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

DISCUSSION Marc MERLIN 1 participant 1 message Jul 24, 2012 Btrfs file system

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Marc MERLIN

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Quoted 2 lines · Marc MERLIN

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

Since yesterday, I tried other kernels, including noprempt, volprempt and preempt for 3.4.4.
I also tried a default 3.2.0 kernel from debian (all amd64), but that did not help. I'm still seeing close to 25 seconds to scan 15K files.

How can it possibly be so slow?
More importantly how I can provide useful debug information.

- I don't think it's a problem with the kernel since I tried 4 kernels, including a default debian one.

- Alignement seem ok, I made sure cylinders was divisible by 512:
/dev/sda2 502272 52930559 26214144 83 Linux

- I tried another brand new btrfs, and thing are even slower now.

gandalfthegreat:/mnt/mnt2# mount -o ssd,discard,noatime /dev/sda2 /mnt/mnt2
gandalfthegreat:/mnt/mnt2# reset_cache 
gandalfthegreat:/mnt/mnt2# time du -sh src/
514M	src/
real	0m29.584s
gandalfthegreat:/mnt/mnt2# find src/| wc -l
15261

This is bad enough that there ought to be a way to debug this, right?

Can you suggest something?

Thanks,
Marc

Quoted 49 lines

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

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