Date: Mon, 02 May 2005 08:27:52 -0500 From: Eric Anderson <anderson@centtech.com> To: Poul-Henning Kamp <phk@phk.freebsd.dk> Cc: freebsd-performance@freebsd.org Subject: Re: Very low disk performance on 5.x Message-ID: <42762AD8.1020607@centtech.com> In-Reply-To: <17479.1115040178@critter.freebsd.dk> References: <17479.1115040178@critter.freebsd.dk>
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Poul-Henning Kamp wrote: > In message <4276281C.6060209@centtech.com>, Eric Anderson writes: > >>Poul-Henning Kamp wrote: >> >>>In message <427626DC.5030702@centtech.com>, Eric Anderson writes: >>> >>> >>> >>>>Don't mean to be terse here, but I'm talking about the same test done an >>>>two different RAID5 configurations, with different disks, and not just >>>>me - other users in this very thread see the same issue.. >>> >>> >>>Uhm, if you are using RAID5 and your requests are not aligned and >>>sized after the RAID5 you should *expect* read performance to be poor. >>> >>>If you your request ends up accessing two different blocks even just >>>once per stripe, this totally kills performance. >> >>Wouldn't this be a problem for writes then too? > > > I presume you would only compare read to write performance on a RAID5 > device which has battery backed cache. > > Without a battery backed cache (or pretending to have one) RAID5 > write performance is abysmall no matter which alignment you use. If I write a 10GB file to disk (RAID array has 1GB cache, system has 1GB memory), then I should definitely see better read performance reading that same file back to /dev/null than writing it, right? How about this - you tell me what test to run, and I'll do it (as long as it doesn't destory my data). Eric -- ------------------------------------------------------------------------ Eric Anderson Sr. Systems Administrator Centaur Technology A lost ounce of gold may be found, a lost moment of time never. ------------------------------------------------------------------------
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