Date: Wed, 12 May 2004 11:48:06 -0600 From: Scott Long <scottl@freebsd.org> To: Don Bowman <don@sandvine.com> Cc: freebsd-scsi@freebsd.org Subject: Re: 2120S Stripe - abysmal performance Message-ID: <40A26356.409@freebsd.org> In-Reply-To: <FE045D4D9F7AED4CBFF1B3B813C85337051D8C0D@mail.sandvine.com> References: <FE045D4D9F7AED4CBFF1B3B813C85337051D8C0D@mail.sandvine.com>
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Don Bowman wrote: > 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 I can't remember how the 2100 BIOS works, I'll have to take a look. I thought that it was an attribute that was set when the array is created. Scott
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