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Date: Thu, 14 Jul 2005 21:52:53 +0200
From: Matthias Buelow <mkb@incubus.de>
To: Kevin Oberman <oberman@es.net>
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Cc: freebsd-stable@freebsd.org, freebsd-questions@freebsd.org
Subject: Re: dangerous situation with shutdown process
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Kevin Oberman wrote:

>> How can I fix it on my system?
>
>SCSI or ATA? If it's ATA, turn off write cache with (atacontrol(8) or
>the sysctl.

You do NOT want to do that. Not only will performance drop brutally
(example: drop to 1/5th of normal write speed for sequential writes,
probably worse for random writes) but it will also significantly
reduce the lifetime of your disk. Modern disks are designed to be
used with the write-back cache enabled, so don't turn it off.

>The problem is that disks lie about whether they have actually written
>data. If the power goes off before the data is in cache, it's lost.

No, the problem is that FreeBSD doesn't implement request barriers
and that softupdates is flawed by design and seemingly could not
make use of them, even if they were available (because, as I
understand it, it relies on a total ordering of all writes, unlike
the partial ordering necessary for a journalled fs).

Until a journalled fs that uses write request barriers is available
for FreeBSD, you better had a reliable UPS.

mkb.