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

[Paolo Bonzini]

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Il 16/07/2013 18:56, Eric Blake ha scritto:
> 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?

Not yet, we had no use case yet.  It may be exposed later to the
guests via the SCSI command GET LBA STATUS.

Paolo

>> 
>> 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 :)
> 

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