Re: [dm-devel] Proper way to test RAID456?

[David Woodhouse <dwmw2@infradead.org>]

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On Sun, 2022-01-09 at 07:55 +0800, Qu Wenruo wrote:
> On 2022/1/9 04:29, Lukas Straub wrote:
> > 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.

PPL writes the P/Q of the unmodified chunks from the stripe, doesn't
it?

An alternative in a true file system which can do its own block
allocation is to just calculate the P/Q of the final stripe after it's
been modified, and write those (and) the updated data out to newly-
allocated blocks instead of overwriting the original.

Then the final step is to free the original data blocks and P/Q.

This means that your RAID stripes no longer have a fixed topology; you
need metadata to be able to *find* the component data and P/Q chunks...
it ends up being non-trivial, but it has attractive properties if we
can work it out.

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