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Re: [Qemu-devel] [PATCH RFC v3 05/14] block: add meta bitmaps

From: Vladimir Sementsov-Ogievskiy
Subject: Re: [Qemu-devel] [PATCH RFC v3 05/14] block: add meta bitmaps
Date: Thu, 19 Feb 2015 14:43:04 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 19.02.2015 02:45, John Snow wrote:

On 02/18/2015 09:00 AM, Vladimir Sementsov-Ogievskiy wrote:
Meta bitmap is a 'dirty bitmap' for the BdrvDirtyBitmap. It tracks
changes (set/unset) of this BdrvDirtyBitmap. It is needed for live
migration of block dirty bitmaps.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
  block.c               | 40 ++++++++++++++++++++++++++++++++++++++++
  include/block/block.h |  5 +++++
  2 files changed, 45 insertions(+)

diff --git a/block.c b/block.c
index a127fd2..aaa08b8 100644
--- a/block.c
+++ b/block.c
@@ -58,9 +58,15 @@
   * (3) successor is set: frozen mode.
* A frozen bitmap cannot be renamed, deleted, anonymized, cleared, set,
   *     or enabled. A frozen bitmap can only abdicate() or reclaim().
+ *
+ * Meta bitmap:
+ * Meta bitmap is a 'dirty bitmap' for the BdrvDirtyBitmap. It tracks changes + * (set/unset) of this BdrvDirtyBitmap. It is needed for live migration of
+ * block dirty bitmaps.
  struct BdrvDirtyBitmap {
HBitmap *bitmap; /* Dirty sector bitmap implementation */
+    HBitmap *meta_bitmap;       /* Meta bitmap */
BdrvDirtyBitmap *successor; /* Anonymous child; implies frozen status */
      char *name;                 /* Optional non-empty unique ID */
int64_t size; /* Size of the bitmap (Number of sectors) */ @@ -5398,6 +5404,31 @@ void bdrv_dirty_bitmap_make_anon(BdrvDirtyBitmap *bitmap)
      bitmap->name = NULL;

+HBitmap *bdrv_create_meta_bitmap(BdrvDirtyBitmap *bitmap,
+                                        uint64_t granularity)
+    uint64_t sector_granularity;
+    assert((granularity & (granularity - 1)) == 0);
+    granularity *= 8 * bdrv_dirty_bitmap_granularity(bitmap);
+    sector_granularity = granularity >> BDRV_SECTOR_BITS;
+    assert(sector_granularity);

The maths here could use a comment, I think.

the "granularity" field here actually describes the desired serialization buffer size; e.g. CHUNK_SIZE (1 << 20) or 1 MiB. This parameter should be renamed to explain what it's actually for. Something like "chunk_size" and a comment explaining that it is in bytes.


That said, let's talk about the default chunk size you're using in correlation with this function.

a CHUNK_SIZE of 1MiB here is going to lead us to, if we have a bitmap with the default granularity of 128 sectors\64KiB bytes, a granularity for the meta_bitmap of one billion sectors (1 << 30) or 512GiB.

That's going to be bigger than most drives entirely, which will generally lead us to only using a single "chunk" per drive. Which means we won't really get a lot of mileage out of the bulk/dirty phases most of the time.

It's wild to think about that the first 1,000,000,000 sectors or 512,000,000,000 bytes will all be represented by the first single bit in this bitmap. If a single hair on the drive changes, we resend the _entire_ bitmap, possibly over and over again. Will we ever make progress? Should we investigate a smaller chunk size?

