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Re: [Qemu-devel] [PATCH v2] spec/qcow2: bitmaps: zero bitmap table offse
Denis V. Lunev
Re: [Qemu-devel] [PATCH v2] spec/qcow2: bitmaps: zero bitmap table offset
Thu, 30 Jun 2016 12:12:29 +0300
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.5.1
On 06/30/2016 10:34 AM, Vladimir Sementsov-Ogievskiy wrote:
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
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
@@ -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
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
no guys, we can not make this change at the moment.
We do have QEMU available in the field which is working
with the currently specified format.