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Re: Measured: btrfs COW and sqlite exclusive locking

From: Peter Samuelson <peter_at_p12n.org>
Date: Tue, 10 Jul 2012 15:38:29 -0500

[Mattias Engdegård]
> The biggest surprise was that COW actually made the checkout slightly
> slower, even though no "true" file copies were made. I'm not sure how
> to explain this---perhaps everything is already in cache so the
> copies aren't very expensive, or the COW operations are somehow
> synchronising?

Whatever it is in the data path that makes it slower, it probably is
not a fair comparison. Even though we normally don't want to do our
own fdatasync(), it is fair to consider the additional I/O load that is
generated by Subversion operations. That the I/O might happen in the
background after an svn operation finishes does not make it "free" from
the user's standpoint.

And of course, this load ought to be significantly lower with COW. It
should be measurable if you run 'sync' from the shell in strategic
places during the test runs. Though even this may not give the whole
picture, if a test run involves creating and deleting a lot of repos
and wcs. (Deleting a repos or a wc before it ever has the chance to
hit the disk platters is typical of testsuites, but rather _atypical_
of real world usage.)

Received on 2012-07-10 22:39:09 CEST

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