ufs+softupdates / consistency

[Arne Wörner]

--- David Schultz <das at FreeBSD.ORG> wrote:
> On Wed, Jan 26, 2005, Arne WXrner wrote:
> > On
> >   http://e2fsprogs.sourceforge.net/ext2intro.html
> > I found the strings
> >   "BSD-like synchronous updates"
> >   "it can cause corruption in the user data" .
> > On
> >   http://www.mckusick.com/softdep/
> > I did not find such a statement.
> > Are soft updates safe for user data? I do not really
> > understand, what the first www page means... Maybe they mean,
> > that the new file size (that would be meta data, I think) is
> > written before the user data, so that the file contains
> > undetermined data in its tail.
> 
> The comments you refer to that seem to imply that synchronous
> updates are unsafe and asynchronous updates are safer are wrong
> in general (synchronous updates are safer), but the authors may
> be referring to bugs in the ext2fs implementation at that time.
> Soft Updates, in contrast, provides asynchronous updates, issued
> in an order that makes them safe.
> 
I would be glad, if somebody explains me, why ext2fs/async in
Linux kernel 2.4.27 (KNOPPIX V3.7) is much faster (about 4 times
faster) than a ufs with soft updates on the same slice of the hard
disc?

Is it due to consistency reasons? In case of a ext2fs/sync in my
Linux setting Linux was about 4 times slower.

Are we already trying to issue write order requests for the disc
blocks (whose write order is arbitrary) sorted by sector number
(in order to move the disc heads as less as possible)? The disc
write cache could do that, but I disabled it in order to decrease
the probability of inconsistency.

-Arne



		
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