[PATCH v9 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 d92c827763..77146b5169 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,
+                                non-default compression is used for compressed
+                                clusters. compression_type field must be
+                                present and not zero.
+
                     Bits 3-63:  Reserved (set to 0)
 
          80 -  87:  compatible_features
@@ -188,7 +193,16 @@ present*, if not altered by specific incompatible bit.
 *. Field is not present when header_length is less or equal to 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.
+                    A single compression type is applied to all compressed image
+                    clusters.
+                    If incompatible compression type bit is set: the field must
+                    be present and non-zero (which means non-zlib compression type)
+                    If incompatible compression type bit is unset: the field
+                    may not exist or it must be zero (which means zlib).
+                    Available compression type values:
+                        0: zlib <https://www.zlib.net/>
 
 Header padding
 
-- 
2.18.0