Date: Wed, 12 May 2004 13:36:14 -0400 From: Don Bowman <don@sandvine.com> To: 'Scott Long' <scottl@freebsd.org>, Pete French <petefrench@keithprowse.com> Cc: freebsd-scsi@freebsd.org Subject: RE: 2120S Stripe - abysmal performance Message-ID: <FE045D4D9F7AED4CBFF1B3B813C85337051D8C0D@mail.sandvine.com>
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From: Scott Long [mailto:scottl@freebsd.org] > Pete French wrote: > > >>Read caching should be turned off unless you have a very > specific need > >>for it. Having it on is going to hurt sequential read > performance by a > >>2x factor. The only time is makes sense is when you have a > relatively > >>small data set that gets read repeatedly, with few other > reads mixed in. > > > > > > Interesting comment. Does that refer to this particular > controller, to > > caches on RAID controllers in general, or to caches on the > drives themselves ? > > > > -pete. > > > With the read cache on, every time that the OS requests a new logical > block that isn't in the cache, the controller has to first DMA that > block from the disks into the cache, then DMA from the cache to host > memory. With the read cache off, it only has to DMA from the disks > straight to host memory. Some older AAC controllers also don't > balance the cache well between read and write, so having both enabled > winds up thrashing both. > > The read cache on the drive is a good thing since it will do a bit of > read-ahead which will help reduce latency. Scott, how does one disable the read cache? Its not available in the bios on the AAC (on the 2100). Is there a modepage for this? --don
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