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

[Alexandre Derumier via pve-devel <pve-devel@lists.proxmox.com>]

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From: Alexandre Derumier <alexandre.derumier@groupe-cyllene.com>
To: pve-devel@lists.proxmox.com
Subject: [PATCH pve-storage 03/10] common: qemu_img_create: add backing_file support
Date: Fri,  4 Jul 2025 08:44:59 +0200
Message-ID: <20250704064507.511884-6-alexandre.derumier@groupe-cyllene.com>

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@groupe-cyllene.com>
---
 src/PVE/Storage/Common.pm | 17 +++++++++++++----
 src/PVE/Storage/Plugin.pm | 17 +++--------------
 2 files changed, 16 insertions(+), 18 deletions(-)

diff --git a/src/PVE/Storage/Common.pm b/src/PVE/Storage/Common.pm
index 29f2e52..78e5320 100644
--- a/src/PVE/Storage/Common.pm
+++ b/src/PVE/Storage/Common.pm
@@ -150,14 +150,23 @@ 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;
 
     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', $backing_path, '-F', 'qcow2';
+        push @$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, $path;
+    push @$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 80bb077..c35d5e5 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -880,20 +880,9 @@ sub clone_image {
     # Note: we use relative paths, so we need to call chdir before qemu-img
     eval {
         local $CWD = $imagedir;
-
-        my $cmd = [
-            '/usr/bin/qemu-img',
-            'create',
-            '-b',
-            "../$basevmid/$basename",
-            '-F',
-            $format,
-            '-f',
-            'qcow2',
-            $path,
-        ];
-
-        run_command($cmd);
+        PVE::Storage::Common::qemu_img_create(
+            $scfg, $format, undef, $path, "../$basevmid/$basename",
+        );
     };
     my $err = $@;
 
-- 
2.39.5



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