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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/qcow2-cache.txt | 33 |
1 files changed, 16 insertions, 17 deletions
diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt index 170191a242..8a09a5cc5f 100644 --- a/docs/qcow2-cache.txt +++ b/docs/qcow2-cache.txt @@ -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 |