Re: [Qemu-devel] [PATCH v19 09/16] block: Support dropping active in bdrv_drop_intermediate

[Eric Blake <eblake@redhat.com>]

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On 05/11/2014 07:35 PM, Fam Zheng wrote:
> Dropping intermediate could be useful both for commit and stream, and
> BDS refcnt plus bdrv_swap could do most of the job nicely. It also needs
> to work with op blockers.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>
> ---
>  block.c        | 139 ++++++++++++++++++++++++++++-----------------------------
>  block/commit.c |   2 +-
>  2 files changed, 70 insertions(+), 71 deletions(-)
> 

> -
>  /*
> - * Drops images above 'base' up to and including 'top', and sets the image
> - * above 'top' to have base as its backing file.
> + * Drops images above 'base' up to and including 'top', and sets new 'base' as
> + * backing_hd of top's overlay (the image orignally has 'top' as backing file).
> + * top's overlay may be NULL if 'top' is active, no such update needed.
> + * Requires that the top's overlay to 'top' is opened r/w.
> + *
> + * 1) This will convert the following chain:
> + *
> + *     ... <- base <- ... <- top <- overlay <-... <- active
>   *
> - * Requires that the overlay to 'top' is opened r/w, so that the backing file
> - * information in 'bs' can be properly updated.
> + * to
> + *
> + *     ... <- base <- overlay <- active

Jeff is working on allowing the user full control over the string
written into overlay; let's make sure these efforts are coordinated.
https://lists.gnu.org/archive/html/qemu-devel/2014-05/msg02949.html

> + *
> + * 2) It is allowed for bottom==base, in which case it converts:
>   *
> - * E.g., this will convert the following chain:
> - * bottom <- base <- intermediate <- top <- active
> + *     base <- ... <- top <- overlay <- ... <- active
>   *
>   * to
>   *
> - * bottom <- base <- active
> + *     base <- overlay <- active
>   *
> - * It is allowed for bottom==base, in which case it converts:
> + * 2) It also allows active==top, in which case it converts:

Shouldn't this be 3) ?

>   *
> - * base <- intermediate <- top <- active
> + *     ... <- base <- ... <- top (active)
>   *
>   * to
>   *
> - * base <- active
> + *     ... <- base == active == top
> + *
> + * i.e. only base and lower remains: *top == *base when return.
> + *
> + * 3) If base==NULL, it will drop all the BDS below overlay and set its

and 4)

> + * backing_hd to NULL. I.e.:
>   *
> - * Error conditions:
> - *  if active == top, that is considered an error
> + *     base(NULL) <- ... <- overlay <- ... <- active
> + *
> + * to
> + *
> + *     overlay <- ... <- active
>   *
>   */

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


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