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

[Markus Frank]

Hello,

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
> 
>> 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.


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

test: (g=0): rw=randrw, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=windowsaio, iodepth=32
Run status group 0 (all jobs):
    READ: bw=11.0MiB/s (11.5MB/s), 11.0MiB/s-11.0MiB/s (11.5MB/s-11.5MB/s), io=665MiB (698MB), run=60667-60667msec
   WRITE: bw=6054KiB/s (6200kB/s), 6054KiB/s-6054KiB/s (6200kB/s-6200kB/s), io=359MiB (376MB), run=60667-60667msec

test: (g=0): rw=rw, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=windowsaio, iodepth=32
Run status group 0 (all jobs):
    READ: bw=11.1MiB/s (11.6MB/s), 11.1MiB/s-11.1MiB/s (11.6MB/s-11.6MB/s), io=665MiB (698MB), run=59933-59933msec
   WRITE: bw=6129KiB/s (6276kB/s), 6129KiB/s-6129KiB/s (6276kB/s-6276kB/s), io=359MiB (376MB), run=59933-59933msec


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

test: (g=0): rw=randrw, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=windowsaio, iodepth=32
Run status group 0 (all jobs):
    READ: bw=9.77MiB/s (10.2MB/s), 9.77MiB/s-9.77MiB/s (10.2MB/s-10.2MB/s), io=665MiB (698MB), run=68094-68094msec
   WRITE: bw=5394KiB/s (5524kB/s), 5394KiB/s-5394KiB/s (5524kB/s-5524kB/s), io=359MiB (376MB), run=68094-68094msec

test: (g=0): rw=rw, bs=(R) 4096B-4096B, (W) 4096B-4096B, (T) 4096B-4096B, ioengine=windowsaio, iodepth=32
Run status group 0 (all jobs):
    READ: bw=10.8MiB/s (11.3MB/s), 10.8MiB/s-10.8MiB/s (11.3MB/s-11.3MB/s), io=665MiB (698MB), run=61461-61461msec
   WRITE: bw=5976KiB/s (6120kB/s), 5976KiB/s-5976KiB/s (6120kB/s-6120kB/s), io=359MiB (376MB), run=61461-61461msec


(/share is a directory on an ext4 filesystem on the host)

> 
>   PVE/QemuServer/Virtiofs.pm | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/PVE/QemuServer/Virtiofs.pm b/PVE/QemuServer/Virtiofs.pm
> index cfde92c9..5a91b23a 100644
> --- a/PVE/QemuServer/Virtiofs.pm
> +++ b/PVE/QemuServer/Virtiofs.pm
> @@ -130,14 +130,17 @@ sub start_all_virtiofsd {
>   	next if !$conf->{$opt};
>   	my $virtiofs = parse_property_string('pve-qm-virtiofs', $conf->{$opt});
>   
> -	my $virtiofs_socket = start_virtiofsd($vmid, $i, $virtiofs);
> +	# See https://github.com/virtio-win/kvm-guest-drivers-windows/issues/1136
> +	my $prefer_inode_fh = PVE::QemuServer::Helpers::windows_version($conf->{ostype}) ? 1 : 0;
> +
> +	my $virtiofs_socket = start_virtiofsd($vmid, $i, $virtiofs, $prefer_inode_fh);
>   	push @$virtiofs_sockets, $virtiofs_socket;
>       }
>       return $virtiofs_sockets;
>   }
>   
>   sub start_virtiofsd {
> -    my ($vmid, $fsid, $virtiofs) = @_;
> +    my ($vmid, $fsid, $virtiofs, $prefer_inode_fh) = @_;
>   
>       mkdir $socket_path_root;
>       my $socket_path = "$socket_path_root/vm$vmid-fs$fsid";
> @@ -175,6 +178,7 @@ sub start_virtiofsd {
>   	    push @$cmd, '--announce-submounts';
>   	    push @$cmd, '--allow-direct-io' if $virtiofs->{'direct-io'};
>   	    push @$cmd, '--cache='.$virtiofs->{cache} if $virtiofs->{cache};
> +	    push @$cmd, '--inode-file-handles=prefer' if $prefer_inode_fh;
>   	    push @$cmd, '--syslog';
>   	    exec(@$cmd);
>   	} elsif (!defined($pid2)) {