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

[Qu Wenruo]

Hi all,

[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.

[What we have]
The nearest infrastructure I found in kernel is bio_integrity_payload.

However I found it's bounded to device, as it's designed to support
SCSI/SATA integrity protocol.
While for such use case, it's more bounded to filesystem, as fs (or
higher layer dm device) is the source of integrity data, and device
(dm-raid) only do the verification and possible repair.

I'm not sure if this is a good idea to reuse or abuse
bio_integrity_payload for this purpose.

Should we use some new infrastructure or enhance existing
bio_integrity_payload?

(Or is this a valid idea or just another crazy dream?)

Thanks,
Qu

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