Signed-off-by: Leonid Bloch <lbloch@janustech.com>
---
 docs/qcow2-cache.txt | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
index 8a09a5cc5f..a0a1267482 100644
--- a/docs/qcow2-cache.txt
+++ b/docs/qcow2-cache.txt
@@ -97,9 +97,9 @@ need:
    l2_cache_size = disk_size_GB * 131072
    refcount_cache_size = disk_size_GB * 32768
 
-QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount
-cache of 256KB (262144 bytes), so using the formulas we've just seen
-we have
+QEMU has a default L2 cache of 1MB (1048576 bytes) or 8 clusters (whichever
+is larger) and a refcount cache of 256KB (262144 bytes), so using the
+formulas we've just seen we have (assuming the L2 cache is 1MB):
 
    1048576 / 131072 = 8 GB of virtual disk covered by that cache
     262144 /  32768 = 8 GB
-- 
2.14.1

On 07/24/2018 07:17 AM, Leonid Bloch wrote:
> Signed-off-by: Leonid Bloch <lbloch@janustech.com>
> ---
>   docs/qcow2-cache.txt | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
> index 8a09a5cc5f..a0a1267482 100644
> --- a/docs/qcow2-cache.txt
> +++ b/docs/qcow2-cache.txt
> @@ -97,9 +97,9 @@ need:
>      l2_cache_size = disk_size_GB * 131072
>      refcount_cache_size = disk_size_GB * 32768
>   
> -QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount
> -cache of 256KB (262144 bytes), so using the formulas we've just seen
> -we have
> +QEMU has a default L2 cache of 1MB (1048576 bytes) or 8 clusters (whichever
> +is larger) and a refcount cache of 256KB (262144 bytes), so using the

Looks suspicious; isn't the refcount cache size also dependent on the 
cluster size?

> +formulas we've just seen we have (assuming the L2 cache is 1MB):
>   
>      1048576 / 131072 = 8 GB of virtual disk covered by that cache
>       262144 /  32768 = 8 GB
> 

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

   On 07/24/2018 06:20 PM, Eric Blake wrote:

     On 07/24/2018 07:17 AM, Leonid Bloch wrote:

     Signed-off-by: Leonid Bloch [1]<lbloch@janustech.com>
     ---
       docs/qcow2-cache.txt | 6 +++---
       1 file changed, 3 insertions(+), 3 deletions(-)
     diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
     index 8a09a5cc5f..a0a1267482 100644
     --- a/docs/qcow2-cache.txt
     +++ b/docs/qcow2-cache.txt
     @@ -97,9 +97,9 @@ need:
          l2_cache_size = disk_size_GB * 131072
          refcount_cache_size = disk_size_GB * 32768
       -QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount
     -cache of 256KB (262144 bytes), so using the formulas we've just
     seen
     -we have
     +QEMU has a default L2 cache of 1MB (1048576 bytes) or 8 clusters
     (whichever
     +is larger) and a refcount cache of 256KB (262144 bytes), so using
     the

     Looks suspicious; isn't the refcount cache size also dependent on
     the cluster size?

   Yes, it's 4*cluster_size. But I realize now that the text here speaks
   about the default sizes, which means with with 64KB clusters. So this
   fix is not necessary. Will drop this patch.

     +formulas we've just seen we have (assuming the L2 cache is 1MB):
            1048576 / 131072 = 8 GB of virtual disk covered by that cache
           262144 /  32768 = 8 GB

References

   1. mailto:lbloch@janustech.com

On 07/24/2018 07:17 AM, Leonid Bloch wrote:
> Signed-off-by: Leonid Bloch <lbloch@janustech.com>
> ---
>   docs/qcow2-cache.txt | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)

Updating the subject line, since this doc fix is appropriate for 3.0, if 
it is correct.

> 
> diff --git a/docs/qcow2-cache.txt b/docs/qcow2-cache.txt
> index 8a09a5cc5f..a0a1267482 100644
> --- a/docs/qcow2-cache.txt
> +++ b/docs/qcow2-cache.txt
> @@ -97,9 +97,9 @@ need:
>      l2_cache_size = disk_size_GB * 131072
>      refcount_cache_size = disk_size_GB * 32768
>   
> -QEMU has a default L2 cache of 1MB (1048576 bytes) and a refcount
> -cache of 256KB (262144 bytes), so using the formulas we've just seen
> -we have
> +QEMU has a default L2 cache of 1MB (1048576 bytes) or 8 clusters (whichever
> +is larger) and a refcount cache of 256KB (262144 bytes), so using the
> +formulas we've just seen we have (assuming the L2 cache is 1MB):
>   
>      1048576 / 131072 = 8 GB of virtual disk covered by that cache
>       262144 /  32768 = 8 GB
> 

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