Re: btrfs mount takes too long time

[Anand Jain <anand.jain@oracle.com>]

On 02/09/2021 00:11, Dāvis Mosāns wrote:
> pirmd., 2021. g. 30. aug., plkst. 16:08 — lietotājs Anand Jain
> (<anand.jain@oracle.com>) rakstīja:
>>
>> open_ctree() took 228254398 us. And 98% of it that is 225418272 us
>> was taken by btrfs_read_block_groups().
>>
>> -------------------
>>    1) $ 225418272 us | } /* btrfs_read_block_groups [btrfs] */
>>    1) * 16934.96 us | btrfs_check_rw_degradable [btrfs]();
>>    0) 0.967 us | btrfs_apply_pending_changes [btrfs]();
>>    0) 0.239 us | btrfs_read_qgroup_config [btrfs]();
>>    0) * 21017.34 us | btrfs_get_root_ref [btrfs]();
>>    0) + 15.717 us | btrfs_start_pre_rw_mount [btrfs]();
>>    0) 0.865 us | btrfs_discard_resume [btrfs]();
>>    0) $ 228254398 us | } /* open_ctree [btrfs] */
>> -------------------
>>
>> Now we need to run the same thing on btrfs_read_block_groups(),
>> could you please run.. [1] (no need of the time).
>>
>> [1]
>>     $ umount /btrfs;
>>     $./ftracegraph btrfs_read_block_groups 2 "*:mod:btrfs" "mount
>> /dev/vg/scratch0 /btrfs"
>>
>> Thanks, Anand
>>
>>
> 
> Hi,
> 
> I also have a btrfs filesystem that takes a while to mount.
> So I'm interested if this could be improved.
> 
> $ ./ftracegraph open_ctree 2 "*:mod:btrfs" "time mount /dev/md127 -o
> space_cache=v2,compress=zstd,acl,subvol=Data /mnt/Data/"

  It is better if we don't use the time prefix for the mount command
  here. The ftrace, traces time syscall as well, which is unessential.
  And we lose a lot of trace-buffer to it.

> kernel.ftrace_enabled = 1
> 
> real    1m33,638s
> user    0m0,000s
> sys     0m1,130s
> 
> Here's the trace output https://dāvis.lv/files/ftracegraph.out.gz
> 
> The filesystem is on top of RAID6 mdadm array which is from 9x 3TB HDDs.

  So here is a case of a non-zoned device.

  Again it is btrfs_read_block_groups() which is taking ~98% of the time.

    3) $ 91607669 us |    } /* btrfs_read_block_groups [btrfs] */
    3) # 9399.566 us |    btrfs_check_rw_degradable [btrfs]();
    3)   0.922 us    |    btrfs_apply_pending_changes [btrfs]();
    3) ! 186.540 us  |    btrfs_read_qgroup_config [btrfs]();
    3) * 26109.92 us |    btrfs_get_root_ref [btrfs]();
    3) + 23.965 us   |    btrfs_start_pre_rw_mount [btrfs]();
    3)   1.192 us    |    btrfs_discard_resume [btrfs]();
    3) $ 93501136 us |  } /* open_ctree [btrfs] */

  Could we pls get this?

  $ ./ftracegraph btrfs_read_block_groups 2 "*:mod:btrfs" "mount ..."

  Hopefully, there won't be a trace-buffer rollover here, as we saw in
  the other case so that we could account for all the time spent.

  Also, let's understand how many block groups are there.

  $ btrfs in dump-tree <dev> | grep BLOCK_GROUP_ITEM | wc -l


Thx, Anand

> Best regards,
> Dāvis
>