Re: [Qemu-devel] [PATCH] qcow2: Order concurrent AIO requests on the same unallocated cluster

[Avi Kivity]

On 08/31/2009 05:48 PM, Kevin Wolf wrote:
> When two AIO requests write to the same cluster, and this cluster is
> unallocated, currently both requests allocate a new cluster and the second one
> merges the first one when it is completed. This means an cluster allocation, a
> read and a cluster deallocation which cause some overhead. If we simply let the
> second request wait until the first one is done, we improve overall performance
> with AIO requests (specifially, qcow2/virtio combinations).
>
> This patch maintains a list of in-flight requests that have allocated new
> clusters. A second request touching the same cluster is limited so that it
> either doesn't touch the allocation of the first request (so it can have a
> non-overlapping allocation) or it waits for the first request to complete.
>    

Can't this cause an even/odd pattern where all even-numbered requests 
run first, then all the odd-numbered requests?

(0 goes to disk, 1 depends on it, 2 depends on 1, which isn't allocating 
now, so it goes to disk, 3 depends on 2, ...)

Do you have performance numbers?

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