Re: write-behind performance ... or how behind can write-behind write

[Georgi Alexandrov <teh@amln.net>]

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Bill Davidsen wrote:
> 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.
<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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