Date: Wed, 24 Sep 1997 22:59:38 +0000 (GMT) From: Terry Lambert <tlambert@primenet.com> To: brandon@roguetrader.com (Brandon Gillespie) Cc: wilko@yedi.iaf.nl, tlambert@primenet.com, freebsd-fs@FreeBSD.ORG Subject: Re: Known problems with async ufs? Message-ID: <199709242259.PAA22462@usr03.primenet.com> In-Reply-To: <Pine.BSF.3.96.970924135516.6508A-100000@roguetrader.com> from "Brandon Gillespie" at Sep 24, 97 01:56:11 pm
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> hrm, true > > Ok... what (if any) plans are there to make async at least as ''stable'' > as ext2fs? I'm under the impression that ext2fs does something so it can > recover from "bad things" better.. I don't want to restart this argument with the Linux/EXT2FS camp. However, the answer is it's fsck is more willing to accept "any known good state" as opposed to "the known good state the Fs would have been had it shutdown normally instead of abnormally". IMO (and this *is* MO, and the EXT2FS proponents have other O's), this is unsatisfactory. They addressed this somewhat by guaranteeing that metadata writes would be ordered. It's unclear whether this infringes on USL's Delayed Ordered Writes patent. The end result of DOW is not the same as the end result of NVRAM or Soft Updates; what it means is that multiple file contents can be corrupted, even though the metadata is set (*not* really restored) to a known good state. For example, I had a slow IDE device that would frequently lose all of /usr (it was on the root partition) when I had a crash, because I was doing a lot of manipulation of the directory block that contained it's directory entry. Your mileage may vary; a particular MTBF (Mean Time Between Failures) does not guarantee that you will have a failure in that time. The upshot is that async *is* as ''stable;; as ext2fs when it is mounted async (the historical default). If you want to make ffs come back as transparently, then make the fsck run twice after a crash, and it will recover equivalently "well". It's my recommendation that unless you are running extraordinary measures, like a UPS that can gracefully shutdown your machine for you, then you probably want to mount your ext2fs sync as well. This will have the side effect of making it look as "graceless" as ffs when a recovery is needed. Regards, 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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