Re: [pve-devel] [PATCH pve-storage 03/10] common: qemu_img_create: add backing_file support

["DERUMIER, Alexandre via pve-devel" <pve-devel@lists.proxmox.com>]

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From: "DERUMIER, Alexandre" <alexandre.derumier@groupe-cyllene.com>
To: "pve-devel@lists.proxmox.com" <pve-devel@lists.proxmox.com>, "f.gruenbichler@proxmox.com" <f.gruenbichler@proxmox.com>
Subject: Re: [pve-devel] [PATCH pve-storage 03/10] common: qemu_img_create: add backing_file support
Date: Fri, 4 Jul 2025 12:31:04 +0000
Message-ID: <cd5d34fd36a79b70a38797c8ca4e85076847fd48.camel@groupe-cyllene.com>

> 
> >>cluster_size is set to 128k, as it reduce qcow2 overhead (reduce
> >>disk,
> >>but also memory needed to cache metadatas)
>>
>>should we make this configurable?

I'm not sure yet, I have choose the best balance between memory <-
>performance (too big block reduce performance, too small increase
memory). they are 32 suballocated clusters of 4k with 128k
cluster_size, so for 4k workload, it's avoid over-amplication when it
need to read the parent snapshot chains.

If the image is really big, the best way to keep good performance, is
to increase the l2-cache-size. (maybe make it tunable)
the default qemu cache is 32MB, and it's able to handle 256GB image
with default 64k cluster without performance degradation.




>  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;

>>should we assert that $backing_path is only used with $fmt eq
'qcow2'?

yes indeed, qcow2 is the only format supporting a backing file

>  
>      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';

>>and then use $fmt here as well?

ok, will do



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