On 01/31/11 04:48 PM, Roy Sigurd Karlsbakk wrote:
As I've said here on the list a few times earlier, the last on the
thread 'ZFS not usable (was ZFS Dedup question)', I've been doing some
rather thorough testing on zfs dedup, and as you can see from the
posts, it wasn't very satisfactory. The docs claim 1-2GB memory usage
per terabyte stored, ARC or L2ARC, but as you can read from the post,
I don't find this very likely.
Sorry about the initial post - it was wrong. The hardware configuration was
right, but for initial tests, I use NFS, meaning sync writes. This obviously
stresses the ARC/L2ARC more than async writes, but the result remains the same.
With 140GB with of L2ARC on two X25-Ms and some 4GB partitions on the same
devices, 4GB each, in a mirror, the write speed was reduced to something like
20% of the origian speed. This was with about 2TB used on the zpool with a
single data stream, no parallelism whatsoever. Still with 8GB ARC and 140GB of
L2ARC on two SSDs, this speed is fairly low. I could not see substantially high
CPU or I/O load during this test.
I would not expect good performance on dedup with write... dedup isn't
going to make write's fast - its something you want on a system with a
lot of duplicated data that sustain a lot of reads. (That said, highly
duplicate date with a DDT that fits entirely in RAM might see a benefit
from not having to write meta data frequently. But I suspect an SLOG
here is going to be critical to get good performance since you'll still
have a lot of synchronous meta data writes.)
- Garrett
Vennlige hilsener / Best regards
roy
--
Roy Sigurd Karlsbakk
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r...@karlsbakk.net
http://blogg.karlsbakk.net/
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