Re: [Qemu-devel] Re: KVM call agenda for Jan 25

[Dushyant Bansal]

On Saturday 29 January 2011 04:20 PM, Dushyant Bansal wrote:
> Or this: which is faster, qemu-img convert -f<format>  -O<format>
> <src-image>  <dst-image>  or cp<src-image>  <dst-image>?  What about for
> raw images, shouldn't that be the same speed as cp(1)?  Poke around
> the source code, profile it, understand what it's doing, think about
> ways to improve it.  No need to do everything, just doing part of this
> will give you background on QEMU's block layer.
>
> Contributing patches is a good way get up to speed and show your
> skills.  If time doesn't permit that, just think about the problem and
> how you intend to solve it, and feel free to bounce ideas off me.
>    
I explored 'qemu-img create and convert' and got a basic understanding 
of how they work.

cp faster than qemu-img convert

For raw->raw
In cp, it just copies all the disk blocks actually occupied by the file.
And, with qemu-img convert, it checks all the sectors and copy those, 
which contains atleast one non-NUL byte.
The better performance of cp over qemu-img convert is the result of 
overhead of this checking.

I tried a few variations:
1. just copy all the sectors without checking
So, actual size becomes equal to virtual size.
2. In is_allocated_sectors,out of n sectors, if any sector has a non-NUL 
byte then break and copy all n sectors.
As expected, resultant raw image was quite large in size.

Looking forward to your comments.

Thanks,
Dushyant