On Dec 18, 2009, at 9:40 AM, Stuart Anderson wrote:
On Dec 17, 2009, at 9:21 PM, Richard Elling wrote:
On Dec 17, 2009, at 9:04 PM, stuart anderson wrote:
As a specific example of 2 devices with dramatically different
performance for sub-4k transfers has anyone done any ZFS
benchmarks between the X25E and the F20 they can share?
I am particularly interested in zvol performance with a blocksize
of 16k and highly compressible data (~10x).
16 KB recordsize? That seems a little unusual, what is the
application?
SAM-QFS metadata whose fundamental disk allocation unit (DAU) size
for metadata is 16kB.
Ah, ok. That explains it. I'm not sure there are a lot of people
doing this.
Most folks don't know QFS exists or is open source.
I am going to run some comparison tests but would appreciate any
initial input on what to look out for or how to tune ZFS to get
the most out of the F20.
AFAICT, no tuning should be required. It is quite fast.
It might be helpful, e.g., if there where some where in the
software stack where I could tell part of the system to lie and
treat the F20 as a 4k device?
The F20 is rated at 84,000 random 4KB write IOPS. The DRAM write
buffer will hide 4KB write effects.
Not from some direct vdbench comparison results I have seen. My main
concern here has to do with ZFS compression, which I need for my
application, breaking up the transfer sizes the F20 sees into
smaller than 4KB writes where there is a critical performance
difference. I also suspect/hope that SAM-QFS is telling ZFS to
aggressively flush/commit any metadata updates to stable storage
which probably aggravates the problem though I have not test this yet.
ZFS will coalesce writes, regardless of the recordsize. However, this
is not
the case for writes to the ZIL (for obvious reasons). Measure the ZIL
activity
to see how that workload looks. If you don't see ZIL activity, then
you should
see (mostly) larger I/Os when the txg commits.
OTOH, the X-25E is rated at 3,300 random 4KB writes. It shouldn't
take
much armchair analysis to come to the conclusion that the F20 is
likely
to win that IOPS battle :-)
Though to be fair you should probably compare a single F20 DOM to an
X25-E, or 4 X25E's to a full F20, and of course my systems don't run
from an armchair :)
...or 1,000 1 TB SATA disks... :-)
-- richard
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