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.
If multiple guests use storage on a filesystem or block device managed by the host operating system, the host may be able to schedule I/O more efficiently because it is aware of requests from all guests and knows the physical layout of storage, which may not map linearly to the guests' virtual storage.
