[wsl2] filesystem performance is much slower than wsl1 in /mnt

[ioweb-gr]

I decided to open this as a separate issue because although it's related to the generic issue of filesystem performance it's directly related to WSL 2 while the other issues are for WSL 1 and it's showing very conflicting results.

• Your Windows build number: (Type ver at a Windows Command Prompt)
  Version 10.0.18917 Build 18917

• What you're doing and what's happening:
  I'm testing filesystem write speed in /mnt using dd command. Performing the following tests

WSL2

root@LUCIANO-PC:/home/# dd if=/dev/zero of=/mnt/e/testfile bs=1M count=1000
1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 25.939 s, 40.4 MB/s

WSL1

root@LUCIANO-PC:/home/# dd if=/dev/zero of=/mnt/e/testfile bs=1M count=20000
20000+0 records in
20000+0 records out
20971520000 bytes (21 GB, 20 GiB) copied, 47.4897 s, 442 MB/s

On / it's actually the reverse. WSL2 is more than 2 times faster than WSL1.

• What's wrong / what should be happening instead:
  I would expect the filesystem performance in /mnt to at least be on the same level but it's over 10 times slower.

Another interesting fact is that if I mount the same drive as a cifs share I get 3x performance

WSL 2 (cifs share)

root@LUCIANO-PC:/mnt/sambae# dd if=/dev/zero of=/mnt/sambae/testfile bs=1M count=10000
10000+0 records in
10000+0 records out
10485760000 bytes (10 GB, 9.8 GiB) copied, 84.001 s, 125 MB/s

An update of the current status because it's way too hidden in this thread.

Latest status report: #4197 (comment)_

[billziss-gh]

On / it's actually the reverse. WSL2 is more than 2 times faster than WSL1.

Can you post the results when doing the same test on a file on / for WSL1 and WSL2?

This is straight I/O on a single file where I would expect to see more or less the same perf between WSL1 and WSL2. My expectation was that WSL2 perf is better than WSL1 only when doing "namespace" operations (i.e. working on lots of small files, listing them, stat'ing them, etc.)

As for the results on /mnt/e they are not very surprising, since I/O has to go through both the Linux and Windows file system stack in likely a new and unoptimized piece of code.

[ioweb-gr]

I see,

I'm assuming you're still refering to / and not /mnt folders about namespace operations.
I've repeated the write tests in both environments a few times now and there's no consistent result.

WSL 2

root@LUCIANO-PC:/home/# dd if=/dev/zero of=/testfile bs=1M count=10000
10000+0 records in
10000+0 records out
10485760000 bytes (10 GB, 9.8 GiB) copied, 7.0873 s, 1.5 GB/s
root@LUCIANO-PC:/home/# dd if=/dev/zero of=/testfile bs=1M count=20000

20000+0 records in
20000+0 records out
20971520000 bytes (21 GB, 20 GiB) copied, 51.3779 s, 408 MB/s
root@LUCIANO-PC:/home/#
root@LUCIANO-PC:/home/# dd if=/dev/zero of=~/testfile bs=1M count=20000
^C12360+0 records in
12360+0 records out
12960399360 bytes (13 GB, 12 GiB) copied, 12.1766 s, 1.1 GB/s

WSL1

root@LUCIANO-PC:/home/# dd if=/dev/zero of=~/testfile bs=1M count=10000
10000+0 records in
10000+0 records out
10485760000 bytes (10 GB, 9.8 GiB) copied, 29.7096 s, 353 MB/s
root@LUCIANO-PC:/home/#

I'm thinking this has to do something with OS Caching because in task manager even though I can see the file being written, the disk usage write speed is unchanged while in wsl1 I can see it constantly at around 350mb/sec .

As another interesting fact, the vmmem usage while writing the file is increasing rapidly. By the 3rd copy action all my RAM is used up (32GB).
It goes back down a while after quitting my shell. So maybe the performance is bottlenecked by this bug as well.

[paultyng]

I'm also experiencing this after an upgrade to WSL2, only slow performance under /mnt, not /, slower than WSL1. I can post numbers as well if necessary. This is on Microsoft Windows [Version 10.0.18936.1000].

[definelicht]

I'm experiencing the same issue, /mnt is extremely slow compared to WSL1 (seconds to run git status on a relatively small repository).

[paultyng]

When I run a git status on a clone of https://github.com/hashicorp/terraform in /mnt I get output like this:

It took 6.97 seconds to enumerate untracked files. 'status -uno'
may speed it up, but you have to be careful not to forget to add
new files yourself (see 'git help status').

[ioweb-gr]

This is an example from the command

pv files.tar.gz | tar -zxf -

The speed is amazingly slow in wsl2 on /mnt

[definelicht]

Do we know what's at the root of this, and whether there are any other workarounds than downgrading?

[benhillis]

@definelicht - Yep we're working on improving the performance. In the meaning working out of your root file system (the ext4 volume) will have MUCH better performance.

[ioweb-gr]

Indeed after the last update I can already see much better performance on WSL2.

root@LUCIANO-PC:~# dd if=/dev/zero of=/mnt/e/testfile bs=1M count=10000
10000+0 records in
10000+0 records out
10485760000 bytes (10 GB, 9.8 GiB) copied, 41.277 s, 254 MB/s

That's on an SSD and it's like 6 times faster than before. Still slower then WSL1 but it's definitely improved.

--correction---
I see much better performance on sequential writes. Untar seems very slow still extracting data at bytes/sec while in wsl1 I can see speeds at kb/sec for the same part

[tuananh]

Indeed after the last update I can already see much better performance on WSL2.

root@LUCIANO-PC:~# dd if=/dev/zero of=/mnt/e/testfile bs=1M count=10000
10000+0 records in
10000+0 records out
10485760000 bytes (10 GB, 9.8 GiB) copied, 41.277 s, 254 MB/s

That's on an SSD and it's like 6 times faster than before. Still slower then WSL1 but it's definitely improved.

--correction---
I see much better performance on sequential writes. Untar seems very slow still extracting data at bytes/sec while in wsl1 I can see speeds at kb/sec for the same part

I got

1000+0 records in
1000+0 records out
1048576000 bytes (1.0 GB, 1000 MiB) copied, 3.57894 s, 293 MB/s

but git status still taking more than 2 seconds on quite small repo