[Qemu-devel] [RFC V3 01/24] qcow2: Add deduplication to the qcow2 specification.

[Benoît Canet]

Signed-off-by: Benoit Canet <address@hidden>
---
 docs/specs/qcow2.txt |   33 ++++++++++++++++++++++++++++++++-
 1 file changed, 32 insertions(+), 1 deletion(-)

diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
index 36a559d..16eafd7 100644
--- a/docs/specs/qcow2.txt
+++ b/docs/specs/qcow2.txt
@@ -80,7 +80,10 @@ in the description of a field.
                                 tables to repair refcounts before accessing the
                                 image.
 
-                    Bits 1-63:  Reserved (set to 0)
+                    Bit 1:      Deduplication bit.  If this bit is set then
+                                deduplication is used on this image.
+
+                    Bits 2-63:  Reserved (set to 0)
 
          80 -  87:  compatible_features
                     Bitmask of compatible features. An implementation can
@@ -116,6 +119,7 @@ be stored. Each extension has a structure like the 
following:
                         0x00000000 - End of the header extension area
                         0xE2792ACA - Backing file format name
                         0x6803f857 - Feature name table
+                        0xCD8E819B - Deduplication
                         other      - Unknown header extension, can be safely
                                      ignored
 
@@ -159,6 +163,33 @@ the header extension data. Each entry look like this:
                     terminated if it has full length)
 
 
+== Deduplication ==
+
+The deduplication extension contains the offset and size of the deduplication
+table.
+
+    Byte   0 - 7:   Offset
+
+          8 - 11:   Size
+
+== Deduplication table ==
+
+The deduplication table contains 64 bits offsets to the level 2 deduplication
+table clusters.
+Each entry of these clusters contains a 32 bytes SHA256 hash followed by the
+64 bits logical offset of the first encountered block having this hash.
+
+Entries in the deduplication table are orderered by physical cluster index.
+
+The number of entries in an l2 deduplication table cluster is :
+l2_dedup_cluster_entries = cluster_size / (32 + 8)
+
+The index in the level 1 deduplication table is :
+l1_dedup_index = physical_cluster_index / l2_dedup_cluster_entries
+
+The index in the level 2 deduplication table is:
+l2_dedup_index = physical_cluster_index % l2_dedup_cluster_entries
+
 == Host cluster management ==
 
 qcow2 manages the allocation of host clusters by maintaining a reference count
-- 
1.7.10.4