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

["DERUMIER, Alexandre via pve-devel" <pve-devel@lists.proxmox.com>]

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From: "DERUMIER, Alexandre" <alexandre.derumier@groupe-cyllene.com>
To: "pve-devel@lists.proxmox.com" <pve-devel@lists.proxmox.com>
Subject: Re: [pve-devel] [PATCH pve-storage 0/2] move qemu_img_create to common helpers and enable preallocation on backed images
Date: Thu, 22 May 2025 12:44:12 +0000
Message-ID: <b4eed2f961b3e093cd4bf0263d5b09a1e9b00aa0.camel@groupe-cyllene.com>

I have done some tests with suballocated cluster and base image without
backing_file, indeed, I'm seeing a small performance degradation on big
1TB image.


with a 30GB image, I'm around 22000 iops 4k randwrite/randread  (with
or without l2_extended=on)

with a 1TB image, the result is different


fio –filename=/dev/sdb –direct=1 –rw=randwrite –bs=4k –iodepth=32
–ioengine=libaio –name=test

default l2-cache-size , extended_l2=off, cluster_size=64k : 2700 iops
default l2-cache-size , extended_l2=on, cluster_size=128k: 1500 iops


I have also play with qemu l2-cache-size option of drive (default value
is 1MB, and it's not enough for a 1TB image to keep all metadatas in
memory)

l2-cache-size=8MB , extended_l2=off, cluster_size=64k: 2900 iops
l2-cache-size=64MB , extended_l2=off, cluster_size=64k: 5100 iops
l2-cache-size=128MB , extended_l2=off, cluster_size=64k : 22000 iops

l2-cache-size=8MB , extended_l2=on, cluster_size=128k: 2000 iops
l2-cache-size=64MB , extended_l2=on, cluster_size=128k: 4500 iops
l2-cache-size=128MB , extended_l2=on, cluster_size=128k: 22000 iops


So no difference in needed memory, with or with extended_l2.

but the l2-cache-size tuning is really something we should add in
another patch I think ,for general performance with qcow2.




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