Hi, we talked about this the other day, so here are the patches to change the default cache sizes in qcow2.
Without this patch: * refcount-cache-size = l2-cache-size / 4 unless otherwise specified by the user. This is wasteful, the refcount cache is accessed sequentially during normal I/O, so there's no point in caching more tables. I measured the effect on the refcount cache size when populating an empty qcow2 image using random writes, and there's no difference between having the minimum or the maximum sizes(*). With this patch: * refcount-cache-size is always 4 clusters by default (the minimum) * If "cache-size" is set then l2-cache-size is set to the maximum if possible (disk_size * 8 / cluster_size) and the remainder is assigned to the refcount cache. Regards, Berto (*) there is, actually: having a very large cache can even make the I/O slightly slower, because the larger the cache the longer it takes longer to find a cached entry. I only noticed this under tmpfs anyway. Alberto Garcia (2): qcow2: Give the refcount cache the minimum possible size by default docs: Document the new default sizes of the qcow2 caches block/qcow2.c | 31 +++++++++++++++++++------------ block/qcow2.h | 4 ---- docs/qcow2-cache.txt | 31 ++++++++++++++----------------- tests/qemu-iotests/137.out | 2 +- 4 files changed, 34 insertions(+), 34 deletions(-) -- 2.11.0