On 07/11/2012 09:58 AM, Ferenc-Levente Juhos wrote:
> Hello all,
> what about the fletcher2 and fletcher4 algorithms? According to the zfs man
> page on oracle, fletcher4 is the current default.
> Shouldn't the fletcher algorithms be much faster then any of the SHA
> algorithms?
> On Wed, Jul 11, 2012 at 9:19 AM, Sašo Kiselkov <skiselkov...@gmail.com>wrote:

Fletcher is a checksum, not a hash. It can and often will produce
collisions, so you need to set your dedup to verify (do a bit-by-bit
comparison prior to deduplication) which can result in significant write
amplification (every write is turned into a read and potentially another
write in case verify finds the blocks are different). With hashes, you
can leave verify off, since hashes are extremely unlikely (~10^-77) to
produce collisions.

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