[PULL,02/18] docs: qcow2: introduce compression type feature

Commit Message

Max Reitz Feb. 20, 2020, 4:06 p.m. UTC

From: Vladimir Sementsov-Ogievskiy <[e-mail]>

The patch adds a new additional field to the 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 <[e-mail]>
Signed-off-by: Vladimir Sementsov-Ogievskiy <[e-mail]>
Message-Id: <[e-mail]>
[mreitz: s/Bits 3-63:  Reserved/Bits 4-63:  Reserved/]
Signed-off-by: Max Reitz <[e-mail]>
---
 docs/interop/qcow2.txt | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

Patch

diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index 823cc266e0..5597e24474 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -109,7 +109,12 @@  the next fields through header_length.
                                 An External Data File Name header extension may
                                 be present if this bit is set.
 
-                    Bits 3-63:  Reserved (set to 0)
+                    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 4-63:  Reserved (set to 0)
 
          80 -  87:  compatible_features
                     Bitmask of compatible features. An implementation can
@@ -190,7 +195,19 @@  present*, if not altered by a specific incompatible bit.
 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 ===