Signed-off-by: Leonid Bloch <lbloch@janustech.com>
---
 docs/qcow2-cache.txt |  3 +++
 qemu-options.hx      | 15 +++++++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 8a09a5cc5f..9d261b7da9 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -130,6 +130,9 @@ There are a few things that need to be taken into account:
    memory as possible to the L2 cache before increasing the refcount
    cache size.
 
+- All three "l2-cache-size", "refcount-cache-size", and "cache-size" options
+  can not be set simultaneously.
+
 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
diff --git a/qemu-options.hx b/qemu-options.hx
index b1bf0f485f..ef0706c359 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -752,15 +752,22 @@ image file)
 
 @item cache-size
 The maximum total size of the L2 table and refcount block caches in bytes
-(default: 1048576 bytes or 8 clusters, whichever is larger)
 
 @item l2-cache-size
-The maximum size of the L2 table cache in bytes
-(default: 4/5 of the total cache size)
+The maximum size of the L2 table cache.
+(default: if cache-size is not defined - 1048576 bytes or 8 clusters,
+whichever is larger; if cache-size is defined and is large enough to
+accommodate enough L2 cache to cover the entire virtual size of the image plus
+the minimal amount of refcount cache - enough to cover the entire image;
+if cache-size is defined and is not large enough - as much as possible while
+leaving space for the needed refcount cache)
 
 @item refcount-cache-size
 The maximum size of the refcount block cache in bytes
-(default: 1/5 of the total cache size)
+(default: 4 times the cluster size, or if cache-size is defined and is large
+enough to accommodate enough L2 cache to cover the entire virtual size of the
+image plus the minimal amount of refcount cache - the part of cache-size which
+is left after allocating the full L2 cache)
 
 @item cache-clean-interval
 Clean unused entries in the L2 and refcount caches. The interval is in seconds.
-- 
2.14.1

On 07/24/2018 03:03 PM, Leonid Bloch wrote:
> Signed-off-by: Leonid Bloch <lbloch@janustech.com>
> ---
>   docs/qcow2-cache.txt |  3 +++
>   qemu-options.hx      | 15 +++++++++++----
>   2 files changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
> index 8a09a5cc5f..9d261b7da9 100644
> --- a/docs/qcow2-cache.txt
> +++ b/docs/qcow2-cache.txt
> @@ -130,6 +130,9 @@ There are a few things that need to be taken into account:
>      memory as possible to the L2 cache before increasing the refcount
>      cache size.
>   
> +- All three "l2-cache-size", "refcount-cache-size", and "cache-size" options
> +  can not be set simultaneously.

Reads awkwardly; maybe:

At most two of "l2-cache-size", "refcount-cache-size", and "cache-size" 
can be set together on the command line (the omitted options are 
calculated accordingly).

> +++ b/qemu-options.hx
> @@ -752,15 +752,22 @@ image file)
>   
>   @item cache-size
>   The maximum total size of the L2 table and refcount block caches in bytes
> -(default: 1048576 bytes or 8 clusters, whichever is larger)
>   
>   @item l2-cache-size
> -The maximum size of the L2 table cache in bytes
> -(default: 4/5 of the total cache size)
> +The maximum size of the L2 table cache.
> +(default: if cache-size is not defined - 1048576 bytes or 8 clusters,
> +whichever is larger; if cache-size is defined and is large enough to
> +accommodate enough L2 cache to cover the entire virtual size of the image plus
> +the minimal amount of refcount cache - enough to cover the entire image;
> +if cache-size is defined and is not large enough - as much as possible while
> +leaving space for the needed refcount cache)

Wordy; maybe:

(default: if cache-size is not defined - 1048576 bytes or 8 clusters, 
whichever is larger; otherwise, as large as possible within cache-size 
while still permitting the requested or minimum refcount cache size).

>   
>   @item refcount-cache-size
>   The maximum size of the refcount block cache in bytes
> -(default: 1/5 of the total cache size)
> +(default: 4 times the cluster size, or if cache-size is defined and is large
> +enough to accommodate enough L2 cache to cover the entire virtual size of the
> +image plus the minimal amount of refcount cache - the part of cache-size which
> +is left after allocating the full L2 cache)

Maybe:

(default: 4 times the cluster size, plus any portion of a specified 
cache-size left over after sizing the L2 cache large enough to cover the 
entire image)

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org