On Thu 22 Oct 2020 10:56:46 PM CEST, Yoonho Park wrote: > I am still seeing the performance degradation, but I did find something > interesting (and promising) with qemu 5.1.50. Enabling the subcluster > allocation support in qemu 5.1.50 (extended_l2=on) eliminates the > performance degradation of adding an overlay. Without subcluster allocation > enabled, I still see the performance degradation in qemu 5.1.50 when adding > an overlay. For these experiments, I used 64K blocks and 2M qcow2 cluster > size.
Well, 2MB clusters have 64KB subclusters, so your request size is equal to the subcluster size. If the requests are aligned there should be no copy-on-write and therefore no performance degradation if you have backing images. Berto
