Re: [PATCH v2 0/6] Chunk level degradable check

[Adam Borowski <kilobyte@angband.pl>]

On Tue, Mar 07, 2017 at 09:35:56AM +0800, Qu Wenruo wrote:
> At 03/07/2017 08:36 AM, Adam Borowski wrote:
> > Not so for -draid5 -mraid1, unfortunately:
> 
> Unfortunately, for raid5 there are still unfixed bugs.
> In fact, some raid5/6 bugs are already fixed, but still not merged yet.
> 
> > [/mnt/btr2/scratch]# btrfs fi us /mnt/vol1
> > Data,RAID5: Size:2.02GiB, Used:1.21GiB
> >    /dev/loop0	   1.01GiB
> >    /dev/loop1	   1.01GiB
> >    /dev/loop2	   1.01GiB

> > [/mnt/btr2/scratch]# umount /mnt/vol1
> > [/mnt/btr2/scratch]# losetup -D
> > [/mnt/btr2/scratch]# losetup -f rb
> > [/mnt/btr2/scratch]# losetup -f rc
> 
> So you're pulling out first device.
> In theory, it should be completely OK for RAID5.
> And the degradable check follows it.
> 
> > [/mnt/btr2/scratch]# mount -noatime,degraded /dev/loop0 /mnt/vol1
> > [/mnt/btr2/scratch]# btrfs fi us /mnt/vol1
> > Data,RAID5: Size:2.02GiB, Used:1.21GiB
> >    /dev/loop0	   1.01GiB
> >    /dev/loop0	   1.01GiB
> >    /dev/loop1	   1.01GiB
> 
> Two loop0 shows up here, which should be detected as missing.
> 
> So it should be a btrfs-progs bug, and it'll be much easier to fix than
> kernel.

Alas, it's not merely a display bug, mounting is enough.

> > Write something, mount degraded again.  Massive data corruption, both on
> > plain reads and on scrub, unrecoverable.
> 
> Yep, same thing here.
> And you'll be surprised that even 2 devices RAID5, which is the same as
> RAID1(parity is the same as data), can still cause the problem.
> 
> So, RAID5/6 definitely has problem in degraded mode.
> While I prefer to focus on normal RAID5/6 bug fix first, and until we solve
> all RAID5/6 normal mode bugs with enough test cases covering them.

Actually, turns out even the _first_ mount gets bad, even without writing a
single data byte.  So it's not related to our single chunks bug.

> > Obviously, this problem is somewhere with RAID5 rather than this patch set,
> > but the safety check can't be removed before that is fixed.
> 
> Do we have *safety check* in original behavior?
> 
> At least v4.11-rc1, btrfs still allows us to mount raid5/6 degraded.
> So the patchset itself is behaving just as old one.

Right.  Thus, there's no regression.

As it's a strict improvement over previous state (ie, fixes raid1 issues),
Tested-by: Adam Borowski <kilobyte@angband.pl> (if you don't mind spamming
commits with too many tags).

> I'm completely fine to add a new patch to prohibit raid5/6 degraded mount,
> but that would be a different enhancement though.

Yeah.  I guess it's more in the "don't use RAID5, there be dragons" land.


Thanks for these patches, they fix the #1 problem people have with RAID1.


[Apologies for that "✔" crap on some lines, my exit code on prompt thingy
is very paste-unfriendly; I keep forgetting it so often that I'd better get
rid of it...]

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