Re: [Qemu-devel] [PATCH 07/20] block: allow customizing the granularity of the dirty bitmap

[Paolo Bonzini <pbonzini@redhat.com>]

----- Messaggio originale -----
> Da: "Eric Blake" <eblake@redhat.com>
> A: "Paolo Bonzini" <pbonzini@redhat.com>
> Cc: qemu-devel@nongnu.org, kwolf@redhat.com, stefanha@redhat.com, jcody@redhat.com
> Inviato: Venerdì, 14 dicembre 2012 22:27:04
> Oggetto: Re: [PATCH 07/20] block: allow customizing the granularity of the dirty bitmap
> 
> On 12/12/2012 06:46 AM, Paolo Bonzini wrote:
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > ---
> >  block-migration.c |  5 +++--
> >  block.c           | 17 ++++++++++-------
> >  block.h           |  5 +----
> >  block/mirror.c    | 14 ++++----------
> >  qapi-schema.json  |  4 +++-
> >  5 files changed, 21 insertions(+), 24 deletions(-)
> > 
> 
> > @@ -4218,16 +4220,17 @@ void *qemu_blockalign(BlockDriverState *bs,
> > size_t size)
> >      return qemu_memalign((bs && bs->buffer_alignment) ?
> >      bs->buffer_alignment : 512, size);
> >  }
> >  
> > -void bdrv_set_dirty_tracking(BlockDriverState *bs, int enable)
> > +void bdrv_set_dirty_tracking(BlockDriverState *bs, int
> > granularity)
> >  {
> >      int64_t bitmap_size;
> >  
> > -    if (enable) {
> > -        if (!bs->dirty_bitmap) {
> > -            bitmap_size = (bdrv_getlength(bs) >>
> > BDRV_SECTOR_BITS);
> > -            bs->dirty_bitmap = hbitmap_alloc(bitmap_size,
> > -
> >                                             BDRV_LOG_SECTORS_PER_DIRTY_CHUNK);
> > -        }
> > +    assert((granularity & (granularity - 1)) == 0);
> > +
> > +    if (granularity) {
> > +        granularity >>= BDRV_SECTOR_BITS;
> 
> Given that granularity is specified in bytes, does it make sense for
> a user to want a granularity of 4G?

No, the point of the HBitmap is to allow small granularities.  Anything
bigger than 1 or 2 MB makes little sense.

Paolo