[dm-devel] Ideas to reuse filesystem's checksum to enhance dm-raid1/10/5/6?

[Andreas Dilger]

On Nov 15, 2017, at 7:18 PM, Qu Wenruo <quwenruo.btrfs at gmx.com> wrote:
> 
> [Background]
> Recently I'm considering the possibility to use checksum from filesystem
> to enhance device-mapper raid.
> 
> The idea behind it is quite simple, since most modern filesystems have
> checksum for their metadata, and even some (btrfs) have checksum for data.
> 
> And for btrfs RAID1/10 (just ignore the RAID5/6 for now), at read time
> it can use the checksum to determine which copy is correct so it can
> return the correct data even one copy get corrupted.
> 
> [Objective]
> The final objective is to allow device mapper to do the checksum
> verification (and repair if possible).
> 
> If only for verification, it's not much different from current endio
> hook method used by most of the fs.
> However if we can move the repair part from filesystem (well, only btrfs
> supports it yet), it would benefit all fs.

I recall Darrick was looking into a mechanism to do this.  Rather than
changing the whole block layer to take a callback to do a checksum, what
we looked at was to allow the upper-layer read to specify a "retry count"
to the lower-layer block device.  If the lower layer is able to retry the
read then it will read a different device (or combination of devices for
e.g. RAID-6) based on the retry count, until the upper layer gets a good
read (based on checksum, or whatever).  If there are no more devices (or
combinations) to try then a final error is returned.

Darrick can probably point at the original thread/patch.

Cheers, Andreas





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