Re: [Qemu-devel] [PATCH] Add qcow2 documentation

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Mar 8, 2011 at 1:31 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> Am 08.03.2011 14:13, schrieb Stefan Hajnoczi:
>> On Tue, Mar 8, 2011 at 11:47 AM, Kevin Wolf <kwolf@redhat.com> wrote:
>>> +         20 - 23:   cluster_bits
>>> +                    Number of bits that are used for addressing an offset
>>> +                    within a cluster (1 << cluster_bits is the cluster size)
>>> +
>>> +         24 - 31:   size
>>> +                    Virtual disk size in bytes
>>
>> Any constraints on these two fields that should be mentioned?
>
> For the size not that I'm aware of.
>
> For cluster_bits qemu restricts it to 512 <= cluster_size <= 2 MB. I
> think we should add 512 as a lower limit, anything smaller doesn't make
> sense and steals us bits that we want to use for flags.
>
> The 2 MB are more of an implementation limitation. Would you mention it
> here? The format shouldn't have any problem with larger sizes.

It could be mentioned as an explicit implementation limit so that a
third party implementing qcow2 support from scratch doesn't use the
format in ways that won't work with QEMU's implementation.

Stefan