Paul Clements wrote:

Georgi Alexandrov wrote:

quoted

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.

quoted

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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