Re: [BULK] Re: [PATCH] xfstests 311: test fsync with dm flakey V2

[Dave Chinner <david@fromorbit.com>]

On Thu, Apr 25, 2013 at 08:24:04PM -0400, Josef Bacik wrote:
> On Thu, Apr 25, 2013 at 04:45:56PM -0600, Dave Chinner wrote:
> > On Thu, Apr 25, 2013 at 10:12:56AM -0400, Josef Bacik wrote:
.....
> > > +	$here/src/fsync-tester -s $SEED -r -t $test_num $extra $testfile
> > > +	if [ $? -ne 0 ]; then
> > > +		_unmount_flakey
> > > +		_cleanup
> > > +		exit
> > > +	fi
> > > +
> > > +	_md5_checksum $testfile
> > > +	_drop_writes
> > > +	_unmount_flakey
> > 
> > So, _drop_writes suspends the dm-flakey device, freezes the
> > filesystem, turns off writes then thaws the filesystem, right?
> >
> > If so, doesn't that mean you're not actually testing fsync() as the
> > freeze will effectively sync the entire filesystem before you start
> > dropping writes?
> > 
> > I can see why you want to stop unmount from writing back metadata to
> > simulate a crash, but if you've already frozen the filesystem then
> > writeback has already occurred before you stop the writes. So I
> > can't see how this is actually testing fsync - what it appears to be
> > testing is the fileystem freeze code...
> > 
> > [ This is precisely the issue that XFS shutdown ioctls deal with to
> > trigger an immediate forced shutdown of the filesystem that prevents
> > *any* further writes from being issued by the filesystem - no sync
> > operations get in the way and change the state of the filesystem
> > after then fsync call, so we know that what is on disk is what was
> > written by the sync/fsync calls being tested.
> > 
> > This is how we test sync/fsync in other XFS tests (e.g.
> > xfs/137-140), and this is the reason why us XFS people have
> > suggested that other filesystems should implement the ioctls for
> > this functionality rather than try to invent new ways of trying
> > to stop filesystems from writing back dirty metadata for fsync/sync
> > testing....
> > 
> > Besides, if a corruption is detected, you need a method of stopping
> > all dirty metadata from being written back in the filesystem to
> > prevent propagation of the corruption.  These ioctls should just be
> > an interface into that mechanism. ]
> >
> 
> So I need to look at what this does.  I don't think it freezes the file system,

`dmsetup suspend` ends up in dm_suspend(). This calls lock_fs(), which
calls freeze_bdev()....

If you do `dmsetup suspend --nolockfs` then it won't freeze the
filesystem during the suspend...

> because I've run this test and I definitely have a tree log on the file system
> after I unmount, which means we didn't actually sync().  Either that or freeze()
> is broken for btrfs and it's not actually causing the transaction to commit.

Entirely possible.

/me muses randomly about why we call them bugs when they are almost
always layered like onions.....

> If
> it turns out that it is freeze() then it's not actually a fsync tester, but it
> seems to still be a valid test since it's definitely causing problems for all
> the file systems I've tested and I'll leave this as it is and then write another
> real fsync tester.

ext4 as well?

> > How long does this take to run? It seems like the quick group would
> > be appropriate if it takes less than a minute. Also, fsync tests
> > fall under the category of "metadata" and "log", so they probably
> > should be added, too.
> > 
> 
> It takes 66 seconds to run on btrfs, is that fast enough for quick?  I'll add
> metadata and log to the list as well.  Thanks for the thorough review,

Probably a bit too long for my liking - it won't be quick when there
are several instances of VMs running xfstests on the same set of
spindles....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com