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

[Peter Lieven <pl@kamp.de>]

On 16.07.2013 18:29, 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
>
> 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.
>
> The patches implement a new operation, bdrv_co_get_block_status, that
> supersedes bdrv_co_is_allocated.  The bdrv_is_allocated API is still
> available of course, and implemented on top of the new operation.
>
> Patches 1-3 touch cow, which uses bdrv_co_is_allocated during its reads
> and writes.  I optimized it a bit because it was unbearably slow during
> testing.  With these patches (also tested with blkverify), booting of
> a cow guest image is not particularly slow.
>
> Patches 4 to 6 clean up the bdrv_is_allocated and bdrv_is_allocated_above
> implementation, eliminating some code duplication.
>
> Patches 7 and 8 tweak bdrv_has_zero_init and its usage in qemu-img in
> a way that helps when implementing the new API.
>
> Patches 9 to 11 implement bdrv_get_block_status and change the formats
> to report all the available information.
>
> Patch 12 adds the "map" subcommand to qemu-img.
>
> Finally, patches 13 to 17 tweak the implementation to extract more
> information from protocols, and combine this information with format
> metadata whenever possible.

maybe i have missed it in your patches, but would it be also possible to issue
DISCARDZEROES ioctl on linux to return zero status for block devices also?

Peter