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Xref: sserve comp.unix.admin:14309 comp.unix.bsd:12771 comp.unix.internals:6355 comp.unix.wizards:31263 Newsgroups: comp.unix.admin,comp.unix.bsd,comp.unix.internals,comp.unix.wizards Path: sserve!newshost.anu.edu.au!munnari.oz.au!news.Hawaii.Edu!ames!agate!doc.ic.ac.uk!uknet!gdt!aber!fronta.aber.ac.uk!pcg From: pcg@aber.ac.uk (Piercarlo Grandi) Subject: Re: why is altering MINFREE discouraged ? In-Reply-To: richard@castle.ed.ac.uk's message of Tue, 19 Oct 1993 13: 57:41 GMT Message-ID: <PCG.93Oct19194620@decb.aber.ac.uk> Sender: news@aber.ac.uk (USENET news service) Nntp-Posting-Host: decb.aber.ac.uk Reply-To: pcg@aber.ac.uk (Piercarlo Grandi) Organization: Prifysgol Cymru, Aberystwyth References: <1993Oct15.212623.14509@aw101.iasl.ca.boeing.com> <PCG.93Oct18191520@decb.aber.ac.uk> <CF5DG9.6F@festival.ed.ac.uk> Date: Tue, 19 Oct 1993 18:46:20 GMT Lines: 26 >>> On Tue, 19 Oct 1993 13:57:41 GMT, richard@castle.ed.ac.uk (Richard >>> Tobin) said: Richard> In article <PCG.93Oct18191520@decb.aber.ac.uk> pcg@aber.ac.uk Richard> (Piercarlo Grandi) writes: pcg> Briefly: when looking for a cylinder group with free blocks to pcg> extend a file, the FFS chooses a cylinder group *at random*, for pcg> some rather good reasons. If the amount of free space is below a pcg> certain threshold, it must try several cylinder groups to find one pcg> with free blocks, and this is rather expensive. Richard> Just how expensive is it? Very expensive -- both in CPU time and reduced thruput. The FFS paper shows that it can halve the latter and substantially raise the former. Richard> It does seem a bit much to waste 200Mb on a 2Gb disk! As Ritchie&Thompson once said of the 5% overhead for metadata in the V7 filesystem, some other oses waste much more in trailing blanks alone. The FFS problem exists with more or less all FS organizations -- if finding a free block is not expensive, then the dispersion over the disks of files as the disk fills up becomes a major problem; when there are few blocks around, chances are they are not neatly clustered.