From owner-freebsd-questions  Wed Jan 30 12:41:15 2002
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From: "Scott Gerhardt" <scott@gerhardt-it.com>
To: "Brian T.Schellenberger" <bts@babbleon.org>,
	"Espen Tagestad" <espen@modula.no>, <questions@FreeBSD.ORG>
Subject: RE: Softupdates
Date: Wed, 30 Jan 2002 14:43:26 -0600
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   >
   > Now it IS a bad idea (even though many people do it without
   > happening to get
   > burned) to run with write caching *and* softupdates both on.
   > In general,
   > it's a bad isea to run with write caching, period, but
   > combining it with
   > softupdates makes things a lot worse, but softupdates without
   > write caching
   > is safer than the other way 'round.  Of course, turning both
   > off is safest
   > and slowest, so pick your poison.  I use softupdates on ALL
   > file systems and
   > turn off write caching myself.
   >
   > (To turn off write-caching, put this in /boot/loader.conf:
   >
   > # write cache considered dangerous
   > hw.ata.wc=0
   > )
   >
   > The reason for the general advice to turn it off on / is
   > because it does
   > introduce a delay and / is traditionally rather small.  The
   > delay effectively
   > gives you less space in a file system since freed space may
   > not be freed yet,
   > but if you make / larger than usual and turn on softupdates
   > you'll get the
   > speed benefit of softupdates and yet won't risk running out
   > of disk space.
   >
   > (It seems an especial shame to turn off softupdates on
   > whatever file system
   > contains /tmp since the benefits are larger on a file system
   > with lots of
   > writes.)

The write caching issue is more of an IDE-ATA issue.
As far as I know write caching is not such an issue with the newer SCSI
drives that have Tagged Queueing Enabled (see dmesg from one of my systems
below).


Excerpt from dmesg:

da0 at ahc0 bus 0 target 0 lun 0
da0: <IBM-PSG DDYS-T18350M  M S9PC> Fixed Direct Access SCSI-3 device
da0: 160.000MB/s transfers (80.000MHz, offset 63, 16bit), Tagged Queueing
Enabled

I could be wrong, so correct me if I am.

Regards,

	- Scott


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