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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--
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