[CentOS] 3Ware 9550SX and latency/system responsiveness

[Feizhou]

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 
>> config,
> 
> 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.