Re: [Xen-devel] [PATCH] Segments can span multiple clusters with tap:qcow

[Mark McLoughlin <markmc@xxxxxxxxxx>]

Hi Keir,

On Thu, 2007-04-26 at 10:09 +0100, Keir Fraser wrote:
> On 25/4/07 21:41, "Mark McLoughlin" <markmc@xxxxxxxxxx> wrote:
> 
> > In blktap's qcow we need split up read/write requests if the requests
> > span multiple clusters. However, with our MAX_AIO_REQUESTS define we
> > assume that there is only ever a single aio request per tapdisk request
> > and under heavy i/o we can run out of room causing us to cancel
> > requests.
> > 
> > The attached patch dynamically allocates (based on cluster_bits) the
> > various io request queues the driver maintains.
> 
> The current code allocates aio-request info for every segment in a request
> ring (MAX_AIO_REQUESTS == BLK_RING_SIZE * MAX_SEGMENTS_PER_REQUEST). This
> patch seems to take into account that each segment (part-of-page) can itself
> be split into clusters, hence the page_size/cluster_size calculation, but
> shouldn't this be multiplied by the existing MAX_AIO_REQUESTS? Otherwise you
> provide only enough aio requests for one segment at a time, rather than a
> request ring's worth of segments?

        Absolutely, well spotted. I fixed that typo after testing, but
obviously forgot to run "quilt refresh" before sending ...

        Fixed version attached.

Thanks,
Mark.

Attachment: block-qcow-multiple-aio-requests-per-segment.patch
Description: Text Data

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