On Mon, Mar 12, 2012 at 5:23 PM, Basil Kurian <[email protected]> wrote:
> I don't know much about the inside stuffs of ZFS.
>
Ok usually it is done by CRC.
Some times CRC is same for two blocks but they have differrent Data
and that is called CRC collission.
This is how hammer resolves it.
The hash function used for comparing data blocks is CRC-32 (CRCs
are computed anyways as part of HAMMER data integrity features,
so there's no additional overhead). Since CRC is a weak hash
function a byte-by-byte comparison is done before actual dedup-
ing. In case of a CRC collision (two data blocks have the same
CRC but different contents) the checksum is upgraded to SHA-256.
When you dedup in Hammer it gives you the Data about CRC collissions
# hammer dedup /home
Dedup /home: objspace 8000000000000000:0000 7fffffffffffffff:ffff pfs_id 4
Dedup /home succeeded
Dedup ratio = 1.22
462 GB referenced
378 GB allocated
14 MB skipped
6869 CRC collisions
0 SHA collisions
0 bigblock underflows
Thanks
Siju
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