Virtualization and Memory Considerations - more
Figures 4 and 5 show Write and Read performance when the virtual machine has only 256MB of RAM as opposed to the virtual machine having 512MB of RAM as used in the head-to-head comparison with the physical machine.
In Figure 4 showing Write Performance, the virtual machine with 512MB RAM clearly outperformed the virtual machine with 256 MB RAM at smaller file sizes, with nearly identical performance as file sizes increased. Again, this effect is primarily due to write caching, so with less memory allocated, the cache performance boost declines sooner, resulting in lower performance for the 32 MB file write.
Figure 4: Write performance of 256MB RAM virtual machine vs. 512MB RAM virtual machine
Read performance in Figure 5 between the 256MB virtual NAS and 512MB virtual NAS shows a tradeoff, with higher performance at lower file sizes for the larger memory virtual device and lower performance for the higher file sizes for the larger memory virtual device. (The 128 MB data point for the 256 MB plot line is another measurement glitch. The real performance is more in line with the other data points.)
Figure 5: Read performance of 256MB RAM virtual machine vs. 512MB RAM virtual machine
Virtualization and Network Interface Considerations
Network performance is a key aspect to optimizing server functionality. For a server to provide value, it must provide high throughput to clients, data stores, and other servers over a high-speed network. In addition to adding RAM to a Host physical server, there are multiple different options to optimize the performance of virtual machines.
Understanding that Guest operating systems interact with the computing hardware through the virtualization layer explains how optimized virtual drivers can improve the performance of a virtual machine. As discussed in my last article, VMware has packages of high performance drivers, called "VMware Tools," for Windows, Linux, FreeBSD, and Netware Guest operating systems that improve the performance of network interfaces and disk I/O operations.
In addition to installing VMware Tools, performance may be enhanced on a high network traffic server by installing additional physical NICs on the Host machine. These additional NICs can then be dedicated to specific virtual machines. I tested this functionality by adding another NIC to my Host server, and configured the network settings to map each physical NIC to different virtual switches within VMware Server's configurations, as shown in Figure 6.