Re: [Qemu-devel] [RFC] qed: Add QEMU Enhanced Disk format

[Avi Kivity <avi@redhat.com>]

  On 09/06/2010 01:04 PM, Stefan Hajnoczi wrote:
> 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).
>
> The format is currently functional with the following features missing:
>   * 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.
>   * Changing the backing filename.
>   * Consistency check (fsck).  This is simple due to the on-disk layout.
>
> Signed-off-by: Anthony Liguori<aliguori@us.ibm.com>
> Signed-off-by: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
> ---
> This code is also available from git (for development and testing the tracing
> and blkverify features are pulled in, whereas this single squashed patch
> applies to mainline qemu.git):
>
> http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/qed
>
> Numbers for RHEL6 install, cache=none disk image on ext3.  This is an
> interactive install on my laptop, so not a proper benchmark but I want to show
> there is real difference today:
>   * raw: 4m4s
>   * qed: 4m21s (107%)
>   * qcow2: 4m46s (117%)
>
>   Makefile.objs        |    1 +
>   block/qcow2.c        |   22 -
>   block/qed-cluster.c  |  136 +++++++
>   block/qed-gencb.c    |   32 ++
>   block/qed-l2-cache.c |  131 ++++++
>   block/qed-table.c    |  242 +++++++++++
>   block/qed.c          | 1103 ++++++++++++++++++++++++++++++++++++++++++++++++++
>   block/qed.h          |  212 ++++++++++

docs/qed.txt ++++++++++++++++++++++++++++++++++++++++++++

?



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