Re: [Lsf-pc] [Topic] Bcache

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Mar 14, 2012 at 09:32:28AM -0400, Kent Overstreet wrote:
> I'm already registered to attend, but would it be too late in the
> process to give a talk? I'd like to give a short talk about bcache, what
> it does and where it's going (more than just caching).

[CCing dm-devel list]

I am curious if you considered writing a device mapper driver for this? If
yes, why that is not a good choice. It seems to be stacked device and device
mapper should be good at that. All the configuration through sysfs seems
little odd to me.

On a side note, I was playing with bcache a bit. I tried to register the
cache device and it crashes. (I guess I should post this on relevant mailing
list).

# echo /dev/sdc > /sys/fs/bcache/register

[ 6758.314093] Pid: 0, comm: kworker/0:1 Not tainted 3.1.0-bcache+ #2
Hewlett-Packard HP xw6600 Workstation/0A9Ch
[ 6758.314093] RIP: 0010:[<ffffffff8146625b>]  [<ffffffff8146625b>]
closure_put+0x5b/0xe0
[ 6758.314093] RSP: 0018:ffff88013fc83c60  EFLAGS: 00010246
[ 6758.314093] RAX: 6b6b6b6b6b6b6b6b RBX: ffff8801281204a0 RCX:
0000000000000000
[ 6758.314093] RDX: 0000000000000000 RSI: 00000000ffffffff RDI:
ffff88013906ec48
[ 6758.314093] RBP: ffff88013fc83c60 R08: 0000000000000000 R09:
0000000000000001
[ 6758.314093] R10: 0000000000000000 R11: 0000000000000000 R12:
0000000000000000
[ 6758.314093] R13: ffff880130b58560 R14: 0000000000080000 R15:
0000000000000000
[ 6758.314093] FS:  0000000000000000(0000) GS:ffff88013fc80000(0000)
knlGS:0000000000000000
[ 6758.314093] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 6758.314093] CR2: 00007f9becec7000 CR3: 0000000137fe0000 CR4:
00000000000006e0
[ 6758.314093] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[ 6758.314093] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7:
0000000000000400
[ 6758.314093] Process kworker/0:1 (pid: 0, threadinfo ffff88013a44a000,
task ffff88013a458000)
[ 6758.314093] Stack:
[ 6758.314093]  ffff88013fc83c80 ffffffff8145ee6d ffffffff00000000
ffff8801281204a0
[ 6758.314093]  ffff88013fc83c90 ffffffff81173a9d ffff88013fc83cc0
ffffffff812d15d3
[ 6758.314093]  ffff88013a44a000 0000000000000000 ffff8801281204a0
0000000000080000
[ 6758.314093] Call Trace:
[ 6758.314093]  <IRQ> 
[ 6758.314093]  [<ffffffff8145ee6d>] uuid_endio+0x3d/0x50
[ 6758.314093]  [<ffffffff81173a9d>] bio_endio+0x1d/0x40
[ 6758.314093]  [<ffffffff812d15d3>] req_bio_endio+0x83/0xc0
[ 6758.314093]  [<ffffffff812d4f71>] blk_update_request+0x101/0x5c0
[ 6758.314093]  [<ffffffff812d51a2>] ? blk_update_request+0x332/0x5c0
[ 6758.314093]  [<ffffffff812d5461>] blk_update_bidi_request+0x31/0x90
[ 6758.314093]  [<ffffffff812d54ec>] blk_end_bidi_request+0x2c/0x80
[ 6758.314093]  [<ffffffff812d5580>] blk_end_request+0x10/0x20
[ 6758.314093]  [<ffffffff81471b7c>] scsi_io_completion+0x9c/0x5f0
[ 6758.314093]  [<ffffffff81468940>] scsi_finish_command+0xb0/0xe0
[ 6758.314093]  [<ffffffff81471965>] scsi_softirq_done+0xa5/0x140
[ 6758.314093]  [<ffffffff812db70b>] blk_done_softirq+0x7b/0x90
[ 6758.314093]  [<ffffffff8104fc65>] __do_softirq+0xc5/0x3a0
[ 6758.314093]  [<ffffffff817f6dac>] call_softirq+0x1c/0x30
[ 6758.314093]  [<ffffffff8100419d>] do_softirq+0x8d/0xc0
[ 6758.314093]  [<ffffffff8105027e>] irq_exit+0xae/0xe0
[ 6758.314093]  [<ffffffff817f74b3>] do_IRQ+0x63/0xe0
[ 6758.314093]  [<ffffffff817ecc30>] common_interrupt+0x70/0x70
[ 6758.314093]  <EOI> 
[ 6758.314093]  [<ffffffff8100a1f6>] ? mwait_idle+0xb6/0x470
[ 6758.314093]  [<ffffffff8100a1ed>] ? mwait_idle+0xad/0x470
[ 6758.314093]  [<ffffffff810011df>] cpu_idle+0x8f/0xd0
[ 6758.314093]  [<ffffffff817da107>] start_secondary+0x1be/0x1c2
[ 6758.314093] Code: 00 48 8b 50 48 83 e2 08 0f 85 9c 00 00 00 48 8b 50 48
83 e2 10 0f 85 8d 00 00 00 48 83 78 18 00 75 46 48 8b 40 40 48 85 c0 74 24 
[ 6758.314093]  8b 50 48 48 c1 ea 04 89 d1 89 f2 83 e1 01 f0 0f c1 50 4c
83 
[ 6758.314093] RIP  [<ffffffff8146625b>] closure_put+0x5b/0xe0
[ 6758.314093]  RSP <ffff88013fc83c60>

Thanks
Vivek