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Date:      Wed, 30 Jan 2002 20:15:42 -0500
From:      Brian T.Schellenberger <bts@babbleon.org>
To:        "Scott Gerhardt" <scott@gerhardt-it.com>
Cc:        <questions@FreeBSD.ORG>
Subject:   Re: Softupdates
Message-ID:  <20020131011543.10D3A4078@i8k.babbleon.org>
In-Reply-To: <KPEMLBLEMPMHGLJOCDEGIEHIDLAA.scott@gerhardt-it.com>
References:  <KPEMLBLEMPMHGLJOCDEGIEHIDLAA.scott@gerhardt-it.com>

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On Wednesday 30 January 2002 03:43 pm, you wrote:
>    > 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.

No, you are exactly correct.  I think that the ideal would probably to be 
have write caching disabled for ata (IDE) and enabled for SCSI.

But I do wish that in marginal cases the defaults were more conservative 
myself.

>
> Regards,
>
> 	- Scott
>
>
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-- 
Brian T. Schellenberger . . . . . . .   bts@wnt.sas.com (work)
Brian, the man from Babble-On . . . .   bts@babbleon.org (personal)
                                ME -->  http://www.babbleon.org
http://www.eff.org   <-- GOOD GUYS -->  http://www.programming-freedom.org 

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