When you’re working on optimizing performance for your virtual machines, storage location is an important factor. A trade-off always exists between expensive storage that offers high performance and high availability and storage with lower cost and lower performance.
Storage can be divided into different tiers depending on a number of factors:
High Tier. Offers high performance and high availability. Might offer built-in snapshots to facilitate backups and point-in-time (PiT) restorations. Supports replication, full storage processor redundancy, and SAS drives. Uses high-cost spindles.
Mid Tier. Offers mid-range performance, lower availability, some storage processor redundancy, and SCSI or SAS drives. May offer snapshots. Uses medium-cost spindles.
Lower Tier. Offers low performance, little internal storage redundancy. Uses low end SCSI drives or SATA (serial low-cost spindles).
Not all applications need to be on the highest-performance, most-available storage—at least not throughout their entire life cycle.
If you need some of the functionality of the high tier, such as snapshots, but do not want to pay for it, you might be able to achieve some of the high-performance characteristics in software. For example, you can create snapshots in software.
When you decide where to place a virtual machine, ask yourself these questions:
How critical is the virtual machine?
What are its performance and availability requirements?
What are its PiT restoration requirements?
What are its backup requirements?
What are its replication requirements?
A virtual machine might change tiers throughout its life cycle because of changes in criticality or changes in technology that push higher-tier features to a lower tier. Criticality is relative and might change for a variety of reasons, including changes in the organization, operational processes, regulatory requirements, disaster planning, and so on.