Date: Mon, 12 Mar 2001 15:10:23 -0800 (PST) From: Matt Dillon <dillon@earth.backplane.com> To: Alfred Perlstein <bright@wintelcom.net> Cc: Kevin Oberman <oberman@es.net>, Soren Schmidt <sos@freebsd.dk>, mobile@FreeBSD.ORG, stable@FreeBSD.ORG Subject: Re: Disk I/O problem in 4.3-BETA Message-ID: <200103122310.f2CNANH77246@earth.backplane.com> References: <200103122036.VAA99695@freebsd.dk> <200103122146.f2CLkLs08952@ptavv.es.net> <20010312140636.A18351@fw.wintelcom.net>
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: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. : :-- :-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org] :Daemon News Magazine in your snail-mail! http://magazine.daemonnews.org/ It's actually worse. Someone, I forget who, ran some tests with write-caching turned on and found that the IDE drive could hold a pending write in its cache 'forever', even in the face of other writes, as long as there was other disk activity going on. So we aren't just talking about issuing I/O's out of order, we are talking about issuing a sequence of writes and having some of them simply not ever commiting to disk (not for a long, long time) in a heavily loaded environment. That's bad news. -Matt To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-mobile" in the body of the message
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