Bill Davidsen wrote:

Paul Clements wrote:

quoted

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.

<snip>

Well, at least write-mostly is suitable for reading from the SSD disk
only in a setup like mine. If writes get really problematic maybe it's
better to consider a SSD-only solution.

-- 
regards,
Georgi Alexandrov
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