From: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
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 <dplotni...@virtuozzo.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
Message-Id: <20200131142219.3264-3-vsement...@virtuozzo.com>
[mreitz: s/Bits 3-63: Reserved/Bits 4-63: Reserved/]
Signed-off-by: Max Reitz <mre...@redhat.com>
---
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
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 ===
--
2.24.1
