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+RBD Export & Import
+===================
+
+This is a file format of an RBD image or snapshot. It's a sparse format
+for the full image. There are three recording sections in the file.
+
+(1) Header.
+(2) Metadata.
+(3) Diffs.
+
+Header
+~~~~~~
+
+"rbd image v2\\n"
+
+Metadata records
+~~~~~~~~~~~~~~~~
+
+Every record has a one byte "tag" that identifies the record type,
+followed by length of data, and then some other data.
+
+Metadata records come in the first part of the image.  Order is not
+important, as long as all the metadata records come before the data
+records.
+
+In v2, we have the following metadata in each section:
+(1 Bytes) tag.
+(8 Bytes) length.
+(n Bytes) data.
+
+In this way, we can skip the unrecognized tag.
+
+Image order
+-----------
+
+- u8: 'O'
+- le64: length of appending data (8)
+- le64: image order
+
+Image format
+------------
+
+- u8: 'F'
+- le64: length of appending data (8)
+- le64: image format
+
+Image Features
+--------------
+
+- u8: 'T'
+- le64: length of appending data (8)
+- le64: image features
+
+Image Stripe unit
+-----------------
+
+- u8: 'U'
+- le64: length of appending data (8)
+- le64: image striping unit
+
+Image Stripe count
+------------------
+
+- u8: 'C'
+- le64: length of appending data (8)
+- le64: image striping count
+
+Final Record
+~~~~~~~~~~~~
+
+End
+---
+
+- u8: 'E'
+
+
+Diffs records
+~~~~~~~~~~~~~~~~~
+Record the all snapshots and the HEAD in this section. 
+
+- le64: number of diffs
+- Diffs ...
+
+Detail please refer to rbd-diff.rst