If you're using Linux 2.6 kernel-based virtual machines and you're experiencing slow storage performance as compared to physical hosts, the might be a problem with the I/O Scheduler. As of the Linux 2.6 kernel, the default I/O Scheduler is Completely Fair Queuing (CFQ).

The default scheduler will affect all disk I/O for VMDK and RDM-based virtual storage solutions. In virtualized environments, it is often not beneficial to schedule I/O at both the host and guest layers.

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