[pve-devel] [PATCH pve-storage 2/9] common: 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 2/9] common: add qemu_img_create_qcow2_backed
Date: Tue,  3 Jun 2025 09:55:40 +0200
Message-ID: <20250603075558.627850-6-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 | 27 +++++++++++++++++++++++++++
 src/PVE/Storage/Plugin.pm |  5 +----
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/src/PVE/Storage/Common.pm b/src/PVE/Storage/Common.pm
index 4e4ad30..8910988 100644
--- a/src/PVE/Storage/Common.pm
+++ b/src/PVE/Storage/Common.pm
@@ -25,6 +25,10 @@ my $RAW_PREALLOCATION = {
     full => 1,
 };
 
+my $QCOW2_CLUSTERS = {
+   backed => ['extended_l2=on','cluster_size=128k']
+};
+
 =pod
 
 =head1 NAME
@@ -179,4 +183,27 @@ 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");
+}
+
 1;
diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index 90627b2..8bf4ed3 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', $path];
-
-	run_command($cmd);
+	PVE::Storage::Common::qemu_img_create_qcow2_backed($scfg, $path, "../$basevmid/$basename", $format);
     };
     my $err = $@;
 
-- 
2.39.5



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