On Sat 18 Mar 2017 10:54:16 AM CET, Daniel P. Berrange wrote: >> If you don't care about that at all, the optimal configuration in >> terms of performance is to give qemu a cache large enough that the >> metadata of the whole image fits in it. When setting >> cache-clean-interval, this could actually be reasonable even for >> large images because the memory wouldn't be used forever but just as >> long as the guest is submitting the problematic I/O patterns - but it >> still means that temporarily qemu could really use all of this >> memory. > > Is there some easy algorithm that libvirt can use to determine the > size of the L2 tables, and thus report what the maximum useful cache > size would be ? Yes, the disk size that can be covered with a certain L2 cache is: disk_size = l2_cache_size * cluster_size / 8 (all sizes in bytes) Note that increasing the L2 cache size also increases the refcount cache, which you might want to keep small. It's all explained in detail here: https://github.com/qemu/qemu/blob/master/docs/qcow2-cache.txt https://blogs.igalia.com/berto/2015/12/17/improving-disk-io-performance-in-qemu-2-5-with-the-qcow2-l2-cache/ Berto -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
