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 <aligu...@us.ibm.com> Signed-off-by: Stefan Hajnoczi <stefa...@linux.vnet.ibm.com> --- Split up for easier reviewing. This code is also available from git: http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/qed
