On 2022/1/9 04:29, Lukas Straub wrote:
On Sat, 8 Jan 2022 19:52:59 +0000
Lukas Straub <lukasstraub2@xxxxxx> wrote:
CC'ing linux-raid mailing list, where md raid development happens.
dm-raid is just a different interface to md raid.
On Fri, 7 Jan 2022 10:30:56 +0800
Qu Wenruo <quwenruo.btrfs@xxxxxxx> wrote:
Hi,
Recently I'm working on refactor btrfs raid56 (with long term objective
to add proper journal to solve write-hole), and the coverage of current
fstests for btrfs RAID56 is not that ideal.
Is there any project testing dm/md RAID456 for things like
re-silvering/write-hole problems?
And how you dm guys do the tests for stacked RAID456?
I really hope to learn some tricks from the existing, tried-and-true
RAID456 implementations, and hopefully to solve the known write-hole
bugs in btrfs.
Just some thoughts:
Besides the journal to mitigate the write-hole, md raid has another
trick:
The Partial Parity Log
https://www.kernel.org/doc/html/latest/driver-api/md/raid5-ppl.html
When a stripe is partially updated with new data, PPL ensures that the
old data in the stripe will not be corrupted by the write-hole. The new
data on the other hand is still affected by the write hole, but for
btrfs that is no problem.
But there is a even simpler solution for btrfs: It could just not touch
stripes that already contain data.
That would waste a lot of space, if the fs is fragemented.
Or we have to write into data stripes when free space is low.
That's why I'm trying to implement a PPL-like journal for btrfs RAID56.
Thanks,
Qu
The big problem will be NOCOW files, since a write to an already
allocated extent will necessarily touch a stripe with old data in it
and the new data also needs to be protected from the write-hole.
Regards,
Lukas Straub
Thanks,
Qu
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