Re: [Qemu-devel] [PATCH v2 2/2] docs: Document the new default sizes of the qcow2 caches

[Max Reitz]

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 <address@hidden>
> Reviewed-by: Eric Blake <address@hidden>
> ---
>  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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