On 24/06/2010 15:54, Bob Friesenhahn wrote:
On Thu, 24 Jun 2010, Ross Walker wrote:
Raidz is definitely made for sequential IO patterns not random. To
get good random IO with raidz you need a zpool with X raidz vdevs
where X = desired IOPS/IOPS of single drive.
Remarkably, I have yet to see mention of someone testing a raidz which
is comprised entirely of FLASH SSDs. This should help with the IOPS,
particularly when reading.
I have.
Briefly:
X4270 2x Quad-core 2.93GHz, 72GB RAM
Open Solaris 2009.06 (snv_111b)
ARC limited to 4GB
44x SSD in a F5100.
4x SAS HBAs, 4x physical SAS connections to the f5100 (16x SAS
channels in total), each to a different domain.
1. RAID-10 pool
22x mirrors across domains
ZFS: 16KB recordsize, atime=off
randomread filebennch benchmark with a 16KB block size with 1, 16,
..., 128 threads, 128GB working set.
maximum performance when 128 threads: ~137,000 ops/s
2. RAID-Z pool
11x 4-way RAID-z, each raid-z vdev across domains
ZFS: recordsize=16k, atime=off
randomread filebennch benchmark with a 16KB block size with 1, 16,
..., 128 threads, 128GB working set.
maximum performance when 64-128 threads: ~34,000 ops/s
With a ZFS recordsize of 32KB it got up-to ~41,000 ops/s.
Larger ZFS record sizes produced worse results.
RAID-Z delivered about 3.3X less ops/s compared to RAID-10 here.
SSDs do not make any fundamental chanage here and RAID-Z characteristics
are basically the same whether it is configured out of SSDs or HDDs.
However SSDs could of course provide a good-enough performance even with
RAID-Z, as at the end of a day it is not about benchmarks but your
environment requirements.
A given number of SSDs in a RAID-Z configuration is able to deliver the
same performance as a much greater number of disk drives in RAID-10
configuration and if you don't need much space it could make sense.
--
Robert Milkowski
http://milek.blogspot.com
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