Date: Sun, 31 Mar 1996 14:26:38 -0700 (MST) From: Terry Lambert <terry@lambert.org> To: taob@io.org (Brian Tao) Cc: freebsd-hackers@FreeBSD.org Subject: Re: Lowering minfree to 1% on large disks Message-ID: <199603312126.OAA11913@phaeton.artisoft.com> In-Reply-To: <Pine.NEB.3.92.960331133320.29121H-100000@zap.io.org> from "Brian Tao" at Mar 31, 96 01:49:15 pm
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> I know the tunefs man page contains warnings about lowering the > minfree threshold on a disk to below 5%, but besides file write > performance, is there any other reason *not* to drop it down to 1 or > 2 percent? Because this is below the hard-coded hysteresis range in which the FS will automatically switch between "time" and "space" optimization. If you do this, it will cause the FS mode to toggle constantly. > The specific application is with FTP filesystems. I have a couple > of 4GB disks for our mirror archives and I wouldn't mind recovering > the 300MB or so on each drive. Yeah, disk is cheap, but it still > seems like a waste to me. > > Since the only writes occuring on those drives come from the > mirror process, I figure network performance will always be the > bottleneck rather than disk writes. The reason for the reserve is fragmentation. Effectively, the choice of where to write is a hash onto the disk. For a decreasing reserve, the liklihood of a hash collision increases expotentially, and this will hurt preformance significantly. The 50% collision probability mark for a straight hash is ~85% (Knuth, _Sorting And Searching_). A free reserve of 10% was settled on by people who, like you, didn't want to "waste space'. In reality, you are trading hash reserve for fragmentation wastage by dropping the reserve. 8% is a little iffy, 5% is downright foolish for the 5% before you hit the reserve, and 1% or 2% *will* cause *severe* fragmentation. Are you prepared to write a defragmenter? No one has written one yet because they didn't need one -- they didn't try tuning their reserve down to 1% or 2%. If you do, you'll need one. Terry Lambert terry@lambert.org --- Any opinions in this posting are my own and not those of my present or previous employers.
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