[pve-devel] [PATCH pve-storage 2/2] common: qemu_img_create: add backing_file support

[Fiona Ebner]

Am 22.05.25 um 15:53 schrieb Alexandre Derumier via pve-devel:
> 
> and use it for plugin linked clone
> 
> This also enable extended_l2=on, as it's mandatory for backing file
> preallocation.
> 
> Preallocation was missing previously, so it should increase performance
> for linked clone now (around x5 in randwrite 4k)
> 
> cluster_size is set to 128k, as it reduce qcow2 overhead (reduce disk,
> but also memory needed to cache metadatas)
> 
> l2_extended is not enabled yet on base image, but it could help too
> to reduce overhead without impacting performance
> 
> bench on 100G qcow2 file:
> 
> fio --filename=/dev/sdb --direct=1 --rw=randwrite --bs=4k --iodepth=32 --ioengine=libaio --name=test
> fio --filename=/dev/sdb --direct=1 --rw=randread --bs=4k --iodepth=32 --ioengine=libaio --name=test
> 
> base image:
> 
> randwrite 4k: prealloc=metadata, l2_extended=off, cluster_size=64k: 20215
> randread 4k: prealloc=metadata, l2_extended=off, cluster_size=64k: 22219
> randwrite 4k: prealloc=metadata, l2_extended=on, cluster_size=64k: 20217
> randread 4k: prealloc=metadata, l2_extended=on, cluster_size=64k: 21742
> randwrite 4k: prealloc=metadata, l2_extended=on, cluster_size=128k: 21599
> randread 4k: prealloc=metadata, l2_extended=on, cluster_size=128k: 22037
> 
> clone image with backing file:
> 
> randwrite 4k: prealloc=metadata, l2_extended=off, cluster_size=64k: 3912
> randread 4k: prealloc=metadata, l2_extended=off, cluster_size=64k: 21476
> randwrite 4k: prealloc=metadata, l2_extended=on, cluster_size=64k: 20563
> randread 4k: prealloc=metadata, l2_extended=on, cluster_size=64k: 22265
> randwrite 4k: prealloc=metadata, l2_extended=on, cluster_size=128k: 18016
> randread 4k: prealloc=metadata, l2_extended=on, cluster_size=128k: 21611
> 
> Signed-off-by: Alexandre Derumier <alexandre.derumier at groupe-cyllene.com>

In general, the approach is fine by me :)

> ---
>  src/PVE/Storage/Common.pm | 17 +++++++++++++----
>  src/PVE/Storage/Plugin.pm |  5 +----
>  2 files changed, 14 insertions(+), 8 deletions(-)
> 
> diff --git a/src/PVE/Storage/Common.pm b/src/PVE/Storage/Common.pm
> index 0770d70..01a995a 100644
> --- a/src/PVE/Storage/Common.pm
> +++ b/src/PVE/Storage/Common.pm
> @@ -148,14 +148,23 @@ sub preallocation_cmd_option {
>  }
>  
>  sub qemu_img_create {
> -    my ($scfg, $fmt, $size, $path) = @_;
> +    my ($scfg, $fmt, $size, $path, $backing_path) = @_;
> +
> +    die "size can't be specified if backing file is used" if $size && $backing_path;

I'd prefer having a second dedicated function
qemu_img_create_qcow2_with_backing() rather than an interface where some
parameters need to be undef depending on how you want to use it. It also
doesn't require a format parameter, because we only allow it for qcow2.

>      my $cmd = ['/usr/bin/qemu-img', 'create'];
>  
> -    my $prealloc_opt = preallocation_cmd_option($scfg, $fmt);
> -    push @$cmd, '-o', $prealloc_opt if defined($prealloc_opt);
> +    my $options = [];
> +
> +    if($backing_path) {
> +	push @$cmd, '-b', $backing_path, '-F', 'qcow2';

You're now using "-F qcow2" and...(continued below)

> +	push @$options, 'extended_l2=on','cluster_size=128k';
> +    };
>  
> -    push @$cmd, '-f', $fmt, $path, "${size}K";
> +    push @$options, preallocation_cmd_option($scfg, $fmt);
> +    push @$cmd, '-o', join(',', @$options) if @$options > 0;
> +    push @$cmd, '-f', $fmt, $path;

..."-f $fmt"...

> +    push @$cmd, "${size}K" if $size;
>  
>      run_command($cmd, errmsg => "unable to create image");
>  }
> diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
> index 7030e4e..a3856ca 100644
> --- a/src/PVE/Storage/Plugin.pm
> +++ b/src/PVE/Storage/Plugin.pm
> @@ -847,10 +847,7 @@ sub clone_image {
>      eval {
>  	local $CWD = $imagedir;
>  
> -	my $cmd = ['/usr/bin/qemu-img', 'create', '-b', "../$basevmid/$basename",
> -		   '-F', $format, '-f', 'qcow2', $path];

...but the old command was using "-F $format -f qcow2"!

> -
> -	run_command($cmd);
> +	PVE::Storage::Common::qemu_img_create($scfg, $format, undef, $path, "../$basevmid/$basename");
>      };
>      my $err = $@;
>  
> -- 
> 2.39.5
> 
>