Re: [Qemu-devel] [PATCH RFC for-2.6 0/3] block: Add meta dirty bitmap for migration/persistence

[Fam Zheng <famz@redhat.com>]

On Mon, 12/07 18:47, John Snow wrote:
> 
> 
> On 12/07/2015 12:59 AM, Fam Zheng wrote:
> > Vladimir,
> > 
> > This is what I propose to implement meta bitmap. It's implemented in the
> > HBitmap level to be more efficient, and the interface slightly varies too.
> > 
> 
> I missed it: What was wrong with Vladimir's approach / what are the
> benefits of this approach?

The only real difference with this series is, only actual bit toggling will
mark meta dirty. Vladimir's approach was in BdrvDirtyBitmap level which can't
tell bit toggling from repetitive bit set/unset. This is from his patch:

@@ -3390,6 +3428,9 @@ void bdrv_set_dirty_bitmap(BdrvDirtyBitmap *bitmap,
 {
     assert(bdrv_dirty_bitmap_enabled(bitmap));
     hbitmap_set(bitmap->bitmap, cur_sector, nr_sectors);
+    if (bitmap->meta_bitmap) {
+        hbitmap_set(bitmap->meta_bitmap, cur_sector, nr_sectors);
+    }
 }

 void bdrv_reset_dirty_bitmap(BdrvDirtyBitmap *bitmap,
@@ -3397,6 +3438,9 @@ void bdrv_reset_dirty_bitmap(BdrvDirtyBitmap *bitmap,
 {
     assert(bdrv_dirty_bitmap_enabled(bitmap));
     hbitmap_reset(bitmap->bitmap, cur_sector, nr_sectors);
+    if (bitmap->meta_bitmap) {
+        hbitmap_set(bitmap->meta_bitmap, cur_sector, nr_sectors);
+    }
 }

 void bdrv_clear_dirty_bitmap(BdrvDirtyBitmap *bitmap)

> 
> > I'd like to use these operations to make dirty bitmap persistence more
> > efficient too: unchanged dirty bits don't need to be flushed to disk. So I'm
> > posting this as a separate series for a common base for both sides.
> > 
> 
> This is a reasonable use of the meta-bitmap strategy in general.
> 
> Keep in mind Vladimir's approach to Meta bitmaps used a different
> granularity such that 1 physical bit implied 1 sector needed to be
> re-transmitted.

Yes, I can fix the meta bitmap granularity.

> 
> A meta-bitmap that keeps track of disk flushes may require a different
> granularity than one used for migration.
> 
> > Posting as RFC as 2.6 dev phase is just starting, we can still tweak the
> > interface and/or implementation to fit the need.
> > 
> > Fam Zheng (3):
> >   HBitmap: Introduce "meta" bitmap to track bit changes
> >   tests: Add test code for meta bitmap
> >   block: Support meta dirty bitmap
> > 
> >  block.c                | 46 ++++++++++++++++++++++++++++++-
> >  block/mirror.c         |  3 +-
> >  blockdev.c             |  3 +-
> >  include/block/block.h  | 11 ++++++++
> >  include/qemu/hbitmap.h |  7 +++++
> >  migration/block.c      |  2 +-
> >  tests/test-hbitmap.c   | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  util/hbitmap.c         | 22 +++++++++++++++
> >  8 files changed, 164 insertions(+), 4 deletions(-)
> >