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Re: Speeding up dpkg, a proposal

On Thursday 03,March,2011 02:45 PM, Marius Vollmer wrote:
> ext Chow Loong Jin <hyperair@ubuntu.com> writes:
>> Could we somehow avoid using sync()? sync() syncs all mounted filesystems, which
>> isn't exactly very friendly when you have a few slow-syncing filesystems like
>> btrfs (or even NFS) mounted.
> Hmm, right.  We could keep a list of all files that need fsyncing, and
> then fsync them all just before writing the checkpoint.

I remember seeing there being some list of files to be fsynced in one of the
older dpkgs. It's probably that which led to the ext4 slowdown -- you'd get the
same effect of one sync() per file on systems with an ext4 root. If you had a
process doing heavy I/O in the background, each of those fsync()s will take a
considerable amount of time.

> Half of that is already done (for the content of the packages), we would
> need to add it for the files in /var/lib/dpkg/, or we could just fsync
> the whole directory.
> But then again, I would argue that the sync() is actually necessary
> always, for correct semantics: You also want to sync everything that the
> postinst script has done before recording that a package is fully
> installed.

Yes, you're right. I completely forgot about that. I don't think most postinst
scripts sync when done. I suppose the best that can be done is to batch the
stuff as best as can be done to reduce the number of sync()s needed.

Kind regards,
Loong Jin

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