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Date: Tue, 22 Nov 2011 16:06:05 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v2 6/8] block: request overlap detection
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To: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel@nongnu.org

Am 17.11.2011 14:40, schrieb Stefan Hajnoczi:
> Detect overlapping requests and remember to align to cluster boundaries
> if the image format uses them.  This assumes that allocating I/O is
> performed in cluster granularity - which is true for qcow2, qed, etc.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

>  static void coroutine_fn wait_for_overlapping_requests(BlockDriverState *bs,
>          int64_t sector_num, int nb_sectors)
>  {
>      BdrvTrackedRequest *req;
> +    int64_t cluster_sector_num;
> +    int cluster_nb_sectors;
>      bool retry;
>  
> +    /* If we touch the same cluster it counts as an overlap */
> +    round_to_clusters(bs, sector_num, nb_sectors,
> +                      &cluster_sector_num, &cluster_nb_sectors);

Is this really required? Image formats must be able to deal with two
concurrent write requests to the same cluster, and I don't think it
makes a difference whether it's a guest write request or a COR one.

Or does the queuing protect more than just that a COR never takes
precedence over a guest write?

Kevin