Re: [Qemu-devel] [patch 3/4] block stream: add support for partial streaming

[Eric Blake <eblake@redhat.com>]

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On 01/04/2012 07:08 AM, Marcelo Tosatti wrote:
> Add support for streaming data from an intermediate section of the 
> image chain (see patch and documentation for details).
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> Index: stefanha/block.c
> ===================================================================
> --- stefanha.orig/block.c
> +++ stefanha/block.c
> @@ -2229,6 +2229,70 @@ int bdrv_is_allocated(BlockDriverState *
>      return data.ret;
>  }
>  
> +/*
> + * Given an image chain: [BASE] -> [INTER1] -> [INTER2] -> [TOP]
> + *
> + * Return true if the given sector is allocated in top or base.
> + * Return false if the given sector is allocated in intermediate images.
> + *
> + * 'pnum' is set to the number of sectors (including and immediately following
> + *  the specified sector) that are known to be in the same
> + *  allocated/unallocated state.

Not a problem with this patch, per say, so much as a question about the
next steps:

How hard would it be to go one step further, and provide a monitor
command where qemu could dump the state of BASE, INTER1, or INTER2
without removing it from the image chain?  Libvirt would really like to
be able to have a command where the user can request to inspect to see
the contents of (a portion of) the disk at the time the snapshot was
created, all while qemu continues to run and the TOP file continues to
be adding deltas to that portion of the disk.

For that matter, I'm still missing out on the ability to extract the
contents of a qcow2 internal snapshot from an image that is in use by
qemu - we have the ability to delete internal snapshots but not to probe
their contents.

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


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