[sheepdog-users] qemu-img convert; vdi write and preallocation

[Valerio Pachera]

2013/3/24 Liu Yuan <namei.unix at gmail.com>:
> Actually there is way, but preallocation is not well documented.
> $ qemu-img create -o size=100M,preallocation=full sheepdog:vdi

Ok.
I don't i'm evere going to use it but it's nice to know it's possible.
Others may find it useful.

> The 'vdi write' isn't intelligent enough to detect all-zero chunks from
> the file descriptor.
> What we can do better is built-in this intelligent or add a new command
> 'vdi punch' similar to file systems 'punch hole' that punch zero chunks
> from data store. Which one is better?

I think it's better to make vdi write "intellingent" and let the
possibility to choose between the two possibility.
I would set ad default method the "intelligent" one.
This way, it also matches the default policy of creation of a vdi:
without --prealloc.

Note:
it's probably a bad idea to use the "intelligent vdi write" on a
pre-allocated vdi.
Is it?
Should collie report an error ot switch automatically to the
non-intelligent modality?