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

From: Max Reitz
Subject: [PULL 02/18] docs: qcow2: introduce compression type feature
Date: Thu, 20 Feb 2020 17:06:54 +0100

From: Vladimir Sementsov-Ogievskiy <address@hidden>

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 <address@hidden>
Signed-off-by: Vladimir Sementsov-Ogievskiy <address@hidden>
Message-Id: <address@hidden>
[mreitz: s/Bits 3-63:  Reserved/Bits 4-63:  Reserved/]
Signed-off-by: Max Reitz <address@hidden>
 docs/interop/qcow2.txt | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

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 
+                                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 
+                    All compressed clusters in an image use the same 
+                    type.
+                    If the incompatible bit "Compression type" is set: the 
+                    must be present and non-zero (which means non-zlib
+                    compression type). Otherwise, this field must not be 
+                    or must be zero (which means zlib).
+                    Available compression type values:
+                        0: zlib <https://www.zlib.net/>
 === Header padding ===

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