The production machine hasn't had a lockup since moving to 3.15.7-031507-generic (it's been up for 4 days) even though we could reproduce the lockup on a new machine with that kernel using a snapshot of the old volume.

Another twist is that on the productino machine I'm now reliably seeing "No space left on device", even though there appears to be in principle 63GB remaining:

$ btrfs fi df /path/to/volume
Data, single: total=489.97GiB, used=427.75GiB
System, DUP: total=8.00MiB, used=60.00KiB
System, single: total=4.00MiB, used=0.00
Metadata, DUP: total=5.00GiB, used=4.50GiB
Metadata, single: total=8.00MiB, used=0.00
unknown, single: total=512.00MiB, used=0.00

$ sudo btrfs fi show /path/to/volume
Label: none  uuid: 3ffd71ab-6c3d-4486-a6b0-5c1eeb9be6b3
	Total devices 1 FS bytes used 432.25GiB
	devid    1 size 500.00GiB used 500.00GiB path /dev/dm-0

The ENOSPC is happening for mkdir and rename syscalls in particular.

I've posted a mail to the BTRFS list about this: http://thread.gmane.org/gmane.comp.file-systems.btrfs/37415

I did a rebalance with `btrfs balance start -dusage=10` (increasing 10) to try and gain more space for metadata, but this didn't fix the situation. I did however get this stack trace in dmesg.

In the end, I had to enlarge the volume before it became usable again.


[375794.106653] ------------[ cut here ]------------
[375794.106676] WARNING: CPU: 1 PID: 24706 at /home/apw/COD/linux/fs/btrfs/extent-tree.c:6946 use_block_rsv+0xfd/0x1a0 [btrfs]()
[375794.106678] BTRFS: block rsv returned -28
[375794.106679] Modules linked in: softdog tcp_diag inet_diag dm_crypt ppdev xen_fbfront fb_sys_fops syscopyarea sysfillrect sysimgblt i2c_piix4 serio_raw parport_pc parport mac_hid isofs xt_tcpudp iptable_filter xt_owner ip_tables x_tables btrfs xor raid6_pq crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 lrw gf128mul glue_helper ablk_helper cryptd floppy psmouse
[375794.106702] CPU: 1 PID: 24706 Comm: twsearch.py Not tainted 3.15.7-031507-generic #201407281235
[375794.106703] Hardware name: Xen HVM domU, BIOS 4.2.amazon 05/23/2014
[375794.106705]  0000000000001b22 ffff88016db437c8 ffffffff8176f115 0000000000000007
[375794.106707]  ffff88016db43818 ffff88016db43808 ffffffff8106ceac ffff8801e4890000
[375794.106709]  ffff8800a71ab9c0 ffff8801aedcd800 0000000000001000 ffff88001c987000
[375794.106711] Call Trace:
[375794.106718]  [<ffffffff8176f115>] dump_stack+0x46/0x58
[375794.106721]  [<ffffffff8106ceac>] warn_slowpath_common+0x8c/0xc0
[375794.106723]  [<ffffffff8106cf96>] warn_slowpath_fmt+0x46/0x50
[375794.106731]  [<ffffffffa00d9d1d>] use_block_rsv+0xfd/0x1a0 [btrfs]
[375794.106739]  [<ffffffffa00de687>] btrfs_alloc_free_block+0x57/0x220 [btrfs]
[375794.106746]  [<ffffffffa00c8a3c>] btrfs_copy_root+0xfc/0x2b0 [btrfs]
[375794.106757]  [<ffffffffa013a583>] ? create_reloc_root+0x33/0x2c0 [btrfs]
[375794.106767]  [<ffffffffa013a743>] create_reloc_root+0x1f3/0x2c0 [btrfs]
[375794.106776]  [<ffffffffa0140eb8>] btrfs_init_reloc_root+0xb8/0xd0 [btrfs]
[375794.106784]  [<ffffffffa00ee967>] record_root_in_trans.part.30+0x97/0x100 [btrfs]
[375794.106792]  [<ffffffffa00ee9f4>] record_root_in_trans+0x24/0x30 [btrfs]
[375794.106800]  [<ffffffffa00efeb1>] btrfs_record_root_in_trans+0x51/0x80 [btrfs]
[375794.106808]  [<ffffffffa00f13d6>] start_transaction.part.35+0x86/0x560 [btrfs]
[375794.106815]  [<ffffffffa00d1ee0>] ? btrfs_reduce_alloc_profile.isra.48+0x80/0x160 [btrfs]
[375794.106818]  [<ffffffff8109be78>] ? finish_task_switch+0x128/0x180
[375794.106826]  [<ffffffffa00f18d9>] start_transaction+0x29/0x30 [btrfs]
[375794.106834]  [<ffffffffa00f19a7>] btrfs_join_transaction+0x17/0x20 [btrfs]
[375794.106841]  [<ffffffffa00d9764>] flush_space+0xf4/0x160 [btrfs]
[375794.106848]  [<ffffffffa00d998a>] reserve_metadata_bytes+0x1ba/0x450 [btrfs]
[375794.106851]  [<ffffffff811dd073>] ? generic_permission+0xf3/0x120
[375794.106854]  [<ffffffff812f010c>] ? security_inode_permission+0x1c/0x30
[375794.106857]  [<ffffffff810b5450>] ? __wake_up_sync+0x20/0x20
[375794.106864]  [<ffffffffa00daf3a>] btrfs_delalloc_reserve_metadata+0x16a/0x4a0 [btrfs]
[375794.106873]  [<ffffffffa0102b3d>] __btrfs_buffered_write+0x15d/0x5c0 [btrfs]
[375794.106877]  [<ffffffff8118bd9c>] ? handle_pte_fault+0x18c/0x1b0
[375794.106886]  [<ffffffffa010319f>] btrfs_file_aio_write+0x1ff/0x3b0 [btrfs]
[375794.106889]  [<ffffffff811d268a>] do_sync_write+0x5a/0x90
[375794.106892]  [<ffffffff811d32db>] vfs_write+0xcb/0x1f0
[375794.106894]  [<ffffffff811d37df>] SyS_write+0x4f/0xb0
[375794.106897]  [<ffffffff817858bf>] tracesys+0xe1/0xe6
[375794.106898] ---[ end trace 1853311c87a5cd93 ]---