Date: Mon, 3 Aug 2015 11:36:17 -0500 (CDT) From: Bob Friesenhahn <bfriesen@simple.dallas.tx.us> To: Matt Churchyard <matt.churchyard@userve.net> Cc: FreeBSD FS <freebsd-fs@freebsd.org> Subject: RE: ZFS: Disabling ARC? Message-ID: <alpine.GSO.2.01.1508031132270.13737@freddy.simplesystems.org> In-Reply-To: <45e74b89dd754991a366a46aa5101822@SERVER.ad.usd-group.com> References: <55BC14B7.9010009@sneakertech.com> <20150801133635.00002ecc@Leidinger.net> <45e74b89dd754991a366a46aa5101822@SERVER.ad.usd-group.com>
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On Mon, 3 Aug 2015, Matt Churchyard wrote: > > I don't know if it's changed, but even changing ARC to metadata only used to cause massive performance issues when reading large files in small chunks. Reading a 128k ZFS record in 4k chunks would cause ZFS to read the same 128k record from disk 32 times. > There's a forum thread about it here - https://forums.freebsd.org/threads/zfs-primarycache-all-versus-metadata.45555/ > > Generally I've found ARC to be one of the most important parts of > ZFS. There's no case I know of where it will actually adversely > affect performance (not that I've really looked). Its only downside > is that it doesn't seem to manage memory as well as it should, and > so a lot of people (including me) have resorted to limiting it. The main reason to disable caching is if it is known in advance that the data will only be read once, and that the block will be read in a single operation. This may be useful for block-oriented databases which do their own caching, and video servers which carefully read full blocks. The reason to disable the caching is to avoid wasting memory. Bob -- Bob Friesenhahn bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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