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

[Daniel J Blueman <daniel.blueman@gmail.com>]

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.

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

The next question, is does filesystem (eg BTRFS) compression come
before or after checksumming?
-- 
Daniel J Blueman