On 2018-04-17 14:37, Alberto Garcia wrote: > 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. > > Changes: > v3: > - Mention that if you use internal snapshots you may want to increase > the cache size [Max] > > v2: https://lists.gnu.org/archive/html/qemu-block/2018-03/msg00822.html > - s/overriden/overridden/ (in both patches) > > v1: https://lists.gnu.org/archive/html/qemu-block/2018-03/msg00709.html > - Initial release > > 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 | 33 ++++++++++++++++----------------- > tests/qemu-iotests/137.out | 2 +- > 4 files changed, 36 insertions(+), 34 deletions(-)
Thanks, applied to my block-next branch: https://github.com/XanClic/qemu/commits/block-next Max
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