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

[Christoph Hellwig]

On Sat, Dec 11, 2010 at 12:50:20PM +0000, Paul Brook wrote:
> > It's guest visible state, so it must not change due to migrations.  For
> > the current implementation all values for it work anyway - if it's
> > smaller than the block size we'll zero out the remainder of the block.
> 
> That sounds wrong. Surely we should leave partial blocks untouched.

While zeroing them is not required for qemu, the general semantics of
the XFS ioctl require it.  It punches a hole, which means it's makes the
new area equivalent to a hole create by truncating a file to a larger
size and then only writing at the larger offset.  The semantics for a
hole in all Unix filesystems is that we read back zeroes from them.
If we write into a sparse file at a not block aligned offset the
zeroing of the partial block also happens.