Re: [Qemu-devel] [PATCH 1/6] libqblock APIs

[Paolo Bonzini <pbonzini@redhat.com>]

Il 03/09/2012 15:56, Eric Blake ha scritto:
> Exactly how does the *pnum argument work?  This interface looks like it
> isn't fully thought out yet.  Either I want to know if a chunk of
> sectors is allocated (I supply start and length of sectors to check),
> regardless of how many sectors beyond that point are also allocated
> (pnum makes no sense);

pnum makes sense if the [start, start+length) range includes both
allocated and unallocated sectors.

> or I want to know how many sectors are allocated
> from a given point (I supply start, and the function returns length, so
> nb_sectors makes no sense).

This operation could be O(number of blocks in disk) worst case, so it
makes sense to provide nb_sectors as an upper bound.  nb_sectors is
typically dictated by the size of your buffer.

That said, QEMU's internal bdrv_is_allocated function does have one not
entirely appealing property: the block at start + *pnum might have the
same state as the block at start + *pnum - 1, even if *pnum < length.
We may want to work around this in libqblock, but we could also simply
document it.

Paolo

> Either way, I think you are supplying too
> many parameters for how I envision checking for allocated sectors.