Paul Clements wrote:
Georgi Alexandrov wrote:
Generally with the healthy array I'm getting the write performance of
the SATA disk alone (in terms of requests/sec issued to the disk and
bytes/sec written). The SATA disk is obviously a bottleneck even with
the write-behind option set(2).
write-behind can help with two things:
1) overcoming latency (say one disk is on the network -- it may be the
same speed as the source disk, but it takes longer round-trip for each
I/O to complete)
2) temporary slowness of a device (say at a peak in I/O) -- the queue
can temporarily hide the slowness of the secondary disk, but this
won't last very long -- if writes continue at a pace faster than the
disk can handle (i.e., the queue gets filled) then the array drops
back to non-write-behind behavior
At least with write-mostly all of the capacity is going into saving
data, not serving data. But as you note below if the writes are
happening at a rate faster than the device can support it will be a
bottleneck.
So the questions is How behind can write-behind write? And can we get a
better performance in a similar setup.
By default, it queues up 256 writes. This can be increased, but I've
actually seen worse performance in some cases -- not sure why. I
haven't had the time to dig into it and figure it out.
--
Paul
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Bill Davidsen <davidsen@xxxxxxx>
"Woe unto the statesman who makes war without a reason that will still
be valid when the war is over..." Otto von Bismark
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