Re: [Qemu-devel] [PATCH 4/7] block: initial VHDX driver support framework - supports open and probe

[Stefan Hajnoczi]

On Thu, Mar 07, 2013 at 10:23:54AM -0500, Jeff Cody wrote:
> On Thu, Mar 07, 2013 at 03:30:44PM +0100, Stefan Hajnoczi wrote:
> > On Wed, Mar 06, 2013 at 09:48:11AM -0500, Jeff Cody wrote:
> > > +    ret = bdrv_pread(bs->file, s->bat_offset, s->bat, s->bat_rt.length);
> > > +
> > > +    for (i = 0; i < s->bat_entries; i++) {
> > > +        le64_to_cpus(&s->bat[i]);
> > > +    }
> > 
> > How does BAT size scale when the image size is increased?  QCOW2 and QED
> > use caches for metadata that would be too large or wasteful to keep in
> > memory.
> 
> The BAT size is dependent on the virtual disk size, and the block
> size.  The block size is allowed to range from 1MB - 256MB.  There is
> one BAT entry per block.
> 
> In practice, the large block size keeps the BAT entry table reasonable
> (for a 127GB file, the block size created by Hyper-V is 32MB, so the
> table is pretty small - 32KB).
> 
> However, I don't see anything in the spec that forces the block size
> to be larger, for a large virtual disk size.  So for the max size of
> 64TB, and the smallest block size of 1MB, keeping the BAT in memory
> would indeed be excessive.
> 
> I'll re-read the spec, and see if there is anything that ties the
> block size and virtual size together.  If not, I'll have to add
> caching.

BTW the qcow2 cache code can be reused.

Stefan