[Qemu-devel] [PATCH 0/7] qed: Add QEMU Enhanced Disk format

[Stefan Hajnoczi]

QEMU Enhanced Disk format is a disk image format that forgoes features
found in qcow2 in favor of better levels of performance and data
integrity.  Due to its simpler on-disk layout, it is possible to safely
perform metadata updates more efficiently.

Installations, suspend-to-disk, and other allocation-heavy I/O workloads
will see increased performance due to fewer I/Os and syncs.  Workloads
that do not cause new clusters to be allocated will perform similar to
raw images due to in-memory metadata caching.

The format supports sparse disk images.  It does not rely on the host
filesystem holes feature, making it a good choice for sparse disk images
that need to be transferred over channels where holes are not supported.

Backing files are supported so only deltas against a base image can be
stored.

The file format is extensible so that additional features can be added
later with graceful compatibility handling.

Internal snapshots are not supported.  This eliminates the need for
additional metadata to track copy-on-write clusters.

Compression and encryption are not supported.  They add complexity and can be
implemented at other layers in the stack (i.e. inside the guest or on the
host).  Encryption has been identified as a potential future extension and the
file format allows for this.

This patchset implements the base functionality.

Later patches will address the following points:
 * Fine-grained L2 cache to allow for better request parallelism.  Allocating
   write requests are currently serialized.  This will also fix the corner
   case where a read request to the same sectors as a pending write request
   looks at the backing file or zeroes instead of using the write request data.
 * Resizing the disk image.  The capability has been designed in but the
  code has not been written yet.
 * Resetting the image after backing file commit completes.

Signed-off-by: Anthony Liguori <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
---
Split up for easier reviewing.  This code is also available from git:

http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/qed