Date: Tue, 13 Mar 2001 01:29:19 -0800 From: Alfred Perlstein <bright@wintelcom.net> To: Helge Oldach <Helge.Oldach@de.origin-it.com> 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: <20010313012919.K29888@fw.wintelcom.net> In-Reply-To: <200103130907.KAA08943@galaxy.de.cp.philips.com>; from Helge.Oldach@de.origin-it.com on Tue, Mar 13, 2001 at 10:07:03AM %2B0100 References: <20010313005811.J29888@fw.wintelcom.net> <200103130907.KAA08943@galaxy.de.cp.philips.com>
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* Helge Oldach <Helge.Oldach@de.origin-it.com> [010313 01:07] wrote: > Alfred Perlstein: > >* Helge Oldach <Helge.Oldach@de.origin-it.com> [010313 00:48] wrote: > >> Alfred Perlstein: > >> >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? :-) > > > >I'm pretty sure you're wrong. > > I think you misunderstood my argument. Agreed, there is practically > no difference in the damage done to softupdates versus write-cacheing > during a power outage. huh? I'm confused about "softupdates versus write-cacheing". > But there should be a difference when the OS dies away while power stays > up. The OS dying away means that the disk has lots of time to spill out > the cached data to the physical medium as it's no longer banged at high > data rates by the host. So at least in theory we should be better off in > this situation. No we shouldn't! Either way we should have a consistant filesystem. > >I'm not 100% certain, but many people working with embedded systems > >have explained to me that it's no longer safe to assume that write > >cached data will be sync'd to the disk's media at crash time. > > That may be correct. But then this breaks my naive understanding of > "write caching"... > > And again: Isn't write-cacheing turned on on SCSI disks? :-) It's suggested that it be turned off, see Justin's answers to my previous questions. -- -Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org] Daemon News Magazine in your snail-mail! http://magazine.daemonnews.org/ To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-mobile" in the body of the message
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