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Subject: Re: Disk I/O problem in 4.3-BETA
In-Reply-To: <20010312140636.A18351@fw.wintelcom.net> from Alfred Perlstein at "Mar 12, 2001  2: 6:36 pm"
To: bright@wintelcom.net (Alfred Perlstein)
Date: Tue, 13 Mar 2001 09:48:26 +0100 (MET)
Cc: oberman@es.net, sos@freebsd.dk, mobile@FreeBSD.ORG,
	stable@FreeBSD.ORG
From: Helge Oldach <Helge.Oldach@de.origin-it.com>
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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

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