Date: Thu, 21 Sep 2000 11:31:51 -0600 From: "Justin T. Gibbs" <gibbs@plutotech.com> To: Mike Smith <msmith@FreeBSD.ORG> Cc: scsi@FreeBSD.ORG Subject: Re: disable write caching with softupdates? Message-ID: <200009211730.LAA42240@pluto.plutotech.com>
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>> > That's the price of having a recoverable file system. See Seltzer, Ganger, >> >> Not necessarily. > > Er, you're being both contrary and plain wrong. It's a fundamental > assumption of the softupdates implementation that it is possible to issue > an ordered write and have it complete in an ordered fashion. Softupdates guarantees write ordering by batching writes that have the same ordering dependency and postponing the writes that depend on queued writes until those writes complete. This is not the same as relying on the ability to queue writes in a particular order. The assumption that softupdates does make, however, is that a write that completes is committed to media. >> Actually, performance-wise, you'd probably want to know the real geometry, >> given all the stuff FFS does to exploit it. > > Since a) drives don't have 'real' geometries, and haven't for the last > half of the last decade at least, and b) we intentionally disable most of > these optimisations because they're founded on assumptions that stopped > being relevant a good five years before *that*... no, you don't care > about the drive's geometry at all. There was a talk (I don't think it was a full blown paper) at a USENIX a few years ago that showed that you could approximate the behavior of modern disks using a liniar model. The speaker also showed some simulated performance improvements by using the model. -- Justin To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-scsi" in the body of the message
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