The patch add new additional field to qcow2 header: compression_type,
which specifies compression type. If field is absent or zero, default
compression type is set: ZLIB, which corresponds to current behavior.
New compression type (ZSTD) is to be added in further commit.
Suggested-by: Denis Plotnikov <dplotni...@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
---
docs/interop/qcow2.txt | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index 3f2855593f..82ec0845d8 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -109,6 +109,12 @@ least next five fields, up to the @header_length field.
An External Data File Name header extension may
be present if this bit is set.
+ Bit 3: Compression type bit. If this bit is set,
+ non-default compression is used for compressed
+ clusters. In this case, @header_length must
+ be at least 108 and @compression_type field
+ must be non-zero.
+
Bits 3-63: Reserved (set to 0)
80 - 87: compatible_features
@@ -183,7 +189,18 @@ This definition implies the following:
3. If we want to add incompatible field (or a field, for which some its values
would be incompatible), it must be accompanied by incompatible feature bit.
- < ... No additional fields in the header currently ... >
+ 104: compression_type
+ Defines the compression method used for compressed
clusters.
+ A single compression type is applied to all compressed
image
+ clusters.
+ If incompatible compression type bit is set: the field must
+ exist (i.e. @header_length >= 105) and must be non-zero (
+ which means non-zlib compression type)
+ If incompatible compression type bit is unset: the field
+ may not exist (if @header_length < 105) or it must be zero
+ (which means zlib).
+ Available compression type values:
+ 0: zlib <https://www.zlib.net/>
Directly after the image header, optional sections called header extensions can
be stored. Each extension has a structure like the following:
--
2.21.0
