Re: [Qemu-devel] [PATCH v3 00/19] Add qemu-img subcommand to dump file metadata

[Stefan Hajnoczi]

On Thu, Jul 25, 2013 at 04:22:58PM +0200, 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.
> 
> Patch 4 fixes the bs->total_sectors file size cache, which is used in
> bdrv_co_get_block_status to clamp invalid input.
> 
> Patches 5 to 7 clean up the bdrv_is_allocated and bdrv_is_allocated_above
> implementation, eliminating some code duplication.
> 
> Patches 8 and 9 tweak bdrv_has_zero_init and its usage in qemu-img in
> a way that helps when implementing the new API.
> 
> Patches 10 to 12 implement bdrv_get_block_status and change the formats
> to report all the available information.
> 
> Patch 13 and 14 adds the "map" subcommand to qemu-img, and the relevant
> documentation.
> 
> Finally, patches 15 to 19 tweak the implementation to extract more
> information from protocols, and combine this information with format
> metadata whenever possible.
> 
> v2->v3:
>         new patch 4 to fix qemu-iotests regression
>         new patch 14 for documentation
> 
> Paolo Bonzini (19):
>   cow: make reads go at a decent speed
>   cow: make writes go at a less indecent speed
>   cow: do not call bdrv_co_is_allocated
>   block: update bs->total_sectors on writes
>   block: make bdrv_co_is_allocated static
>   block: remove bdrv_is_allocated_above/bdrv_co_is_allocated_above
>     distinction
>   block: expect errors from bdrv_co_is_allocated
>   qemu-img: always probe the input image for allocated sectors
>   block: make bdrv_has_zero_init return false for copy-on-write-images
>   block: introduce bdrv_get_block_status API
>   block: define get_block_status return value
>   block: return get_block_status data and flags for formats
>   qemu-img: add a "map" subcommand
>   docs, qapi: document qemu-img map
>   block: use bdrv_has_zero_init to return BDRV_BLOCK_ZERO
>   raw-posix: return get_block_status data and flags
>   raw-posix: detect XFS unwritten extents
>   block: add default get_block_status implementation for protocols
>   block: look for zero blocks in bs->file

Looking forward to v4 :).  It seems there are still some things to tidy
up but soon this can be merged.

Stefan