[PATCH pve-storage] qcow2 format: enable subcluster allocation by default

[Alexandre Derumier]

extended_l2 is an optimisation to reduce write amplification.
Currently,without it, when a vm write 4k, a full 64k cluster
need to be writen.

When enabled, the cluster is splitted in 32 subclusters.

We use a 128k cluster by default, to have 32 * 4k subclusters

https://blogs.igalia.com/berto/2020/12/03/subcluster-allocation-for-qcow2-images/
https://static.sched.com/hosted_files/kvmforum2020/d9/qcow2-subcluster-allocation.pdf

some stats for 4k randwrite benchmark

Cluster size   Without subclusters     With subclusters
16 KB          5859 IOPS               8063 IOPS
32 KB          5674 IOPS               11107 IOPS
64 KB          2527 IOPS               12731 IOPS
128 KB         1576 IOPS               11808 IOPS
256 KB         976 IOPS                 9195 IOPS
512 KB         510 IOPS                 7079 IOPS
1 MB           448 IOPS                 3306 IOPS
2 MB           262 IOPS                 2269 IOPS

Signed-off-by: Alexandre Derumier <alexandre.derumier at groupe-cyllene.com>
---
 src/PVE/Storage/Plugin.pm | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/PVE/Storage/Plugin.pm b/src/PVE/Storage/Plugin.pm
index 6444390..31b20fe 100644
--- a/src/PVE/Storage/Plugin.pm
+++ b/src/PVE/Storage/Plugin.pm
@@ -561,7 +561,7 @@ sub preallocation_cmd_option {
 	die "preallocation mode '$prealloc' not supported by format '$fmt'\n"
 	    if !$QCOW2_PREALLOCATION->{$prealloc};
 
-	return "preallocation=$prealloc";
+	return "preallocation=$prealloc,extended_l2=on,cluster_size=128k";
     } elsif ($fmt eq 'raw') {
 	$prealloc = $prealloc // 'off';
 	$prealloc = 'off' if $prealloc eq 'metadata';
-- 
2.39.2