[pve-devel] [PATCH qemu-server] virtiofs: prevent issue with Windows OS and too many files

[Markus Frank]

On 2025-05-02 14:13, Fiona Ebner wrote:
> Am 02.05.25 um 13:52 schrieb Markus Frank:
>> On 2025-04-28 12:17, Fiona Ebner wrote:
>>> As reported in the community forum [0] and the virtio-win project [1],
>>> virtiofsd will run into its open file limit when used with a Windows
>>> guest that reads too many files. It's also reported that the issue
>>> does not occur with Linux guests and a workaround is using
>>> '--inode-file-handles=mandatory' on virtiofsd command line.
>> I was able to reproduce the issue with a directory containing a million
>> files.
>> The virtiofs share became unusable when I tried to list the files with
>> 'ls'.
>> This patch allows to list all the files without problems.
>>>
>>> The option is described as follows in the vritiofsd help:
>> typo: virtiofsd
> 
> Will fix!
> 
>>>
>>>> When to use file handles to reference inodes instead of O_PATH file
>>>> descriptors (never, prefer, mandatory)
>>>
>>> and the default is 'never'.
>>>
>>> Fix the above issue by using 'prefer' rather than 'mandatory', because
>>> that should not break other edge cases:
>>>
>>>> prefer: Attempt to generate file handles, but fall back to O_PATH
>>>> file descriptors where the underlying filesystem does not support
>>>> file handles. Useful when there are various different filesystems
>>>> under the shared directory and some of them do not support file
>>>> handles.
>>>
>>> [0]: https://forum.proxmox.com/threads/165565/
>>> [1]: https://github.com/virtio-win/kvm-guest-drivers-windows/issues/1136
>>>
>>> Signed-off-by: Fiona Ebner <f.ebner at proxmox.com>
>> Tested-by: Markus Frank <m.frank at proxmox.com>
>>> ---
>>>
>>> Didn't get around to measure the performance impact yet, so feel free
>>> to check that if you test this patch. While not being broken is more
>>> important than good performance, it would still be good to know for
>>> completeness.
>> I made a few read/write tests with fio in a Windows 11 guest.
> 
> Since the option affects file handles, it would be more interesting to
> test something that involves handling many files rather than just IO.

Okay, here is a test that creates 10000 4kB files.

without this patch (/usr/libexec/virtiofsd --fd=15 --shared-dir=/share --announce-submounts --syslog):

PS Z:\> fio --name=filetest --rw=write --bs=4k --size=4k --numjobs=10 --filesize=4k --directory="testdir" --ioengine=windowsaio --create_on_open=1 --file_service_type=random --nrfiles=10000 --runtime=30 --time_based --group_reporting
fio: this platform does not support process shared mutexes, forcing use of threads. Use the 'thread' option to get rid of this warning.
filetest: (g=0): rw=write, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=windowsaio, iodepth=1
...
fio-3.39
Starting 10 threads
Jobs: 4 (f=18): [_(3),f(1),_(2),f(1),_(1),f(2)][100.0%][eta 00m:00s]
filetest: (groupid=0, jobs=10): err= 0: pid=8000: Fri May 2 15:45:44 2025
   write: IOPS=967, BW=3870KiB/s (3962kB/s)(114MiB/30049msec); 0 zone resets
     slat (nsec): min=0, max=632300, avg=20663.88, stdev=13715.30
     clat (usec): min=494, max=12174, avg=2302.34, stdev=676.20
      lat (usec): min=512, max=12195, avg=2323.00, stdev=675.62
     clat percentiles (usec):
      |  1.00th=[ 1037],  5.00th=[ 1352], 10.00th=[ 1532], 20.00th=[ 1745],
      | 30.00th=[ 1926], 40.00th=[ 2073], 50.00th=[ 2212], 60.00th=[ 2376],
      | 70.00th=[ 2573], 80.00th=[ 2802], 90.00th=[ 3195], 95.00th=[ 3523],
      | 99.00th=[ 4228], 99.50th=[ 4490], 99.90th=[ 5211], 99.95th=[ 5669],
      | 99.99th=[ 8455]
    bw (  KiB/s): min= 1621, max= 4387, per=99.24%, avg=3840.90, stdev=41.34, samples=600
    iops        : min=  401, max= 1094, avg=957.38, stdev=10.37, samples=600
   lat (usec)   : 500=0.01%, 750=0.10%, 1000=0.70%
   lat (msec)   : 2=34.57%, 4=62.77%, 10=1.86%, 20=0.01%
   cpu          : usr=0.00%, sys=0.00%, ctx=0, majf=0, minf=0
   IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
      submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
      complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
      issued rwts: total=0,29069,0,0 short=0,0,0,0 dropped=0,0,0,0
      latency   : target=0, window=0, percentile=100.00%, depth=1

