How can btrfs take 23sec to stat 23K files from an SSD?

[Marc MERLIN <marc@merlins.org>]

I just realized that the older thread got a bit confusing, so I'll keep
problems separate and make things simpler :)

On an _unencrypted_ partition on the SSD, running du -sh on a directory
with 15K files, takes 23 seconds on unencrypted SSD and 4 secs on
encrypted spinning drive, both with a similar btrfs filesystem, and 
the same kernel (3.4.4).

Unencrypted btrfs on SSD:
gandalfthegreat:~# mount -o compress=lzo,discard,nossd,space_cache,noatime /dev/sda2 /mnt/mnt2
gandalfthegreat:/mnt/mnt2# echo 3 > /proc/sys/vm/drop_caches; time du -sh src
514M	src
real	0m22.667s

Encrypted btrfs on spinning drive of the same src directory:
gandalfthegreat:/var/local# echo 3 > /proc/sys/vm/drop_caches; time du -sh src
514M	src
real	0m3.881s

I've run this many times and get the same numbers.
I've tried deadline and noop on /dev/sda (the SSD) and du is just as slow.  

I also tried with:
- space_cache and nospace_cache
- ssd and nossd
- noatime didn't seem to help even though I was hopeful on this one.

In all cases, I get:
gandalfthegreat:/mnt/mnt2# echo 3 > /proc/sys/vm/drop_caches; time du -sh src
514M	src
real	0m22.537s


I'm having the same slow speed on 2 btrfs filesystems on the same SSD.
One is encrypted, the other one isnt:
Label: 'btrfs_pool1'  uuid: d570c40a-4a0b-4d03-b1c9-cff319fc224d
	Total devices 1 FS bytes used 144.74GB
	devid    1 size 441.70GB used 195.04GB path /dev/dm-0

Label: 'boot'  uuid: 84199644-3542-430a-8f18-a5aa58959662
	Total devices 1 FS bytes used 2.33GB
	devid    1 size 25.00GB used 5.04GB path /dev/sda2

If instead of stating a bunch of files, I try reading a big file, I do get speeds
that are quite fast (253MB/s and 423MB/s).

22 seconds for 15K files on an SSD is super slow and being 5 times
slower than a spinning disk with the same data.
What's going on?

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/