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

On Fri, 2010-11-05 at 23:18:47 +0100, Philipp Kern wrote:
> On Fri, Oct 22, 2010 at 11:35:54AM +0200, Guillem Jover wrote:
> > Those are related to the fsync()/sync() changes in dpkg from some time
> > ago, the patches would:
> > 
> >   1) Switch back from sync() to fsync() before rename() (while keeping
> >      the sync() code around for the benefit of other distributions
> >      that might not want to switch just yet). So to avoid unrelated
> >      I/O when there's background work being done for example. This
> >      hack also only works on Linux where sync() is synchronous, so
> >      it would unify that code path for all dpkg supported platforms.
> > 
> >      Bug: #588339
> > 
> >   <http://git.hadrons.org/?p=debian/dpkg/dpkg.git;a=commitdiff;h=87740373>
> I do have to say that I'm not overly happy about this change this late in the
> freeze.

My idea was to get those on the planned 1.15.9, but after the sudden
freeze and the initial reactions to the first unblock request I didn't
feel like asking for any more changes. Anyway, the blame would be
obviously on me in case these do not get accepted due to the delay.

> I suppose this resembles what we currently have in Lenny?

No, lenny's dpkg does not do sync() nor fsync() before renames on
extracted files from archives.

dpkg 1.15.6 started doing fsync() for extracted files.
dpkg switched from fsync() to sync() on Linux.

> What are the implications for ext4 and btrfs?

They are going to be slower with this change.

> Why was it changed in the first place?

I wrote about this in <http://lwn.net/Articles/392599/>. Extremely short
summary: because fsync() was considered too slow on ext4 (#578635).

> >   2) Add a new --force-unsafe-io to disable those fsync() for people
> >      using certain file systems where the only option is to choose
> >      between reliability or acceptable performance. Or in cases where
> >      the data is cached and it might not be important to lose it.
> > 
> >      Bug: #584254
> > 
> >   <http://git.hadrons.org/?p=debian/dpkg/dpkg.git;a=commitdiff;h=0484cd7d>
> So even when you don't do sync() anymore you still want to disable those
> fsyncs()?

Given the performance degradation the first patch will introduce on at
least ext4, users might want to disable fsync()s. Arguably they seem
to want to do that now even with sync(), as people are already using
hacks like eatmydata to disable them, the problem is that this will
disable *all* syncs, including the ones for the database among others.
If the file system gets corrupted you might still have a chance of
recovering, if the databases do, you most probably need either backups
or a reinstall.

> Granted, it's a self-contained new feature.  What's the use case
> for squeeze?  d-i I heared, but I'm not sure that they'd use it in this
> cycle.  And tmpfs users, or who?

Other users are buildds using fresh unpacked snapshots for each build
session. ext4 users who would prefer faster dpkg runs rather than data

The problem with sync() is that even if it gives better performance
than fsync()s on ext4, it's just the wrong thing to use, sync()
affects all mount points, so if you happen to have for example a server
with lots of writes on say a /srv mount point, and the admin decides to
do a large install/upgrade, it will force all that data down to all
mount points, for each archive unpacked.

The target for this is ext4 users mainly, the syncs were included mostly
to protect them from data loss. They are also the ones suffering the
I/O performance degradation. I don't think it's fair to make users not
suffering such problems suffer unrelated issues like the ones imposed by
sync(). As such I think it should be them who have to decide between
performance or data loss.

Also I found it interesting that the ext4 developers were complaining
about userspace developers not doing the right thing regarding atomic
renames (which would include fsync()) during the huge zero-length
discussions, to then state that fsync() is obviously too slow and heavy
handed and thus should better not be used.


Ultimately I just think this is a problem with ext4. Regardless of that,
I'll try to come up with a better, portable alternative to improve the
performance for 1.16.x if possible.

Finally, while accepting patch 2) alone might make sense, accepting 1)
alone would not.


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