On 12/02/2015 11:09 PM, Dave Chinner wrote:
On Wed, Dec 02, 2015 at 01:07:56PM +0200, Avi Kivity wrote:
Removing a directory with ~900 32MB files, we saw this:
[ 5645.684464] INFO: task xfsaild/md0:12247 blocked for more than
120 seconds.
[ 5645.686488] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[ 5645.687713] xfsaild/md0 D ffff88103f9d3680 0 12247 2
0x00000080
[ 5645.687729] ffff8810136f7d40 0000000000000046 ffff882026d82220
ffff8810136f7fd8
[ 5645.687732] ffff8810136f7fd8 ffff8810136f7fd8 ffff882026d82220
ffff882026d82220
[ 5645.687734] ffff88103f9d44c0 0000000000000001 0000000000000000
ffff8820285aa928
[ 5645.687737] Call Trace:
[ 5645.687747] [<ffffffff816098d9>] schedule+0x29/0x70
[ 5645.687768] [<ffffffffa06cd880>] _xfs_log_force+0x230/0x290 [xfs]
[ 5645.687773] [<ffffffff810a9510>] ? wake_up_state+0x20/0x20
[ 5645.687796] [<ffffffffa06cd906>] xfs_log_force+0x26/0x80 [xfs]
[ 5645.687808] [<ffffffffa06d2380>] ?
xfs_trans_ail_cursor_first+0x90/0x90 [xfs]
[ 5645.687818] [<ffffffffa06d24d1>] xfsaild+0x151/0x5e0 [xfs]
[ 5645.687828] [<ffffffffa06d2380>] ?
xfs_trans_ail_cursor_first+0x90/0x90 [xfs]
[ 5645.687831] [<ffffffff8109727f>] kthread+0xcf/0xe0
[ 5645.687834] [<ffffffff810971b0>] ? kthread_create_on_node+0x140/0x140
[ 5645.687837] [<ffffffff81614358>] ret_from_fork+0x58/0x90
[ 5645.687852] [<ffffffff810971b0>] ? kthread_create_on_node+0x140/0x140
'rm' did not complete, but was killable. Nothing else was running
on the system at the time.
Which means the filesystem was not hung, nor was rm blocked in XFS.
That implies the directory/inode reads that rm does were running
really slowly. Something else going on here.
The filesystem was mounted with the discard option set, but since
that is discouraged, we'll retry without it.
Ah, yes, that could cause exactly these symptoms.
I'd guess you are using storage that has unqueued TRIM operations
(i.e. SATA 3.0 hardware somewhere in your storage path, as queued
TRIM only came along with SATA 3.1 and AFAIA there's not a lot of
3.1 hardware out there yet) which means while discards are
being issued all other IO tanks and goes really slow.
It's bare-metal cloud hardware so I don't immediately know (I don't
control the machine). I could find out -
We have seen individual TRIM requests on some SSDs take tens of
milliseconds to complete, regardless of their size. Hence if you
have one of these devices and you're running thousands of TRIM
commands across ~30GB of data being freed, then you'd see things
like rm being really slow on the read side and log forces waiting an
awful long time for journal IO completion processing to take
place...
- but it's probably better to just drop discard and see if it happens again.
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