[CentOS] 3Ware 9550SX and latency/system responsiveness
feizhou at graffiti.net
Sat Sep 15 00:18:31 UTC 2007
Simon Banton wrote:
> At 11:16 -0400 14/9/07, Ross S. W. Walker wrote:
>> Yes, a write-back cache with a BBU will definitely help, also your
> The write-cache is enabled, but what I've not known up to now is that
> the absence of a BBU will impact IO performance in this way - which
> seems to be what you and Feizhou are saying. Is there any way to tell
> the card to forget about not having a BBU and behave as if it did?
Short of modifying the code...I do not know of any.
> The main problem here is the latency when under IO load not the
> throughput (or lack of). I don't care if it can't achieve 300MB/s
> sustained write speeds, only that it shouldn't bring the machine to its
> knees in the process of getting 35MB/s.
SATA 7200RPM disks are not exactly fantastic at random i/o. The cache
only boosts writes in a significant way and less so for reads. Maybe you
could consider RAID10.
That way, them 7200RPM disks are a match if not better than hardware
raid with BBU cache for a mirror of scsi drives.
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