Date: Sun, 21 Mar 2010 17:53:16 +0100 From: Ulrich =?utf-8?B?U3DDtnJsZWlu?= <uqs@spoerlein.net> To: Scott Long <scottl@samsco.org> Cc: freebsd-current@freebsd.org, freebsd-arch@freebsd.org Subject: Re: Increasing MAXPHYS Message-ID: <20100321165316.GU99813@acme.spoerlein.net> In-Reply-To: <9524C333-F191-4F7A-A5FA-BD52498169C0@samsco.org> References: <4BA4E7A9.3070502@FreeBSD.org> <201003201753.o2KHrH5x003946@apollo.backplane.com> <891E2580-8DE3-4B82-81C4-F2C07735A854@samsco.org> <20100321163051.GT99813@acme.spoerlein.net> <9524C333-F191-4F7A-A5FA-BD52498169C0@samsco.org>
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[CC trimmed] On Sun, 21.03.2010 at 10:39:10 -0600, Scott Long wrote: > On Mar 21, 2010, at 10:30 AM, Ulrich Spörlein wrote: > > On Sat, 20.03.2010 at 12:17:33 -0600, Scott Long wrote: > >> Windows has a MAXPHYS equivalent of 1M. Linux has an equivalent of an > >> odd number less than 512k. For the purpose of benchmarking against these > >> OS's, having comparable capabilities is essential; Linux easily beats FreeBSD > >> in the silly-i/o-test because of the MAXPHYS difference (though FreeBSD typically > >> stomps linux in real I/O because of vastly better latency and caching algorithms). > >> I'm fine with raising MAXPHYS in production once the problems are addressed. > > > > Hi Scott, > > > > while I'm sure that most of the FreeBSD admins are aware of "silly" > > benchmarks where Linux I/O seems to dwarf FreeBSD, do you have some > > pointers regarding your statement that FreeBSD triumphs for real-world > > I/O loads? Can this be simulated using iozone, bonnie, etc? More > > importantly, is there a way to do this file system independently? > > > > iozone and bonnie tend to be good at testing serialized I/O latency; each read and write is serialized without any buffering. My experience is that they give mixed results, sometimes they favor freebsd, sometime linux, sometimes it's a wash, all because they are so sensitive to latency. And that's where is also gets hard to have a "universal" benchmark; what are you really trying to model, and how does that model reflect your actual workload? Are you running a single-instance, single threaded application that is sensitive to latency? Are you running a multi-instance/multi-threaded app that is sensitive to bandwidth? Are you operating on a single file, or on a large tree of files, or on a raw device? Are you sharing a small number of relatively stable file descriptors, or constantly creating and deleting files and truncating space? All true, that's why I wanted to know from you, which real world situations you encountered where FreeBSD did/does outperform Linux in regards to I/O throughput and/or latency (depending on scenario, of course). I hope you don't mind, Uli
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