[Qemu-devel] Re: [PATCH 1/5] block: add discard support

[Paolo Bonzini]

On 12/10/2010 02:38 PM, Christoph Hellwig wrote:
> if it's smaller than the block size we'll zero out the remainder of
> the block.

I think it should fail at VM startup time, or even better do nothing at all.

When you write in the middle of an absent block, and a partially-zero 
block is created, this is not visible: a read cannot see the difference 
between "all zeros because it's sparse" and "all zeros because it's zero".

If I ask you to (optionally) punch a 1kb hole but all you can do is 
punch a 2kb hole, I do care about the second kilobyte of data.  Since 
the hole punching of bdrv_discard is completely optional, it should not 
be done in this case.

Paolo