On Jun 10, 2011, at 17:25, Volodymyr Kostyrko wrote:
> Am I missing something? How about using fletcher[24] for dedup?
Fletcher is fairly weak as things go, and so even though two checksums are the
same, there's a decent chance that the data is actually different. At least
with recent releases of (Open)Solaris, when you enable do a 'dedup=on' the has
used is SHA-256, which has very, very, very, low odds of having the same value
occur from two different blocks of data.
When ZFS dedupe originally came out you could have one of the following values:
. off
. on (== sha256)
. flecther4 with verify/compare
. sha256 (without verify/compare)
. sha256 with verify
There was a long-ish thread on zfs-discuss fairly recently on whether SHA-256
was "good enough" where you could trust it, or whether one should do a verify
step in addition to SHA-256:
http://mail.opensolaris.org/pipermail/zfs-discuss/2011-January/046875.html
While some people argued that it was prudent to use "verify" (especially with
your data/job on the line), a good portion of folks though said that it's not
worth it (i.e., if you're not worried about being hit by lightning (2^-17 to
2^-18), you shouldn't be worried about a hash collision (2^-128)).
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