[pve-devel] [PATCH pve-storage 02/13] plugin: add qemu_img_create_qcow2_backed

[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 02/13] plugin: add qemu_img_create_qcow2_backed
Date: Wed,  9 Jul 2025 18:21:49 +0200
Message-ID: <20250709162202.2952597-5-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/Plugin.pm | 52 ++++++++++++++++++++++++++++-----------
 1 file changed, 38 insertions(+), 14 deletions(-)

diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index 65a34b1..c4b4cd3 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -51,6 +51,10 @@ our $RAW_PREALLOCATION = {
     full => 1,
 };
 
+my $QCOW2_CLUSTERS = {
+    backed => ['extended_l2=on', 'cluster_size=128k'],
+};
+
 our $MAX_VOLUMES_PER_GUEST = 1024;
 
 cfs_register_file(
@@ -654,6 +658,39 @@ sub qemu_img_create {
     run_command($cmd, errmsg => "unable to create image");
 }
 
+=pod
+
+=head3 qemu_img_create_qcow2_backed
+
+    qemu_img_create_qcow2_backed($scfg, $path, $backing_path, $backing_format)
+
+Create a new qemu qcow2 image C<$path> using an existing backing image C<$backing_path> with backing_format C<$backing_format>.
+
+=cut
+
+sub qemu_img_create_qcow2_backed {
+    my ($scfg, $path, $backing_path, $backing_format) = @_;
+
+    my $cmd = [
+        '/usr/bin/qemu-img',
+        'create',
+        '-F',
+        $backing_format,
+        '-b',
+        $backing_path,
+        '-f',
+        'qcow2',
+        $path,
+    ];
+
+    my $options = $QCOW2_CLUSTERS->{backed};
+
+    push @$options, preallocation_cmd_option($scfg, 'qcow2');
+    push @$cmd, '-o', join(',', @$options) if @$options > 0;
+
+    run_command($cmd, errmsg => "unable to create image");
+}
+
 # Storage implementation
 
 # called during addition of storage (before the new storage config got written)
@@ -941,20 +978,7 @@ 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);
+        qemu_img_create_qcow2_backed($scfg, $path, "../$basevmid/$basename", $format);
     };
     my $err = $@;
 
-- 
2.39.5



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