Signed-off-by: Peter Lieven <p...@kamp.de>
---
docs/interop/qcow2.txt | 43 ++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 42 insertions(+), 1 deletion(-)
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index 80cdfd0..c01daf3 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -85,7 +85,12 @@ in the description of a field.
be written to (unless for regaining
consistency).
- Bits 2-63: Reserved (set to 0)
+ Bit 2: Compress format bit. If and only if this bit
+ is set then the compress format extension
+ MUST be present and MUST be parsed and checked
+ for compatibility.
+
+ Bits 3-63: Reserved (set to 0)
80 - 87: compatible_features
Bitmask of compatible features. An implementation can
@@ -135,6 +140,7 @@ be stored. Each extension has a structure like the
following:
0xE2792ACA - Backing file format name
0x6803f857 - Feature name table
0x23852875 - Bitmaps extension
+ 0xC03183A3 - Compress format extension
other - Unknown header extension, can be safely
ignored
@@ -208,6 +214,41 @@ The fields of the bitmaps extension are:
starts. Must be aligned to a cluster boundary.
+== Compress format extension ==
+
+The compress format extension is an optional header extension. It provides
+the ability to specify the compress algorithm and compress parameters
+that are used for compressed clusters. This new header MUST be present if
+the incompatible-feature bit "compress format bit" is set and MUST be absent
+otherwise.
+
+The fields of the compress format extension are:
+
+ Byte 0 - 15: compress_format_name (padded with zeros, but not
+ necessarily null terminated if it has full length)
+
+ 16: compress_level (uint8_t)
+ 0 = default compress level
+ 1 = lowest compress level
+ x = highest compress level (the highest compress
+ level may vary for different compress formats)
+
+ 17 - 19: Reserved for future use, must be zero.
+
+ 20 - 23: extra_data_size
+ Size of compress format specific extra data.
+ For now, as no format specifies extra data,
+ extra_data_size is reserved and should be zero.
+
+ variable: extra_data
+ Extra data with additional parameters for the compress
+ format, occupying extra_data_size bytes.
+
+ variable: Padding to round up the size of compress format extension
+ to the next multiple of 8. All bytes of the padding must be
+ zero.
+
+
== Host cluster management ==
qcow2 manages the allocation of host clusters by maintaining a reference count
--
1.9.1
