[PATCH] qcow2: Expose bitmaps' size during measure

[Eric Blake]

It's useful to know how much space can be occupied by qcow2 persistent
bitmaps, even though such metadata is unrelated to the guest-visible
data.  Report this value as an additional field.

Reported-by: Nir Soffer <address@hidden>
Signed-off-by: Eric Blake <address@hidden>
---

This is independent from my 'qemu-img convert --bitmaps' series, but
highly related.  As an example, if I create a 100M image, then 2
persistent bitmaps, all with default cluster/granularity sizing, I now
see:

$ ./qemu-img measure -f qcow2 -O qcow2 build/img.top
required size: 52756480
fully allocated size: 105185280
bitmaps size: 327680

which argues that I should allocate 52756480 + 327680 bytes prior to
attempting qemu-img convert --bitmaps to a pre-sized destination.

If we like the idea, I probably need to submit a 2/1 patch adding
iotest coverage of the new measurement.

 qapi/block-core.json | 15 ++++++++++-----
 block/qcow2.c        | 25 +++++++++++++++++++++++++
 qemu-img.c           |  3 +++
 3 files changed, 38 insertions(+), 5 deletions(-)

diff --git a/qapi/block-core.json b/qapi/block-core.json
index 943df1926a91..b47c6d69ba27 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -633,18 +633,23 @@
 # efficiently so file size may be smaller than virtual disk size.
 #
 # The values are upper bounds that are guaranteed to fit the new image file.
-# Subsequent modification, such as internal snapshot or bitmap creation, may
-# require additional space and is not covered here.
+# Subsequent modification, such as internal snapshot or further bitmap
+# creation, may require additional space and is not covered here.
 #
-# @required: Size required for a new image file, in bytes.
+# @required: Size required for a new image file, in bytes, when copying just
+#            guest-visible contents.
 #
 # @fully-allocated: Image file size, in bytes, once data has been written
-#                   to all sectors.
+#                   to all sectors, when copying just guest-visible contents.
+#
+# @bitmaps: Additional size required for bitmap metadata not directly used
+#           for guest contents, when that metadata can be copied in addition
+#           to guest contents. (since 5.1)
 #
 # Since: 2.10
 ##
 { 'struct': 'BlockMeasureInfo',
-  'data': {'required': 'int', 'fully-allocated': 'int'} }
+  'data': {'required': 'int', 'fully-allocated': 'int', '*bitmaps': 'int'} }

 ##
 # @query-block:
diff --git a/block/qcow2.c b/block/qcow2.c
index b524b0c53f84..eba6c2511e60 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -4657,6 +4657,7 @@ static BlockMeasureInfo *qcow2_measure(QemuOpts *opts, 
BlockDriverState *in_bs,
     PreallocMode prealloc;
     bool has_backing_file;
     bool has_luks;
+    uint64_t bitmaps_size = 0; /* size occupied by bitmaps in in_bs */

     /* Parse image creation options */
     cluster_size = qcow2_opt_get_cluster_size_del(opts, &local_err);
@@ -4732,6 +4733,8 @@ static BlockMeasureInfo *qcow2_measure(QemuOpts *opts, 
BlockDriverState *in_bs,

     /* Account for input image */
     if (in_bs) {
+        BdrvDirtyBitmap *bm;
+        size_t bitmap_overhead = 0;
         int64_t ssize = bdrv_getlength(in_bs);
         if (ssize < 0) {
             error_setg_errno(&local_err, -ssize,
@@ -4739,6 +4742,26 @@ static BlockMeasureInfo *qcow2_measure(QemuOpts *opts, 
BlockDriverState *in_bs,
             goto err;
         }

+        FOR_EACH_DIRTY_BITMAP(in_bs, bm) {
+            if (bdrv_dirty_bitmap_get_persistence(bm)) {
+                uint64_t bmsize = bdrv_dirty_bitmap_size(bm);
+                uint32_t granularity = bdrv_dirty_bitmap_granularity(bm);
+                const char *name = bdrv_dirty_bitmap_name(bm);
+                uint64_t bmclusters = DIV_ROUND_UP(bmsize / granularity
+                                                   / CHAR_BIT, cluster_size);
+
+                /* Assume the entire bitmap is allocated */
+                bitmaps_size += bmclusters * cluster_size;
+                /* Also reserve space for the bitmap table entries */
+                bitmaps_size += ROUND_UP(bmclusters * sizeof(uint64_t),
+                                         cluster_size);
+                /* Guess at contribution to bitmap directory size */
+                bitmap_overhead += ROUND_UP(strlen(name) + 24,
+                                            sizeof(uint64_t));
+            }
+        }
+        bitmaps_size += ROUND_UP(bitmap_overhead, cluster_size);
+
         virtual_size = ROUND_UP(ssize, cluster_size);

         if (has_backing_file) {
@@ -4795,6 +4818,8 @@ static BlockMeasureInfo *qcow2_measure(QemuOpts *opts, 
BlockDriverState *in_bs,
      * still counted.
      */
     info->required = info->fully_allocated - virtual_size + required;
+    info->has_bitmaps = !!bitmaps_size;
+    info->bitmaps = bitmaps_size;
     return info;

 err:
diff --git a/qemu-img.c b/qemu-img.c
index 6541357179c2..d900bde89911 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -5084,6 +5084,9 @@ static int img_measure(int argc, char **argv)
     if (output_format == OFORMAT_HUMAN) {
         printf("required size: %" PRIu64 "\n", info->required);
         printf("fully allocated size: %" PRIu64 "\n", info->fully_allocated);
+        if (info->has_bitmaps) {
+            printf("bitmaps size: %" PRIu64 "\n", info->bitmaps);
+        }
     } else {
         dump_json_block_measure_info(info);
     }
-- 
2.26.1