Run status group 0 (all jobs):
   WRITE: bw=3870KiB/s (3962kB/s), 3870KiB/s-3870KiB/s (3962kB/s-3962kB/s), io=114MiB (119MB), run=30049-30049msec



with this patch (/usr/libexec/virtiofsd --fd=15 --shared-dir=/share --announce-submounts --inode-file-handles=prefer --syslog):

PS Z:\> fio --name=filetest --rw=write --bs=4k --size=4k --numjobs=10 --filesize=4k --directory="testdir" --ioengine=windowsaio --create_on_open=1 --file_service_type=random --nrfiles=10000 --runtime=30 --time_based --group_reporting
fio: this platform does not support process shared mutexes, forcing use of threads. Use the 'thread' option to get rid of this warning.
filetest: (g=0): rw=write, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=windowsaio, iodepth=1
...
fio-3.39
Starting 10 threads
Jobs: 6 (f=318): [_(1),f(1),_(1),f(1),_(1),f(2),_(1),f(2)][100.0%][eta 00m:00s]
filetest: (groupid=0, jobs=10): err= 0: pid=8872: Fri May 2 15:55:39 2025
   write: IOPS=964, BW=3859KiB/s (3952kB/s)(113MiB/30028msec); 0 zone resets
     slat (nsec): min=0, max=1623.2k, avg=20705.88, stdev=16208.95
     clat (usec): min=462, max=25441, avg=2312.40, stdev=778.69
      lat (usec): min=489, max=25458, avg=2333.11, stdev=778.39
     clat percentiles (usec):
      |  1.00th=[  996],  5.00th=[ 1336], 10.00th=[ 1516], 20.00th=[ 1745],
      | 30.00th=[ 1926], 40.00th=[ 2089], 50.00th=[ 2245], 60.00th=[ 2409],
      | 70.00th=[ 2573], 80.00th=[ 2802], 90.00th=[ 3163], 95.00th=[ 3523],
      | 99.00th=[ 4228], 99.50th=[ 4490], 99.90th=[ 5473], 99.95th=[ 7832],
      | 99.99th=[25035]
    bw (  KiB/s): min= 1180, max= 4385, per=98.89%, avg=3816.80, stdev=44.92, samples=600
    iops        : min=  291, max= 1094, avg=952.10, stdev=11.25, samples=600
   lat (usec)   : 500=0.01%, 750=0.14%, 1000=0.86%
   lat (msec)   : 2=33.73%, 4=63.43%, 10=1.79%, 20=0.02%, 50=0.03%
   cpu          : usr=0.00%, sys=0.00%, ctx=0, majf=0, minf=0
   IO depths    : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%, >=64=0.0%
      submit    : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
      complete  : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
      issued rwts: total=0,28969,0,0 short=0,0,0,0 dropped=0,0,0,0
      latency   : target=0, window=0, percentile=100.00%, depth=1

Run status group 0 (all jobs):
   WRITE: bw=3859KiB/s (3952kB/s), 3859KiB/s-3859KiB/s (3952kB/s-3952kB/s), io=113MiB (119MB), run=30028-30028msec