Re: file/extent checksums for dedup/sync...

[Chris Mason <chris.mason@oracle.com>]

On Wed, Jan 27, 2010 at 01:23:28PM +0000, Daniel J Blueman wrote:
> On Wed, Jan 27, 2010 at 12:30 PM, Andi Kleen <andi@firstfloor.org> wrote:
> > Daniel J Blueman <daniel.blueman@gmail.com> writes:
> >
> >> For purposes of data deduplication and data synchronisation, it would
> >> be a powerful tool to expose file data checksums.
> >>
> >> Since eg BTRFS uses the crc32c algorithm [1], it's possible to compute
> >> the file's overall CRC from the accumulation of the CRCs from all it's
> >> extents' CRCs.
> >>
> >> For now, exposing this via an IOCTL may be sufficient, though any
> >> ideas for introducing it in a more standard way? (it's a pity that
> >> when stat64 was introduced, reserved fields weren't added)
> >
> > The problem of doing it in any "standard way" is that it would
> > hard code the way the file system does checksums in the applications.
> > So the file system could never change it without breaking
> > user space.

At the end of the day the checksums are also hard coded on disk.  We
can't add a new way without continuing to support the old one.

> 
> I guess the filesystem would need to express this in the resulting
> data-structure, eg:
>  - type 1 corresponds to using the crc32c algorithm with starting seed
> N and accumulating ascending over data extents, padding with modulus
> remainder or sparse holes with 0
>  - type 2 etc

Yes, if they were exported to userland we'd need to export version info.

> 
> The next question, is does filesystem (eg BTRFS) compression come
> before or after checksumming?

The checksums are based on what is on disk, so they are done on the
compressed data.

-chris