[Qemu-devel] [RFC PATCH 01/11] qcow2: Extend spec for external data files

[Kevin Wolf <kwolf@redhat.com>]

This adds external data file to the qcow2 spec as a new incompatible
feature.

Signed-off-by: Kevin Wolf <kwolf@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