Date: Wed, 27 Aug 2008 08:01:38 +1000 From: Andrew Snow <andrew@modulus.org> To: freebsd-fs@freebsd.org Subject: Poor ZFS prefetch performance Message-ID: <48B47D42.9060307@modulus.org>
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On latest 8-current with latest patches, I am seeing poor data prefetch hit rate with my workload of rsync on millions of files. ZFS has been quite good apart from this - UFS was much faster at this particular workload. ZFS just seems to be reading many megabytes over and over from the filesystem with little to show for it. Arcstats reports the following: kstat.zfs.misc.arcstats.prefetch_data_hits: 475476 kstat.zfs.misc.arcstats.prefetch_data_misses: 5325057 kstat.zfs.misc.arcstats.prefetch_metadata_hits: 126411515 kstat.zfs.misc.arcstats.prefetch_metadata_misses: 3998476 I don't really want to turn of prefetching completely because it does actually help in many situations. From this I deduce it would be really nice to provide an extra tunable that allows you to turn off prefetching for data but leave it on for metadata. This would be easy to do because data prefetches come in through dmu_zfetch, but metadata comes through dmu_prefetch() in dmu.c - Andrew
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