Date: Tue, 13 Mar 2001 10:07:03 +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: <200103130907.KAA08943@galaxy.de.cp.philips.com> In-Reply-To: <20010313005811.J29888@fw.wintelcom.net> from Alfred Perlstein at "Mar 13, 2001 0:58:11 am"
next in thread | previous in thread | raw e-mail | index | archive | help
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. 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. >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? :-) Helge To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-stable" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200103130907.KAA08943>