This adds external data file to the qcow2 spec as a new incompatible
feature.
Signed-off-by: Kevin Wolf <kw...@redhat.com>
---
docs/interop/qcow2.txt | 19 ++++++++++++++++---
1 file changed, 16 insertions(+), 3 deletions(-)
diff --git a/docs/interop/qcow2.txt b/docs/interop/qcow2.txt
index fb5cb47245..90af0d02e5 100644
--- a/docs/interop/qcow2.txt
+++ b/docs/interop/qcow2.txt
@@ -97,7 +97,17 @@ in the description of a field.
be written to (unless for regaining
consistency).
- Bits 2-63: Reserved (set to 0)
+ Bit 2: External data file. If this bit is set, an
+ external data file name header extension must
+ be present as well. Guest clusters are then
+ stored in the external data file. For such
+ images, clusters in the external data file are
+ not refcounted. The offset field in the
+ Standard Cluster Descriptor must match the
+ guest offset and neither compressed clusters
+ nor internal snapshots are supported.
+
+ Bits 3-63: Reserved (set to 0)
80 - 87: compatible_features
Bitmask of compatible features. An implementation can
@@ -148,6 +158,7 @@ be stored. Each extension has a structure like the
following:
0x6803f857 - Feature name table
0x23852875 - Bitmaps extension
0x0537be77 - Full disk encryption header pointer
+ 0x44415441 - External data file name
other - Unknown header extension, can be safely
ignored
@@ -450,8 +461,10 @@ Standard Cluster Descriptor:
1 - 8: Reserved (set to 0)
9 - 55: Bits 9-55 of host cluster offset. Must be aligned to a
- cluster boundary. If the offset is 0, the cluster is
- unallocated.
+ cluster boundary. If the offset is 0 and bit 63 is clear,
+ the cluster is unallocated. The offset may only be 0 with
+ bit 63 set (indicating a host cluster offset of 0) when an
+ external data file is used.
56 - 61: Reserved (set to 0)
--
2.20.1
