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

[Keir Fraser <keir@xensource.com>]

On 25/4/07 21:41, "Mark McLoughlin" <markmc@redhat.com> 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?

 -- Keir