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Re: Pre-approval request for dpkg sync() changes for squeeze

On Sun, Nov 21, 2010 at 09:18:04AM +0100, Raphael Hertzog wrote:
> On Sun, 21 Nov 2010, Ben Hutchings wrote:
> > I'm coming to this late.  It sounds like dpkg has changed its behaviour
> > several times recently.  Please can you summarise dpkg's current and
> > proposed use of fsync() vs sync(), and the reasons for this.
> Jonathan made a good summary of the history. I should add that dpkg uses
> sync() instead of fsync() only on systems where we know that sync() is
> synchronous (i.e. Linux only).
> Now we want to stop using sync() because of the bad side-effects:
> - using on a tmpfs is slower because it syncs changes on unrelated
>   filesystems
> - there are those reports of dpkg blocked due to the sync
>   see http://bugs.debian.org/595927 http://bugs.debian.org/600075
> > Also do I understand correctly that fsync() is more expensive when ext4
> > delayed allocation is in use?
> Apparently, at least for dpkg's usage pattern. But the performance are so
> much slower that you have been asked whether it would make sense to change
> the defaults on ext4 to include "nodelalloc".

Something that might be worth trying is using fallocate, which /might/
mitigate the delayed allocation effects.


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