[Qemu-devel] [RFC PATCH 00/11] qcow2: External data files

[Kevin Wolf]

There are use cases where raw images are given (e.g. existing physical
disks), but advanced features like dirty bitmaps or backing files are
wanted that require use of a proper image format like qcow2.

This series adds an incompatible feature bit to qcow2 which allows to
use an external data file: Metadata is kept in the qcow2 file like
usual, but guest data is written to an external file. Clusters in the
data file are not reference counted, instead we use a flat layout where
host cluster offset == guest cluster offset. The external data file is
therefore readable as a raw image (though writing to it invalidates the
associated qcow2 metadata). Features that require refcounting such as
internal snapshots or compression are not supposed in such setups.

There are a few reasons why this is still RFC:

- The resulting code passes qemu-iotests, so we don't regress on normal
  qcow2 files, but testing with external data files is still minimal
  (converting an existing image and confirming it reads back unmodified;
  installing a guest OS and making sure it boots). We need at least some
  qemu-iotests cases.

- QAPI documentation is missing

- Discard isn't passed through to the data file yet

- s->image_data_file isn't correct, it gets the value from the attached
  node rather than just from the image file. This means that on header
  updates we might be writing "back" a path that wasn't there before.

- Probably something else I just can't remember now :-)

Kevin Wolf (11):
  qcow2: Extend spec for external data files
  qcow2: Basic definitions for external data files
  qcow2: Pass bs to qcow2_get_cluster_type()
  qcow2: Prepare qcow2_get_cluster_type() for external data file
  qcow2: Prepare count_contiguous_clusters() for external data file
  qcow2: Don't assume 0 is an invalid cluster offset
  qcow2: External file I/O
  qcow2: Add basic data-file infrastructure
  qcow2: Creating images with external data file
  qcow2: Store data file name in the image
  qcow2: Add data file to ImageInfoSpecificQCow2

 qapi/block-core.json       |   5 +-
 docs/interop/qcow2.txt     |  19 ++++-
 block/qcow2.h              |  40 +++++++--
 include/block/block_int.h  |   1 +
 block/qcow2-bitmap.c       |   7 +-
 block/qcow2-cache.c        |   6 +-
 block/qcow2-cluster.c      | 144 +++++++++++++++++++-------------
 block/qcow2-refcount.c     |  40 ++++++---
 block/qcow2-snapshot.c     |   7 +-
 block/qcow2.c              | 166 ++++++++++++++++++++++++++++++++++---
 tests/qemu-iotests/031.out |   8 +-
 tests/qemu-iotests/036.out |   4 +-
 tests/qemu-iotests/061.out |  14 ++--
 tests/qemu-iotests/082.out |  27 ++++++
 14 files changed, 372 insertions(+), 116 deletions(-)

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2.20.1