After running ceph on XFS for some time, I decided to try btrfs again.
Performance with the current "for-linux-min" branch and big metadata
is much better. The only problem (?) I'm still seeing is a warning
that seems to occur from time to time:

[87703.784552] ------------[ cut here ]------------
[87703.789759] WARNING: at fs/btrfs/inode.c:2103
btrfs_orphan_commit_root+0xf6/0x100 [btrfs]()
[87703.799070] Hardware name: ProLiant DL180 G6
[87703.804024] Modules linked in: btrfs zlib_deflate libcrc32c xfs
exportfs sunrpc bonding ipv6 sg serio_raw pcspkr iTCO_wdt
iTCO_vendor_support i7core_edac edac_core ixgbe dca mdio
iomemory_vsl(PO) hpsa squashfs [last unloaded: scsi_wait_scan]
[87703.828166] Pid: 929, comm: kworker/1:2 Tainted: P           O
3.3.2-1.fits.1.el6.x86_64 #1
[87703.837513] Call Trace:
[87703.840280]  [<ffffffff8104df6f>] warn_slowpath_common+0x7f/0xc0
[87703.847016]  [<ffffffff8104dfca>] warn_slowpath_null+0x1a/0x20
[87703.853533]  [<ffffffffa0355686>] btrfs_orphan_commit_root+0xf6/0x100 [btrfs]
[87703.861541]  [<ffffffffa0350a06>] commit_fs_roots+0xc6/0x1c0 [btrfs]
[87703.868674]  [<ffffffffa0351bcb>]
btrfs_commit_transaction+0x5db/0xa50 [btrfs]
[87703.876745]  [<ffffffff810127a3>] ? __switch_to+0x153/0x440
[87703.882966]  [<ffffffff81070a90>] ? wake_up_bit+0x40/0x40
[87703.888997]  [<ffffffffa0352040>] ?
btrfs_commit_transaction+0xa50/0xa50 [btrfs]
[87703.897271]  [<ffffffffa035205f>] do_async_commit+0x1f/0x30 [btrfs]
[87703.904262]  [<ffffffff81068949>] process_one_work+0x129/0x450
[87703.910777]  [<ffffffff8106b7eb>] worker_thread+0x17b/0x3c0
[87703.916991]  [<ffffffff8106b670>] ? manage_workers+0x220/0x220
[87703.923504]  [<ffffffff810703fe>] kthread+0x9e/0xb0
[87703.928952]  [<ffffffff8158c224>] kernel_thread_helper+0x4/0x10
[87703.935555]  [<ffffffff81070360>] ? kthread_freezable_should_stop+0x70/0x70
[87703.943323]  [<ffffffff8158c220>] ? gs_change+0x13/0x13
[87703.949149] ---[ end trace b8c31966cca731fa ]---
[91128.812399] ------------[ cut here ]------------
[91128.817576] WARNING: at fs/btrfs/inode.c:2103
btrfs_orphan_commit_root+0xf6/0x100 [btrfs]()
[91128.826930] Hardware name: ProLiant DL180 G6
[91128.831897] Modules linked in: btrfs zlib_deflate libcrc32c xfs
exportfs sunrpc bonding ipv6 sg serio_raw pcspkr iTCO_wdt
iTCO_vendor_support i7core_edac edac_core ixgbe dca mdio
iomemory_vsl(PO) hpsa squashfs [last unloaded: scsi_wait_scan]
[91128.856086] Pid: 6806, comm: btrfs-transacti Tainted: P        W  O
3.3.2-1.fits.1.el6.x86_64 #1
[91128.865912] Call Trace:
[91128.868670]  [<ffffffff8104df6f>] warn_slowpath_common+0x7f/0xc0
[91128.875379]  [<ffffffff8104dfca>] warn_slowpath_null+0x1a/0x20
[91128.881900]  [<ffffffffa0355686>] btrfs_orphan_commit_root+0xf6/0x100 [btrfs]
[91128.889894]  [<ffffffffa0350a06>] commit_fs_roots+0xc6/0x1c0 [btrfs]
[91128.897019]  [<ffffffffa03a2b61>] ?
btrfs_run_delayed_items+0xf1/0x160 [btrfs]
[91128.905075]  [<ffffffffa0351bcb>]
btrfs_commit_transaction+0x5db/0xa50 [btrfs]
[91128.913156]  [<ffffffffa03524b2>] ? start_transaction+0x92/0x310 [btrfs]
[91128.920643]  [<ffffffff81070a90>] ? wake_up_bit+0x40/0x40
[91128.926667]  [<ffffffffa034cfcb>] transaction_kthread+0x26b/0x2e0 [btrfs]
[91128.934254]  [<ffffffffa034cd60>] ?
btrfs_destroy_marked_extents.clone.0+0x1f0/0x1f0 [btrfs]
[91128.943671]  [<ffffffffa034cd60>] ?
btrfs_destroy_marked_extents.clone.0+0x1f0/0x1f0 [btrfs]
[91128.953079]  [<ffffffff810703fe>] kthread+0x9e/0xb0
[91128.958532]  [<ffffffff8158c224>] kernel_thread_helper+0x4/0x10
[91128.965133]  [<ffffffff81070360>] ? kthread_freezable_should_stop+0x70/0x70
[91128.972913]  [<ffffffff8158c220>] ? gs_change+0x13/0x13
[91128.978826] ---[ end trace b8c31966cca731fb ]---

I'm able to reproduce this with ceph on a single server with 4 disks
(4 filesystems/osds) and a small test program based on librbd. It is
simply writing random bytes on a rbd volume (see attachment).

Is this something I should care about? Any hint's on solving this
would be appreciated.

Thanks,
Christian