@@ -116,31 +116,30 @@ There are three options available, and all of them take bytes:
"refcount-cache-size": maximum size of the refcount block cache
"cache-size": maximum size of both caches combined
-There are two things that need to be taken into account:
+There are a few things that need to be taken into account:
- Both caches must have a size that is a multiple of the cluster size
(or the cache entry size: see "Using smaller cache sizes" below).
- - If you only set one of the options above, QEMU will automatically
- adjust the others so that the L2 cache is 4 times bigger than the
- refcount cache.
+ - The default L2 cache size is 8 clusters or 1MB (whichever is more),
+ and the minimum is 2 clusters (or 2 cache entries, see below).
-This means that these options are equivalent:
+ - The default (and minimum) refcount cache size is 4 clusters.
- -drive file=hd.qcow2,l2-cache-size=2097152
- -drive file=hd.qcow2,refcount-cache-size=524288
- -drive file=hd.qcow2,cache-size=2621440
+ - If only "cache-size" is specified then QEMU will assign as much
+ memory as possible to the L2 cache before increasing the refcount
+ cache size.
-The reason for this 1/4 ratio is to ensure that both caches cover the
-same amount of disk space. Note however that this is only valid with
-the default value of refcount_bits (16). If you are using a different
-value you might want to calculate both cache sizes yourself since QEMU
-will always use the same 1/4 ratio.
+Unlike L2 tables, refcount blocks are not used during normal I/O but
+only during allocations and internal snapshots. In most cases they are
+accessed sequentially (even during random guest I/O) so increasing the
+refcount cache size won't have any measurable effect in performance
+(this can change if you are using internal snapshots, so you may want
+to think about increasing the cache size if you use them heavily).
-It's also worth mentioning that there's no strict need for both caches
-to cover the same amount of disk space. The refcount cache is used
-much less often than the L2 cache, so it's perfectly reasonable to
-keep it small.
+Before QEMU 2.12 the refcount cache had a default size of 1/4 of the
+L2 cache size. This resulted in unnecessarily large caches, so now the
+refcount cache is as small as possible unless overridden by the user.
Using smaller cache entries
We have just reduced the refcount cache size to the minimum unless the user explicitly requests a larger one, so we have to update the documentation to reflect this change. Signed-off-by: Alberto Garcia <berto@igalia.com> --- docs/qcow2-cache.txt | 33 ++++++++++++++++----------------- 1 file changed, 16 insertions(+), 17 deletions(-)