Re: [RFC PATCH v3 05/27] qcow2: Document the Extended L2 Entries feature

[Eric Blake]

On 12/22/19 5:36 AM, Alberto Garcia wrote:
> Subcluster allocation in qcow2 is implemented by extending the
> existing L2 table entries and adding additional information to
> indicate the allocation status of each subcluster.
>
> This patch documents the changes to the qcow2 format and how they
> affect the calculation of the L2 cache size.
>
> Signed-off-by: Alberto Garcia <address@hidden>
> ---

> @@ -437,7 +445,7 @@ cannot be relaxed without an incompatible layout change).
>   Given an offset into the virtual disk, the offset into the image file can be
>   obtained as follows:
>   
> -    l2_entries = (cluster_size / sizeof(uint64_t))
> +    l2_entries = (cluster_size / sizeof(uint64_t))        [*]
>   
>       l2_index = (offset / cluster_size) % l2_entries
>       l1_index = (offset / cluster_size) / l2_entries
> @@ -447,6 +455,8 @@ obtained as follows:
>   
>       return cluster_offset + (offset % cluster_size)
>   
> +    [*] this changes if Extended L2 Entries are enabled, see next section

> +The size of an extended L2 entry is 128 bits so the number of entries per table
> +is calculated using this formula:
> +
> +    l2_entries = (cluster_size / (2 * sizeof(uint64_t)))

Is it worth unifying these statements by writing:

l2_entries = (cluster_size / ((1 + extended_l2) * sizeof(uint64_t)))

or is that too confusing?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org