Re: [Qemu-devel] [PATCH v6] spec: add qcow2 bitmaps extension specification

[John Snow <jsnow@redhat.com>]

On 01/11/2016 07:20 AM, Vladimir Sementsov-Ogievskiy wrote:
> 
> Are you sure? What about creation\last change dates, file links, user
> data, etc?
> For now, formally, current "For now, as no extra data is defined,
> extra_data_size is reserved and must be zero." is equal to such table,
> but provides more flexibility for future..

Oh, I see what you're trying to do.

In this case, perhaps we need a versioning system for the type-specific
data? We won't be able to just add data arbitrarily, we need to change
some field somewhere.

Maybe we can say something like...

"If extra_data_size is 0, there is no type-specific data and the version
of that data layout is 0. If extra_data_size is non-zero, the first byte
of the type-specific-data must be a version number greater than 0 that
indicates the layout of the data to follow.

For the Dirty Tracking bitmap type, only version 0 is currently valid."

This way it's explicit that data *could* show up for dirty tracking in
the future, but currently it does not.

--js