Re: btrfs mount takes too long time

On 2021/9/2 上午12:11, Dāvis Mosāns wrote:

pirmd., 2021. g. 30. aug., plkst. 16:08 — lietotājs Anand Jain
([off-list ref]) rakstīja:

quoted

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] */

As long as it's not zoned device, this is a known problem.

Block group items by its original design is scattered across the huge
extent tree.

Grabbing them all needs quite a lot of random IO, for large fs it's very
time consuming.

I have purposed a skinny bg tree design to put all block group items
into one tree, so that would greatly speed up the whole mount process.
And with user-space tool to do the convert.

But recently Josef is developing a completely new extent tree design,
which will include a similar design for it.

For know I'm not sure what's the way to go, either we can wait for
extent-tree-v2, or should we push for the middle ground skinny bg tree?

David, Any idea on this?

Thanks,
Qu

quoted

   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/"
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.

Best regards,
Dāvis