On 2018-03-14 09:29, Alberto Garcia wrote: > 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 <be...@igalia.com> > Reviewed-by: Eric Blake <ebl...@redhat.com> > --- > docs/qcow2-cache.txt | 31 ++++++++++++++----------------- > 1 file changed, 14 insertions(+), 17 deletions(-) > > diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt > index 170191a242..c640d45d06 100644 > --- a/docs/qcow2-cache.txt > +++ b/docs/qcow2-cache.txt > @@ -116,31 +116,28 @@ 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.
Could you add a note that it can have a measurable effect when using internal snapshots and the user may want to think about increasing its size when making heavy use of them? Max > -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 >
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