Date: Wed, 22 Mar 2000 15:44:20 +0000 (GMT) From: Richard Wendland <richard@netcraft.com> To: Paul Richards <paul@originative.co.uk> Cc: Richard Wendland <richard@netcraft.com>, Alfred Perlstein <bright@wintelcom.net>, Poul-Henning Kamp <phk@critter.freebsd.dk>, Matthew Dillon <dillon@apollo.backplane.com>, current@FreeBSD.ORG, fs@FreeBSD.ORG Subject: Re: FreeBSD random I/O performance issues Message-ID: <200003221544.PAA08760@ns0.netcraft.com> In-Reply-To: <38D833BC.A082DF09@originative.co.uk> from Paul Richards at "Mar 22, 2000 02:45:16 am"
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> > With sync it's ~20,000 operations matching the total of reads & > > writes. This demonstrates another aspect of the bug, sync behaviour > > should cause 10,000 operations; the reads aren't being cached. > > This isn't quite true. It's 20,000 *write* operations. I put this down > to the mtime update for each write doubling the number of actual write > operations. No read operations take place, the data *does* come out of > the cache. There's nothing wrong with reading as far as I can tell. Yes, you're absolutely right, I should have looked at my own data more closely. If I change the test program to call fsync after write, and run on a default mount filesystem I also see 20,000 I/O operations from 10,000 writes. This probably impacts more real programs out there than sync mounts. If this is mtime updates being does synchronously, that seems a separate issue to the clustering/VM issue, and seems to me it should be fixed. It'll normally double the number of all writes won't it, possibly forcing seeks between otherwise localised access. Can anyone offer an alternative hypothesis to mtime updates being done synchronously? Looking at my logs for the sync filesystem test, mount output before and after shows all ~20,000 operations are writes:: mount /dev/wd0s2e on /var (local, synchronous, writes: sync 182 async 10) time ./seekreadwrite xxx 10000 0.1u 7.8s 1:47.61 7.4% 5+179k 0+20000io 0pf+0w mount /dev/wd0s2e on /var (local, synchronous, writes: sync 20190 async 15) But when using fsync on a default mount filesystems, 10000 writes are sync and 10000 async: mount /dev/wd0s2e on /var (local, writes: sync 682 async 2764) time ./seekreadwrite xxx 10000 0.0u 1.7s 0:54.34 3.3% 4+171k 0+20000io 0pf+0w mount /dev/wd0s2e on /var (local, writes: sync 10682 async 12777) This is on the ATA machine that could run the test in 4 seconds without fsync, 54 seconds with fsync, suggesting some head movements may be being forced, though not 20000 as that would imply 2.7ms per seek. Richard -- Richard Wendland richard@netcraft.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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