[Qemu-devel] Feature request for qcow v2

[Anthony Liguori]

Hi,

I've been playing with some code to implement an HTTP block device in 
QEMU.  I'll post when I've got something working.  As I am building this 
though, I thought of something that would be nice to have for qcow v2.

What I'd like is essentially a copy-on-read flag.  If this flag is set, 
and there is a base image, then whenever a read occurs to an unallocated 
sector, the sector is copied from the base (similar to a write) and the 
sector is allocated.

The idea is that copy-on-read is useful when read is really slow (for 
instance, with an HTTP block device).  This way, you could essentially 
always use qcow + HTTP so that your block device was locally cached.  It 
seems a bit more elegant than trying to implement a caching mechanism in 
the HTTP block device.

Regards,

Anthony Liguori