Re: [Qemu-devel] Add support for new image type

[Nate Bushman]

You make really good points on opening up the image file format.  I see real 
value there in that it would foster development of other solutions around that 
format.  The problem in this case is that StorageCraft's backup image format 
uses proprietary compression and encryption, and so, regrettably, we can't 
(currently) open up that code.  This is an interesting lesson in retrospect.  
We could possibly publish a spec of the format when encryption and compression 
aren't used, but in practice this doesn't happen (compression speeds up the 
backups because the CPU time it takes to compress is far less than the time 
saved by writing the smaller amount of compressed data to the disk), so because 
compression saves both time and space nearly all of our customers use the 
compression options at a minimum, and many also use the encryption.

Interesting and educational discussion.  Thanks for your help Kevin!

Nate

-----Original Message-----
From: Kevin Wolf [mailto:address@hidden 
Sent: Thursday, March 08, 2012 8:18 AM
To: Kai Meyer
Cc: address@hidden; Nate Bushman
Subject: Re: [Qemu-devel] Add support for new image type

(Sorry, missed the rest of this thread as I was expecting to be kept in the CC 
list)

Am 01.03.2012 20:45, schrieb Kai Meyer:
> On 03/01/2012 08:03 AM, Kevin Wolf wrote:
>> Am 29.02.2012 22:52, schrieb Kai Meyer:
>>> Is it possible to extend qemu to support a new image type? I have an 
>>> image type that is ready for consumption and I'm looking for the 
>>> integration point between qemu and the new image format.
>> Which image format do you want to get integrated?
>>
>> Have a look at block/qcow2.c to get an idea of what a qemu block 
>> driver looks like. At the bottom of the file there is a struct that 
>> contains function pointers to all exported functions, so this is 
>> usually a good place to start exploring a driver.
>>
>> Kevin
> 
> Great, this is exactly what we're after. I work for StorageCraft, and 
> we would like to figure out some way to allow qemu to directly consume 
> our image-based backups. It would provide us with user-space mounting 
> (via
> libguestfs) as well as booting VMs directly from Backup images. We 
> already have a proprietary image access library that provides 
> block-wise access to our image files. I have been able to scratch 
> together a proof of concept already, which I am really pleased with.
> 
> I see only two roadblocks for which I don't have immediate answers for.
> 
> 1) Licensing
> Is it possible to license our contributions in such a way that we do 
> not need to open the source code of our image access library?

As other people already said, there's no way that we would accept this.

However, what do you really gain from keeping your file format secret? I can't 
imagine that the information required for a (possibly read-only) properly 
licensed block driver contains anything of what makes the core of your 
products. I can understand that you want to keep sophisticated source code 
closed, but I really can't for file formats.

So if you tried to get permission to publish a specification of your image 
format and wrote a minimal qemu block driver from scratch, I believe this would 
be best for everyone.

> 2) External dependency on our image access library.
> We do not want to force qemu to require our image access library to be 
> present to build. Would it be better to do a conditional build 
> (./configure --with-spf) or a run-time check for our image access library?

A conditional build would be required and I'd consider a run-time check 
optional.

Kevin