Performance of SAN Virtualization

There are two primary performance topics to discuss with SAN virtualization systems:

• Latency

• Caching

Latency

As mentioned previously, one of the disadvantages of virtualization is the possibility of introducing latency into I/O operations and slowing down application performance. This certainly is a possibility. Companies purchasing SAN virtualization products should test and verify what the performance impact may be of the systems they are considering.

The issue is that the virtualization system must first read the entire incoming I/O and interpret it and then create one or more I/Os that are retransmitted to downstream storage. Obviously this is not an architecture for SAN networks that may be stretching supported distance limits or where errors are higher for some reason.

SAN virtualization products can be designed to minimize latency through designs that minimize the processing time involved. It may be that the latency injected by a virtualization product is inconsequential compared to other latencies that occur in normal I/O processing. For example, a delay of a few hundred microseconds is relatively smallless than 5% of the total latency incurred in a normal I/O transmission. If application performance does not need every possible microsecond, I/O latency is not a problem.

Caching

For those cases where latency is a problem, performance can be significantly improved through the use of caching in SAN virtualization systems. Memory caches in SAN virtualization systems are designed to significantly improve I/O performance rates of high-throughput applications like transaction processing.

One of the advantages of caching in SAN virtualization systems is the flexibility in assigning the cache to different LUs and block ranges within those LUs. For instance, it may be possible to assign cache memory to a small area of block addresses in order to eliminate an important application's hot spots. Testing with actual applications and data is the best way to determine if caching in a SAN virtualization system will deliver the desired performance benefits.