single on one device: nothing was marked as missing or 
unwriteable. Is this behavour expected? How can a device be unwriteable but not marked as missing?

Was my course of action to correct the issue correct - is there a better way to re-sync a raid1 
device which has temporarily been removed?

(Afterwards I realised what caused the issue - missing libraries in the initrd - and I can 
reproduce it if necessary.)

-- 
Steven Davies

Re: Device missing with RAID1 on boot - observations

[Qu Wenruo]

On 2021/4/5 下午11:18, Steven Davies wrote:
> Kernel: 5.11.8 vanilla, btrfs-progs 5.11.1
>
> I booted a box with a root btrfs raid1 across two devices,
> /dev/nvme0n1p2 (devid 2) and /dev/sda2 (devid 3). For whatever reason
> during the initrd stage, btrfs device scan was unable to see the NVMe
> device and mounted the rootfs degraded after multiple retries as I had
> designed in the init script.

It looks more like a problem in your initramfs environment.

The more possible cause is, your initramfs only has driver for SATA
disks, but no NVME modules.

You may try to include nvme module in your initramfs to see if that
solves the problem.

Thanks,
Qu

>
> Once booted apparently the kernel was able to see nvme0n1p2 again (with
> no intervention from me) and btrfs device usage / btrfs filesystem show
> did not report any missing devices. btrfs scrub reported that devid 2
> was unwriteable but the scrub completed successfully on devid 3 with no
> errors. New block groups for data and metadata were being created as
> single on devid 3.
>
> I balanced with -dconvert=single -mconvert=dup which moved all block
> groups to devid 3 and completed successfully; there was nothing
> remaining on devid 2 so I removed the device from the filesystem and
> re-added it as devid 4. Once I'd balanced the filesystem back to
> -dconvert=raid1 -mconvert=raid1 everything was back to normal.
>
> My main observation was that it was very hard to notice that there was
> an issue. Yes, I'd purposefully mounted as degraded, but there was no
> indication from the btrfs tools as to why new block groups were only
> being created as single on one device: nothing was marked as missing or
> unwriteable. Is this behavour expected? How can a device be unwriteable
> but not marked as missing?
>
> Was my course of action to correct the issue correct - is there a better
> way to re-sync a raid1 device which has temporarily been removed?
>
> (Afterwards I realised what caused the issue - missing libraries in the
> initrd - and I can reproduce it if necessary.)
>