Date: Thu, 21 Sep 2000 06:09:50 -0700 From: Stephen Byan <Stephen.Byan@quantum.com> To: "'Soren Schmidt'" <sos@freebsd.dk>, Stephen Byan <Stephen.Byan@quantum.com> Cc: fs@FreeBSD.ORG, sos@FreeBSD.ORG, freeBSD-scsi@FreeBSD.ORG Subject: RE: disable write caching with softupdates? Message-ID: <8133266FE373D11190CD00805FA768BF055BD1D3@shrcmsg1.tdh.qntm.com>
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Soren Schmidt [mailto:sos@freebsd.dk] wrote: > > Wouldn't it be acceptable to mark the meta-data writes as > non-cacheable > > (i.e. write though to the media before signalling > completion), and let the > > remaining writes (user data writes) be cacheable? I think > this would improve > > the performance of the file system. > > > > SCSI has supported this for years, in the form of the FUA > bit in the CDB for > > the write command. Somewhat similar behavior can be had in > the newer flavors > > of ATA by issuing a "flush cache" command after each > meta-data write, and > > waiting until the flush command completes before signalling > the completion > > of the non-cacheable write. > > OK, I played a bit with that, the only info I can see I get from the > higher levels is the BIO_ORDERED bit, so I tried to flush the cache > each time I get one of those, _bad_ idea, 10% performance loss... That's the price of having a recoverable file system. See Seltzer, Ganger, McKusick, Smith, Soules, and Stein, "Journaling Versus Soft Updates: Asynchronous Meta-data Protection in File Systems", 2000 USENIX Annual Technical Conference, June 2000, San Diego. Contrast this 10% performance hit versus what you get when you disable caching entirely. Regards, -Steve Steve Byan <stephen.byan@quantum.com> Design Engineer MS 1-3/E23 333 South Street Shrewsbury, MA 01545 (508)770-3414 fax: (508)770-2604 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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