Date: Tue, 13 Sep 2011 11:31:35 +0200 From: Ivan Voras <ivoras@freebsd.org> To: freebsd-fs@freebsd.org Subject: Re: ZFS-lighttpd2-sendfile, too high IO Message-ID: <j4n7tn$r1u$1@dough.gmane.org> In-Reply-To: <alpine.GSO.2.01.1109121428570.12948@freddy.simplesystems.org> References: <freemail.20110912210908.38291.2@xmldata09.freemail.hu> <alpine.GSO.2.01.1109121428570.12948@freddy.simplesystems.org>
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On 12/09/2011 21:32, Bob Friesenhahn wrote: > On Mon, 12 Sep 2011, Laszlo KAROLYI wrote: > >> But does this explain the 4-5mbyte/s reads when having a 15mbit/s >> network load? > > There are only two viable explanations: > > o Insuffient caching due to insufficient resources > > o Data is not being cached at all > > Zfs reads whole 128K blocks (or whatever the filesystem blocksize is) at > a time. It does not read partial blocks from underlying storage. This > makes it very expensive to perform many small read accesses if the reads > are not subsequently cached in the ARC. Yes! Which makes it particularly "interesting" if you try to run a database on it while forgetting to reset the block size to e.g. 8K before the database is initialized - you get 16x more IO than you expected.
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