Re: [PATCH v6 11/28] btrfs: make unmirroed BGs readonly only if we have at least one writable BG

Josef Bacik
2019-12-18T14:54:50Z

On 12/18/19 2:35 AM, Naohiro Aota wrote:
> On Tue, Dec 17, 2019 at 02:25:37PM -0500, Josef Bacik wrote:
>> On 12/12/19 11:08 PM, Naohiro Aota wrote:
>>> If the btrfs volume has mirrored block groups, it unconditionally makes
>>> un-mirrored block groups read only. When we have mirrored block groups, but
>>> don't have writable block groups, this will drop all writable block groups.
>>> So, check if we have at least one writable mirrored block group before
>>> setting un-mirrored block groups read only.
>>>
>>> This change is necessary to handle e.g. xfstests btrfs/124 case.
>>>
>>> When we mount degraded RAID1 FS and write to it, and then re-mount with
>>> full device, the write pointers of corresponding zones of written block
>>> group differ. We mark such block group as "wp_broken" and make it read
>>> only. In this situation, we only have read only RAID1 block groups because
>>> of "wp_broken" and un-mirrored block groups are also marked read only,
>>> because we have RAID1 block groups. As a result, all the block groups are
>>> now read only, so that we cannot even start the rebalance to fix the
>>> situation.
>>
>> I'm not sure I understand.  In degraded mode we're writing to just one mirror
>> of a RAID1 block group, correct?  And this messes up the WP for the broken
>> side, so it gets marked with wp_broken and thus RO.  How does this patch
>> help?  The block groups are still marked RAID1 right? Or are new block groups
>> allocated with SINGLE or RAID0?  I'm confused. Thanks,
>>
>> Josef
>
> First of all, I found that some recent change (maybe commit
> 112974d4067b ("btrfs: volumes: Remove ENOSPC-prone
> btrfs_can_relocate()")?) solved the issue, so we no longer need patch
> 11 and 12. So, I will drop these two in the next version.
>
> So, I think you may already have no interest on the answer, but just
> for a note... The situation was like this:
>
> * before degrading
>   - All block groups are RAID1, working fine.
>
> * degraded mount
>   - Block groups allocated before degrading are RAID1. Writes goes
>     into RAID1 block group and break the write pointer.
>   - Newly allocated block groups are SINGLE, since we only have one
>     available device.
>
> * mount with the both drive again
>   - RAID1 block groups are markd RO because of broken write pointer
>   - SINGLE block groups are also marked RO because we have RAID1 block
>     groups
>
> and at this point, btrfs was somehow unable to allocate new block
> group or to start blancing.

Oooh ok I see, I had it in my head we would still allocate RAID1 chunks, but we
allocate SINGLE, so that makes sense. Go ahead and drop those patches, and
thanks for the explanation.

Josef