Date: Mon, 26 Nov 2001 23:57:36 -0500 From: Brian T.Schellenberger <bts@babbleon.org> To: Bob Willcox <bob@immure.com> Cc: "Patrick O'Reilly" <patrick@mip.co.za>, Nils Holland <nils@tisys.org>, FreeBSD Question List <freebsd-questions@FreeBSD.ORG> Subject: Re: Softupdates Message-ID: <01112623573601.00696@i8k.babbleon.org> In-Reply-To: <20011126172050.A63285@luke.immure.com> References: <NDBBIMKICMDGDMNOOCAIOENHDPAA.patrick@mip.co.za> <01112609195201.00903@i8k.babbleon.org> <20011126172050.A63285@luke.immure.com>
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On Monday 26 November 2001 18:20, Bob Willcox wrote:
> On Mon, Nov 26, 2001 at 09:19:52AM -0500, Brian T.Schellenberger wrote:
>
> [snip]
>
> > Also, you most definately should turn off write-caching if you turn on
> > softupdates. In fact, you should do this anyway: softupdates are really
> > rather safe, but write caching is quite dangerous, and doubly so with
> > softupdates enabled.
> >
> > To do this, set
> >
> > hw.ata.wc=0
> >
> > in your /boot/loader.conf (assuming IDE devices).
If you use softupdates *and* enable write caching then you will screw up your
disk with a simple sequence like
rm -r foo/*
shutdown -p now
because the softupdates process will just finish writing its updates before
the power is clobbered.
At least that happens with my hardward, so softupdates is not compatible with
write-cache-enabled.
And from what I can tell (no rigorous testing), softupdates w/o write cache
is just as fast as non-softupdates w/ write cache, but lots safer. I know
that I've crashed 8 times in the last 36 hours and I'm darn glad I don't
enable write caching! (Of course that's not the norm, but sometimes I manage
that sort of thing.)
>
> In my experience, the current crop of ATA disks (I mostly have IBM)
> write _much_ slower with write caching disabled (on the order of a
> magnitude or more). Consequently, I find disabling it most undesirable.
> Instead, I suggest that you invest in a UPS so that your chance of
> suddenly dropping power to the disk is acceptably low...this is what I
> have done. (I don't think your average system crash where power stays
> up should be a problem since the disk would then have time to flush its
> cache.)
>
> BTW, the performance on SCSI disks doesn't seem to be nearly as adversly
> affected by disabling write caching (perhaps due to their higher-level
> command interface, esp. command tag queuing).
>
> Any disk experts out there should correct me on this if I'm off base
> (but this has been my experience).
>
> [snip]
>
>
> Bob
--
Brian T. Schellenberger . . . . . . . bts@wnt.sas.com (work)
Brian, the man from Babble-On . . . . bts@babbleon.org (personal)
http://www.babbleon.org
-------> Free Dmitry Sklyarov! (let him go home) <-----------
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