Date: Tue, 13 Mar 2001 09:48:26 +0100 (MET) From: Helge Oldach <Helge.Oldach@de.origin-it.com> To: bright@wintelcom.net (Alfred Perlstein) Cc: oberman@es.net, sos@freebsd.dk, mobile@FreeBSD.ORG, stable@FreeBSD.ORG Subject: Re: Disk I/O problem in 4.3-BETA Message-ID: <200103130848.JAA08707@galaxy.de.cp.philips.com> In-Reply-To: <20010312140636.A18351@fw.wintelcom.net> from Alfred Perlstein at "Mar 12, 2001 2: 6:36 pm"
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Alfred Perlstein: >* Kevin Oberman <oberman@es.net> [010312 13:46] wrote: >> How serious is the possible corruption issue, anyway. The loss in >> performance is pretty drastic although it may be that dd is an >> especially bad case, but I really don't like to corrupt my disks, >> either. > >If basically running with blind write caching turned on is akin to >running your filesystem in async mode. This is because write >caching gives the drive license to lie about completing a write, >the various ordering of writes are effectively bypassed. If you >crash without these dependancies actually written to the disk, when >you come back up you have a good chance of losing large portions >of your filesystem. I'd say this is a bit too pessimistic. There is a fundamental difference between softupdates and ATA write-cacheing: Softupdates holds the async data in main RAM while ATA write-cacheing already has it in the (cache memory of the) disk device. Obviously a power outage would affect both situations in a similar way. But during just an operating system crash (assuming power stays up), one should be better off with ATA write-cacheing, as the disk should be able to dump the data from the cache chips to the physical medium. With softupdates async data is just lost. Generally I'd say it's not a bad idea to have write caching on the disk enabled - assuming that it is decently implemented. BTW, don't SCSI disks use write cacheing as well? :-) Just my 2¢, Helge To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-mobile" in the body of the message
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