Re: ceph on btrfs [was Re: ceph on non-btrfs file systems]

[Chris Mason <chris.mason@oracle.com>]

On Tue, Oct 25, 2011 at 04:22:48PM -0400, Josef Bacik wrote:
> On Tue, Oct 25, 2011 at 04:15:45PM -0400, Chris Mason wrote:
> > On Tue, Oct 25, 2011 at 11:05:12AM -0400, Josef Bacik wrote:
> > > On Tue, Oct 25, 2011 at 04:25:02PM +0200, Christian Brunner wrote:
> > > > 
> > > > Attached is a perf-report. I have included the whole report, so that
> > > > you can see the difference between the good and the bad
> > > > btrfs-endio-wri.
> > > >
> > > 
> > > We also shouldn't be running run_ordered_operations, man this is screwed up,
> > > thanks so much for this, I should be able to nail this down pretty easily.
> > > Thanks,
> > 
> > Looks like we're getting there from reserve_metadata_bytes when we join
> > the transaction?
> >
> 
> We don't do reservations in the endio stuff, we assume you've reserved all the
> space you need in delalloc, plus we would have seen reserve_metadata_bytes in
> the trace.  Though it does look like perf is lying to us in at least one case
> sicne btrfs_alloc_logged_file_extent is only called from log replay and not
> during normal runtime, so it definitely shouldn't be showing up.  Thanks,

Whoops, I should have read that num_items > 0 check harder.

btrfs_end_transaction is doing it by setting ->blocked = 1

        if (lock && !atomic_read(&root->fs_info->open_ioctl_trans) &&
            should_end_transaction(trans, root)) {
                trans->transaction->blocked = 1;
		^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                smp_wmb();
        }

       if (lock && cur_trans->blocked && !cur_trans->in_commit) {
                   ^^^^^^^^^^^^^^^^^^^
                if (throttle) {
                        /*
                         * We may race with somebody else here so end up having
                         * to call end_transaction on ourselves again, so inc
                         * our use_count.
                         */
                        trans->use_count++;
                        return btrfs_commit_transaction(trans, root);
                } else {
                        wake_up_process(info->transaction_kthread);
                }
        }

perf is definitely lying a little bit about the trace ;)

-chris