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Path: euryale.cc.adfa.oz.au!newshost.anu.edu.au!harbinger.cc.monash.edu.au!news.mel.connect.com.au!munnari.OZ.AU!metro!metro!inferno.mpx.com.au!news.mel.aone.net.au!imci4!newsfeed.internetmci.com!howland.reston.ans.net!math.ohio-state.edu!magnus.acs.ohio-state.edu!csn!ub!newsstand.cit.cornell.edu!news.tc.cornell.edu!newsserver.sdsc.edu!news.cerf.net!proton.llumc.edu!galaxy.ucr.edu!corsa!grif From: grif@corsa.ucr.edu (Michael Griffith) Newsgroups: comp.unix.bsd.freebsd.misc,comp.os.linux.development.system Subject: Re: The better (more suitable)Unix?? FreeBSD or Linux Date: 14 Feb 1996 01:30:03 GMT Organization: UC Riverside, Dept. of Computer Science Lines: 67 Message-ID: <4frdur$hq@galaxy.ucr.edu> References: <4er9hp$5ng@orb.direct.ca> <311C5EB4.2F1CF0FB@freebsd.org> <4fjodg$o8k@venger.snds.com> <4fo1tu$n31@news.jf.intel.com> NNTP-Posting-Host: cs.ucr.edu Xref: euryale.cc.adfa.oz.au comp.unix.bsd.freebsd.misc:13762 comp.os.linux.development.system:17306 In article <4fo1tu$n31@news.jf.intel.com>, Mike Haertel <haertel@ichips.intel.com> wrote: |In article <4fjodg$o8k@venger.snds.com>, |Michael Griffith <grif@hill.ucr.edu> wrote: |> Writing metadata synchronously and data asynchronously |> can put a filesystem in a state that has undetectable |> errors. | |For some reason, in this sync-vs-async metadata discussion, |someone always brings up the claim that synchronous metadata |updates can leave you in a situation where fsck reports no |errors, yet you can have bogus data. | |True. Ahh. |Then they claim that async metadata update is superior, |because it doesn't have this problem. | |FALSE! You are quite correct. If I was misleading in this regard, I apologize. The real intent of the discussion was to show that async was no worse than sync metadata. However, if you add ordered writes, you eliminate the problem. |It could perfectly well happen that your async metadata |update might purely by chance choose a block ordering |which would leave the file system structure consistent, |yet leave data blocks out of date. | |So this claim is bogus. | | |As far as I know, the only practical difference between |sync and async metadata update is that async results in |faster write performance, especially for small files. The performance implications are quite substantial AND sync metadata doesn't really gain you anything in terms of reliability (it may actually hurt a bit, because you are more likely to have unordered writes.) Given this, why bother with sync metadata? |There may be reliability differences, but I have yet to |see any adequate empirical results to convince me. My gut |feeling is that async requires a more sophisticated fsck, |and that even so the resulting fsck may be more easily |confused. However, this is just a gut feeling. It would |be really neat if someone would do the reliability study |that Jordon suggested... Having inconsistent filesystem structures really isn't the issue. A hard failure where you know you have to restore from backups because fsck can't figure things out is a lot better than silently corrupting data. The study may be worthwhile, but I am holding out for ordered writes. A comparison between properly ordered writes and the current situation would me much more interesting. Anybody have good ideas for the setup of the experiments? -- Michael A. Griffith (grif@cs.ucr.edu) | http://www.cs.ucr.edu/~grif/ Department of Computer Science | PGP public key available. University of California, Riverside | "My freedom of speech implies (909) 787-3803 | your freedom to be offended."