Am 22.05.25 um 15:53 schrieb Alexandre Derumier via pve-devel: > 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 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 > > 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 (32MB) , extended_l2=off, cluster_size=64k : 2700 iops > default l2-cache-size (32MB) , extended_l2=on, cluster_size=128k: 1500 iops
It was not Fabian but me, who reported the regression regarding read performance and performance for initial allocation back then: https://lore.proxmox.com/pve-devel/d5e11d01-f54e-4dd9-b1c0-a02077a0c...@proxmox.com/ The space usage on the underlying storage is greatly improved however. > I have also play with qemu l2-cache-size option of drive (default value > is 32MB, and it's not enough for a 1TB image to keep all metadatas in > memory) > https://github.com/qemu/qemu/commit/80668d0fb735f0839a46278a7d42116089b82816 > > > 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. If we want to enable extended_l2=on, cluster_size=128k by default for all new qcow2 image, I think we should do it together with an increased l2-cache-size then. But yes, should be its own patch. The above results sound promising, but we'll need to test a bigger variety of workloads. If we don't find settings that improve most workloads, we can still make it configurable. _______________________________________________ pve-devel mailing list pve-devel@lists.proxmox.com https://lists.proxmox.com/cgi-bin/mailman/listinfo/pve-devel
