[dm-devel] Proper way to test RAID456?

[Lukas Straub]

On Sat, 8 Jan 2022 19:52:59 +0000
Lukas Straub <lukasstraub2 at web.de> 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 at gmx.com> 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.

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
> > 
> > 
> > --
> > dm-devel mailing list
> > dm-devel at redhat.com
> > https://listman.redhat.com/mailman/listinfo/dm-devel
> >   
> 
> 
> 



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