Re: btrfs-cleaner 100% busy on an idle filesystem with 4.19.3

[Tomasz Chmielewski <tch@virtall.com>]

On 2018-11-22 21:46, Nikolay Borisov wrote:

>> # echo w > /proc/sysrq-trigger
>> 
>> # dmesg -c
>> [  931.585611] sysrq: SysRq : Show Blocked State
>> [  931.585715]   task                        PC stack   pid father
>> [  931.590168] btrfs-cleaner   D    0  1340      2 0x80000000
>> [  931.590175] Call Trace:
>> [  931.590190]  __schedule+0x29e/0x840
>> [  931.590195]  schedule+0x2c/0x80
>> [  931.590199]  schedule_timeout+0x258/0x360
>> [  931.590204]  io_schedule_timeout+0x1e/0x50
>> [  931.590208]  wait_for_completion_io+0xb7/0x140
>> [  931.590214]  ? wake_up_q+0x80/0x80
>> [  931.590219]  submit_bio_wait+0x61/0x90
>> [  931.590225]  blkdev_issue_discard+0x7a/0xd0
>> [  931.590266]  btrfs_issue_discard+0x123/0x160 [btrfs]
>> [  931.590299]  btrfs_discard_extent+0xd8/0x160 [btrfs]
>> [  931.590335]  btrfs_finish_extent_commit+0xe2/0x240 [btrfs]
>> [  931.590382]  btrfs_commit_transaction+0x573/0x840 [btrfs]
>> [  931.590415]  ? btrfs_block_rsv_check+0x25/0x70 [btrfs]
>> [  931.590456]  __btrfs_end_transaction+0x2be/0x2d0 [btrfs]
>> [  931.590493]  btrfs_end_transaction_throttle+0x13/0x20 [btrfs]
>> [  931.590530]  btrfs_drop_snapshot+0x489/0x800 [btrfs]
>> [  931.590567]  btrfs_clean_one_deleted_snapshot+0xbb/0xf0 [btrfs]
>> [  931.590607]  cleaner_kthread+0x136/0x160 [btrfs]
>> [  931.590612]  kthread+0x120/0x140
>> [  931.590646]  ? btree_submit_bio_start+0x20/0x20 [btrfs]
>> [  931.590658]  ? kthread_bind+0x40/0x40
>> [  931.590661]  ret_from_fork+0x22/0x40
>> 
> 
> It seems your filesystem is mounted with the DSICARD option meaning
> every delete will result in discard this is highly suboptimal for 
> ssd's.
> Try remounting the fs without the discard option see if it helps.
> Generally for discard you want to submit it in big batches (what fstrim
> does) so that the ftl on the ssd could apply any optimisations it might
> have up its sleeve.

Spot on!

Removed "discard" from fstab and added "ssd", rebooted - no more 
btrfs-cleaner running.

Do you know if the issue you described ("discard this is highly 
suboptimal for ssd") affects other filesystems as well to a similar 
extent? I.e. if using ext4 on ssd?


> Would you finally care to share the smart data + the model and make of
> the ssd?

2x these:

Model Family:     Samsung based SSDs
Device Model:     SAMSUNG MZ7LM1T9HCJM-00005
Firmware Version: GXT1103Q
User Capacity:    1,920,383,410,176 bytes [1.92 TB]
Sector Size:      512 bytes logical/physical
Rotation Rate:    Solid State Device

1x this:

Device Model:     Micron_5200_MTFDDAK1T9TDC
Firmware Version: D1MU004
User Capacity:    1,920,383,410,176 bytes [1.92 TB]
Sector Sizes:     512 bytes logical, 4096 bytes physical
Rotation Rate:    Solid State Device
Form Factor:      2.5 inches


But - seems the issue was unneeded discard option, so not pasting 
unnecessary SMART data, thanks for finding this out.


Tomasz Chmielewski