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

  • To: Mark McLoughlin <markmc@xxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <keir@xxxxxxxxxxxxx>
  • Date: Thu, 26 Apr 2007 10:09:10 +0100
  • Delivery-date: Thu, 26 Apr 2007 02:08:00 -0700
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AceH4ozsy6dcevPVEduFMQAX8io7RQ==
  • Thread-topic: [Xen-devel] [PATCH] Segments can span multiple clusters with tap:qcow

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
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?

 -- Keir

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