Subject: Re: soft updates Re: Summer of code ideas
To: None <>
From: Greg A. Woods <>
List: netbsd-users
Date: 04/07/2007 08:47:11
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At Fri, 06 Apr 2007 13:37:32 +0100, Dieter wrote:
Subject: soft updates Re: Summer of code ideas=20
> Greg A. Woods writes:
> > Then for the real test start over and do it all again, but this time
> > after "systat bufcache" shows that most of available RAM is used up
> > for metadata an file data, pull the power plugs.  Then try to clean
> > up the mess.
> What mess?  My understanding is that with soft updates, the only
> thing that can possibly happen is that disk space can be lost.
> The background fsck is for reclaiming this lost space.

Huh?  I think you missed the point.

If there are _tens_ of seconds worth of pure _write_ activity still in
_metadata_ buffers in core when the power plug is pulled then you can
lose megabytes of _metadata_ alone, never minde the potentially hundreds
of megabytes of file data from dirty file cache buffers too.

Try it if you don't believe it.

The very concept of delaying metadata writes guarantees less safety,
regardless of whether they are eventualy done in the correct order or

It would be nice if "systat bufcache" could show how much of the various
caches are dirty vs. just unmodified copies, but I think that would
require changes to the VM accounting machinery all the way through the
whole system.

						Greg A. Woods

H:+1 416 218-0098 W:+1 416 489-5852 x122 VE3TCP RoboHack <>
Planix, Inc. <>       Secrets of the Weird <>

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