[pve-devel] [PATCH pve-storage 0/2] move qemu_img_create to common helpers and enable preallocation on backed images

[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 0/2] move qemu_img_create to common helpers and enable preallocation on backed images
Date: Mon, 19 May 2025 12:23:08 +0200
Message-ID: <20250519102310.911326-1-alexandre.derumier@groupe-cyllene.com>

This is part of my work on qcow2 external snapshot, but could improve current qcow2 linked clone

This patch serie move qemu_img_create to common helpers,
and enable preallocation on backed_image to increase performance

This require l2_extended=on on the backed image

I have done some benchmarks on localssd with 100gb qcow2, the performance in randwrite 4k is 5x faster

some presentation of l2_extended=on are available here
https://www.youtube.com/watch?v=zJetcfDVFNw
https://www.youtube.com/watch?v=NfgLCdtkRus

I don't have enabled it for base image, as I think that Fabian see performance regression some month ago.
but I don't see performance difference in my bench. (can you could test on your side again ?)

It could help to reduce qcow2 overhead on disk,
and allow to keep more metadatas in memory for bigger image, as qemu default memory l2_cache_size=1MB)
https://www.ibm.com/products/tutorials/how-to-tune-qemu-l2-cache-size-and-qcow2-cluster-size
Maybe more test with bigger image (>1TB) could be done too to see if it's help



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

linked 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

Alexandre Derumier (2):
  common: add qemu_img_create an preallocation_cmd_option
  common: qemu_img_create: add backing_file support

 src/PVE/Storage/Common.pm          | 62 ++++++++++++++++++++++++++++++
 src/PVE/Storage/GlusterfsPlugin.pm |  2 +-
 src/PVE/Storage/Plugin.pm          | 52 +------------------------
 3 files changed, 65 insertions(+), 51 deletions(-)

-- 
2.39.5



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