Re: [Qemu-devel] [PATCH 3/4] docs: document how to configure the qcow2 L2/refcount caches

[Max Reitz <mreitz@redhat.com>]

On 02.06.2015 14:22, Alberto Garcia wrote:
> QEMU has options to configure the size of the L2 and refcount
> caches for the qcow2 format. However, choosing the right sizes for
> a particular disk image is not a straightforward operation since
> the ratio between the cache size and the allocated disk space is
> not obvious and depends on the size of the cluster and the refcount
> entries.
>
> This document attempts to give an overview of both caches and how to
> configure their sizes.
>
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> Cc: Max Reitz <mreitz@redhat.com>
> ---
>   docs/qcow2-cache.txt | 163 +++++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 163 insertions(+)
>   create mode 100644 docs/qcow2-cache.txt
>
> diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
> new file mode 100644
> index 0000000..7476f73
> --- /dev/null
> +++ b/docs/qcow2-cache.txt
> @@ -0,0 +1,163 @@

[snip]

> +Reducing the memory usage
> +-------------------------
> +It is possible to clean unused cache entries in order to reduce the
> +memory usage during periods of low I/O activity.
> +
> +The parameter "cache-clean-interval" defines an interval (in seconds)
> +after which all unused cache entries are removed from memory.
> +
> +This example removes all unused cache entries every 15 minutes:
> +
> +   -drive file=hd.qcow2,cache-clean-interval=900
> +
> +If unset, the default value for this parameter is 0 and it disables
> +this feature.
> +
> +Note that this functionality currently relies on the MADV_DONTNEED
> +argument for madvise() to actually free the memory, so it is not
> +useful in systems that don't follow that behavior.

I'd like a definition of what "unused" means (I know because of this 
series, but I wouldn't know just from this document).

Max