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Re: [Qemu-devel] [PATCH v4 4/6] introduce new vma archive format


From: Kevin Wolf
Subject: Re: [Qemu-devel] [PATCH v4 4/6] introduce new vma archive format
Date: Thu, 21 Feb 2013 14:03:28 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Thu, Feb 21, 2013 at 08:20:28AM +0000, Dietmar Maurer wrote:
> Honestly, 64k*4G = 128T is not a limit for me. And we still have one byte 
> reserved,
> so we can have up to 1P per image, and up to 253 images.
> 
> For now I have no plans to backup such large VMs.

640k ought to be enough for anybody?

Code is easy enough to change that "works for now" can be good enough.
Changing file formats and external interfaces is hard, though, so better
get it right the first time. I don't want to get patches for a new
format in a year or two just because 128T was enough for the first few
users.

> > Hint - look at how the qcow2 file format is specified.  You need a lot more
> > details - enough that someone could independently implement a program to
> > read and create vma images that would be compatible with what your
> > implementation produces.
> 
> The format is really simple, you have the definitions in the header file,
> and a reference implementation. I am quite sure any experienced developer 
> can write an implementation in a few hours.
> 
> We do not talk about an extremely complex format like 'qcow2' here.

This is not an excuse for lacking details in the spec. Quite the
opposite. If someone has to look at your code in order to implement the
format, you have failed as a spec writer.

Kevin



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