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[Qemu-block] [PATCH 0/2] Give the refcount cache the minimum possible si

From: Alberto Garcia
Subject: [Qemu-block] [PATCH 0/2] Give the refcount cache the minimum possible size by default
Date: Tue, 13 Mar 2018 17:02:10 +0200


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

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.



(*) 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(-)


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