[PATCH v10 2/2] docs: qcow2: introduce compression type feature

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

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 <dplotnikov@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 docs/interop/qcow2.txt | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index 355925c35e..4569f0dba3 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -109,6 +109,11 @@ the next fields through header_length.
                                 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,
+                                a non-default compression is used for compressed
+                                clusters. The compression_type field must be
+                                present and not zero.
+
                     Bits 3-63:  Reserved (set to 0)
 
          80 -  87:  compatible_features
@@ -189,7 +194,16 @@ present*, if not altered by a specific incompatible bit.
 *. A field is considered not present when header_length is less or equal to the
 field's offset. Also, all additional fields are not present for version 2.
 
-        < ... No additional fields in the header currently ... >
+              104:  compression_type
+                    Defines the compression method used for compressed clusters.
+                    All compressed clusters in an image use the same compression
+                    type.
+                    If the incompatible bit "Compression type" is set: the field
+                    must be present and non-zero (which means non-zlib
+                    compression type). Otherwise, this field must not be present
+                    or must be zero (which means zlib).
+                    Available compression type values:
+                        0: zlib <https://www.zlib.net/>
 
 
 === Header padding ===
-- 
2.21.0