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

From: Naohiro Aota <naohiro.aota@wdc.com>
To: Josef Bacik <josef@toxicpanda.com>
Cc: linux-btrfs@vger.kernel.org, David Sterba <dsterba@suse.com>,
	Chris Mason <clm@fb.com>, Nikolay Borisov <nborisov@suse.com>,
	Damien Le Moal <damien.lemoal@wdc.com>,
	Johannes Thumshirn <jthumshirn@suse.de>,
	Hannes Reinecke <hare@suse.com>,
	Anand Jain <anand.jain@oracle.com>,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v6 11/28] btrfs: make unmirroed BGs readonly only if we have at least one writable BG
Date: Wed, 18 Dec 2019 16:35:18 +0900	[thread overview]
Message-ID: <20191218073518.zqtzfdgz7ctwlicn@naota.dhcp.fujisawa.hgst.com> (raw)
In-Reply-To: <78769962-9094-3afc-f791-1b35030c67dc@toxicpanda.com>

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.