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

[Christoph Hellwig]

On Thu, Dec 02, 2010 at 01:12:13PM +0100, Kevin Wolf wrote:
> >      DEFINE_PROP_UINT16("physical_block_size", _state,                   \
> >                         _conf.physical_block_size, 512),                 \
> >      DEFINE_PROP_UINT16("min_io_size", _state, _conf.min_io_size, 0),  \
> > -    DEFINE_PROP_UINT32("opt_io_size", _state, _conf.opt_io_size, 0)
> > +    DEFINE_PROP_UINT32("opt_io_size", _state, _conf.opt_io_size, 0), \
> > +    DEFINE_PROP_UINT32("discard_granularity", _state, \
> > +                       _conf.discard_granularity, 0)
> 
> Is there no way to get this value automatically?
> 
> At least for non-raw images (and it should be trivial to implement this
> in qcow2) I guess we'll want to set it to the cluster size of the image
> instead of requiring the user to set this value.

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.