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[Qemu-block] [PATCH v2] spec/qcow2: bitmaps: zero bitmap table offset

From: Vladimir Sementsov-Ogievskiy
Subject: [Qemu-block] [PATCH v2] spec/qcow2: bitmaps: zero bitmap table offset
Date: Thu, 30 Jun 2016 10:34:02 +0300

After loading bitmap from image and setting IN_USE flag in it's header,
corresponding data (bitmap table and data clusters) becomes inconsistent
and is no longer needed. It is better to free bitmap table and
corresponding clusters from the image immediately after loading the
bitmap than free them when the bitmap is saved, or deleted or set

For now it is impossible to store only bitmap header without bitmap
table, as specification requires it. Storing zeroed bitmap table (one or
more clusters) is the only option to implement the behaviour similar to
specified above.

The same problem is for just storing empty bitmaps.

This patch allows storing only bitmap header for empty bitmaps.

Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>

Additional note. Should we also allow here bitmap_table_offset = 1, like
in bitmap table, for the bitmap with all bits set? I am not sure that it
is needed at all, but just to keep the company..

 docs/specs/qcow2.txt | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 80cdfd0..9826222 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -435,9 +435,12 @@ Structure of a bitmap directory entry:
                     Offset into the image file at which the bitmap table
                     (described below) for the bitmap starts. Must be aligned to
                     a cluster boundary.
+                    Zero value means that bitmap table is not allocated and the
+                    bitmap should be considered as empty (all bits are zero).
          8 - 11:    bitmap_table_size
-                    Number of entries in the bitmap table of the bitmap.
+                    Number of entries in the bitmap table of the bitmap. It
+                    must be zero if bitmap_table_offset is zero.
         12 - 15:    flags

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