Re: [Qemu-devel] [PATCH v2 00/17] Add qemu-img subcommand to dump file metadata

[Eric Blake]

On 07/16/2013 10:29 AM, Paolo Bonzini wrote:
> This series adds a subcommand to "map" that can dump file metadata.
> Metadata that is dumped includes:
> 
> - whether blocks are allocated in bs->file and, if so, where
> 
> - whether blocks are zero
> 
> - whether data is read from bs or bs->backing_hd

Is this command only usable from qemu-img, or is there a QMP counterpart
for getting at the same information on a live image while qemu is running?

> 
> Metadata is dumped for an entire chain of images.  One example usage is
> (for non-compressed, non-encrypted images) to transform the metadata
> into a Linux device-mapper setup, and make a qcow2 image available (for
> read only) as a block device.  Another possible usage is to determine
> the used areas of a file, and convert it in place to another format.
> Alternatively, the richer data can be used by block jobs to quickly
> determine if a block is zero without reading it.

Definitely useful information!  I haven't looked at the implementation
yet, but there had better be a JSON representation of the output :)

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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