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:
v2:
- 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 | 31 ++++++++++++++-----------------
tests/qemu-iotests/137.out | 2 +-
4 files changed, 34 insertions(+), 34 deletions(-)
--
2.11.0
