Date: Mon, 02 May 2005 15:53:34 +0200 From: "Poul-Henning Kamp" <phk@phk.freebsd.dk> To: "Steven Hartland" <killing@multiplay.co.uk> Cc: Eric Anderson <anderson@centtech.com> Subject: Re: Very low disk performance on 5.x Message-ID: <17813.1115042014@critter.freebsd.dk> In-Reply-To: Your message of "Mon, 02 May 2005 14:25:04 BST." <002201c54f1a$5c05dfc0$b3db87d4@multiplay.co.uk>
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>Interesting stuff so: >1. How to we test if this is happening? Calculate by hand what the offset of the striped/raid part of the disk is (ie: take slice+partition stats into account). >2. How do we prevent it from happening? Make sure that the first sector of a partition/slice is always the first sector in a stripe on your raid/stripe/whatever. >3. Why would this be effecting reads and not writes as surely the same >blocking is being done for both? Write on RAID5 uses a cache which lies to you about when things are safely stored on the disk. Good RAID5 has battery backup for that cache. The MBR slice format is stupid because it more often than not gets this exactly wrong. Typically there are 63 "sectors per track" and that ruins any alignment in 99% of the cases. Sysinstall, fdisk and bsdlabel should know about all this and try to help the user get it right. Fixing them to do so may be more trouble than writing a better too bottom up. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 phk@FreeBSD.ORG | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
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