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Re: [Qemu-block] [PATCH V2 1/8] docs: add compress format extension to q

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH V2 1/8] docs: add compress format extension to qcow2 spec
Date: Mon, 10 Jul 2017 09:31:02 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 07/10/2017 08:50 AM, Kevin Wolf wrote:

> We have regretted enough things that were defined a bit sloppily just
> for convenience that I'd rather not make this mistake again when we can
> avoid it.

Agreed.  Another consideration: defining redundant information is
trickier to maintain because correct implementations have to check that
both points are consistent with one another.

>>>> +         17 - 19:  Reserved for future use, must be zero.
>>> We don't need this for now because byte 16 will be the final one in this
>>> struct.
>>>> +         20 - 23:  extra_data_size
>>>> +                   Size of compress format specific extra data.
>>>> +                   For now, as no format specifies extra data,
>>>> +                   extra_data_size is reserved and should be zero.
>>>> +
>>>> +        variable:  extra_data
>>>> +                   Extra data with additional parameters for the compress
>>>> +                   format, occupying extra_data_size bytes.
>>> extra_data_size isn't necessary because we already have the length of
>>> the header extension, so we know how long the whole thing is.
>>> extra_data isn't necessary because we'll just add new fields at the end
>>> if we want to add something.
>> I was asked to add it altough it is redundant.
> Who asked this, and did they have a good reason for it?

May have been me, or may have been implied by my comments, but I agree
that the overall extension header covers this, so we don't need a
redundant form.  So as long as the overall extension header length is
good enough, I'm okay avoiding redundancies.

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

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