[zfs-discuss] zfs diff performance

Edward Ned Harvey
Tue, 28 Feb 2012 14:44:35 -0000

> From: zfs-discuss-bounces at opensolaris.org [mailto:zfs-discuss-
> bounces at opensolaris.org] On Behalf Of Ulrich Graef
>
> Do you use de-duplication? (does not directly harm the performance, but
> needs memory
> and slows down zfs diff through that)?

Yikes.  That couldn''t be more wrong.  Yes, dedup hurts performance,
badly.  Yes, in theory dedup should be able to accelerate performance, but the
way it''s presently implemented, it gets hurt too dramatically by hard
disk seek/latency access time.  The way it''s presently implemented,
there is one and only one way dedup improves performance, which is when you read
duplicate blocks, then you get about 2-4x read performance gain.  For all other
operations - write duplicate, read nonduplicate, write nonduplicate...
Performance is worse with dedup.  As little as 2x, as high as 10x or 20x if you
have sufficient memory *and* you optimize (because the out-of-the-box
configuration is very nearly unusable), and infinite-x if you''re having
insufficient memory or you fail to optimize.