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

From: Vladimir Sementsov-Ogievskiy
Subject: [Qemu-block] [PATCH] spec/qcow2: bitmaps: zero bitmap table offset
Date: Wed, 29 Jun 2016 15:22:47 +0300

This allows effectively free in_use bitmap clusters including bitmap
table without loss of meaningful data.

Now it is possible only to free end-point clusters and zero-out (not
free) bitmap table

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

Hi all!

Here is one small but significant addition to specification of bitmaps in qcow2.

Can we apply it just like this or I'll have to inroduce new incompatible 
feature flag?

If there is existing implementation of the format, it may break image, saved by
software, using extended spec. But is there are any implementations except not
finished my one?

 docs/specs/qcow2.txt | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 80cdfd0..dd07a82 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -435,6 +435,8 @@ 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.

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