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 d7fdb1f..ef5abb2 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -86,7 +86,12 @@ 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
@@ -137,6 +142,7 @@ be stored. Each extension has a structure like the
following:
0x6803f857 - Feature name table
0x23852875 - Bitmaps extension
0x0537be77 - Full disk encryption header pointer
+ 0xC03183A3 - Compress format extension
other - Unknown header extension, can be safely
ignored
@@ -311,6 +317,41 @@ The algorithms used for encryption vary depending on the
method
in the LUKS header, with the physical disk sector as the
input tweak.
+
+== 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.
+
+Note: Ommitting the incompatible "Compress format bit" results in the usage
+of zlib compression with default compression level (default before QEMU 2.10).
+However, this old default has a smaller compression window size which results
in
+lower compression ratio and slightly worse compression speed compared to
+explicity specifying zlib compression with default compression level in the
+compress format extension.
+
+The fields of the compress format extension are:
+
+ Byte 0 - 14: compress_format_name (padded with zeros, but not
+ necessarily null terminated if it has full length).
+ Valid compression format names currently are:
+
+ zlib: Standard zlib deflate compression
+
+ 15: compress_level (uint8_t)
+
+ 0 = default compress level (valid for all formats, default)
+
+ Additional valid compression levels for zlib compression:
+
+ All values between 1 and 9. 1 gives best speed, 9 gives best
+ compression. The default compression level is defined by
zlib
+ and currently defaults to 6.
+
+
== Host cluster management ==
qcow2 manages the allocation of host clusters by maintaining a reference count
--
1.9.1
