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I do the experiment in Xen 4.0.1 with Linux 18.104.22.168 using blktap AIO. Two sequential read applications (sysbench) run on a VM. I compare the performance of application under AS, noop and CFQ in the domU, respectively (disk scheduler of dom0 is CFQ). The results of three schedulers are almost the same. I did not know why AS does not work for sequentail access applications, even the performance of AS is same as noop in two sequential applications. In the physical machine, the as is better than noop, cfq when running two sequential applications.
Does this mean that scheduling I/O requests in VM is not useful?