Here's some quick mappings of chunk size (bytes) to effective meta_bitmap byte granularities, assuming the meta_bitmap is tracking a bitmap with the default granularity of 64KiB:

(1 << 20) 1MiB   -- 512GiB  // This is too high of a granularity
(1 << 17) 128KiB --  64GiB
(1 << 15) 32KiB  --  16GiB
(1 << 11) 2KiB   --   1GiB
(1 << 10) 1KiB   -- 512MiB
(1 << 9)  512B   -- 256MiB
(1 << 8)  256B   -- 128MiB
(1 << 5)  32 B   --  16MiB  // This is too small of a chunk size.
(1 << 1)   1 B   --   1MiB

We want to make the chunk sends efficient, but we also want to make sure that the dirty phase doesn't resend more data than it needs to, so we need to strike a balance here, no?

I think arguments could be made for most granularities between 128MiB through 1GiB. Anything outside of that is too lopsided, IMO.

What are your thoughts on this?
Ok, interesting thing to discuss.

My thoughts:
* the chunk size for block-migration is 1mb, than the bitmap (64kb granularity) for this chunk is 16bit=2bytes long. It's an intuitive reason for choosing the chunk size about 2 bytes. But in this case the data/metadata ratio is very bad (about 20bytes for the header of the chunk). So, taking the nearest value with adequate ratio gives (IMHO) '1kb -- 512mb': 20b/1k ~ 2%. Or 512b => 4%.

* for ndb+mirror migration scheme the default chunk is 64kb instead of 1mb. So the bitmap is more smaller. But the same reason of data/metadata ratio leads to 1kb chunk for dirty bitmap migration.

So, what about default to 1kb and additional parameter for migration (migration capabilities) to give the user a possibility of chose?

* Yes, in most of user cases the bitmap (64kb granularity) will be small (< 1mb). In these cases, I think, it would be better to send the data only in complete step, only once. (for exmaple, if pending <= 1mb, dosn't send anything in incremental phase). Live migration is actually needed only for migration of bitmaps for disks of several TBs size.

+    bitmap->meta_bitmap =
+        hbitmap_alloc(bitmap->size, ffsll(sector_granularity) - 1);
+    return bitmap->meta_bitmap;
+void bdrv_release_meta_bitmap(BdrvDirtyBitmap *bitmap)
+    if (bitmap->meta_bitmap) {
+        hbitmap_free(bitmap->meta_bitmap);
+        bitmap->meta_bitmap = NULL;
+    }
  BdrvDirtyBitmap *bdrv_create_dirty_bitmap(BlockDriverState *bs,
                                            uint32_t granularity,
                                            const char *name,
@@ -5532,6 +5563,9 @@ void bdrv_release_dirty_bitmap(BlockDriverState *bs, BdrvDirtyBitmap *bitmap)
              QLIST_REMOVE(bitmap, list);
+            if (bitmap->meta_bitmap) {
+                hbitmap_free(bitmap->meta_bitmap);
+            }
@@ -5659,6 +5693,9 @@ void bdrv_set_dirty_bitmap(BdrvDirtyBitmap *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,
@@ -5666,6 +5703,9 @@ void bdrv_reset_dirty_bitmap(BdrvDirtyBitmap *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)
diff --git a/include/block/block.h b/include/block/block.h
index c6a928d..f2c62f6 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -4,6 +4,7 @@
  #include "block/aio.h"
  #include "qemu-common.h"
  #include "qemu/option.h"
+#include "qemu/hbitmap.h"
  #include "block/coroutine.h"
  #include "block/accounting.h"
  #include "qapi/qmp/qobject.h"
@@ -487,6 +488,10 @@ void bdrv_dirty_bitmap_deserialize_zeroes(BdrvDirtyBitmap *bitmap, uint64_t start, uint64_t count);
  void bdrv_dirty_bitmap_deserialize_finish(BdrvDirtyBitmap *bitmap);

+HBitmap *bdrv_create_meta_bitmap(BdrvDirtyBitmap *bitmap,
+                                        uint64_t granularity);
+void bdrv_release_meta_bitmap(BdrvDirtyBitmap *bitmap);
  void bdrv_enable_copy_on_read(BlockDriverState *bs);
  void bdrv_disable_copy_on_read(BlockDriverState *bs);

Best regards,

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