What should go into a storage SLA? An engineering group might need certain
amounts of storage to ensure that automated release builds have enough space
to run daily. A finance division might have minimal day-to-day storage needs
but require a certain amount of storage quarterly for generating reports. A
QA group or a group administering timed exams to students might express
its needs in response time as well as raw disk space.
SLAs are typically expressed in terms of availability and response time.
Availability for storage could be thought of as both reachability and usable
space. Response time is usually measured as latency—the time it takes to
complete a response—at a given load. An SLA should also specify MTTR
expectations.
Use standard benchmarking tools to measure these metrics. This has the
advantage of repeatability as you change platforms. The system should still
be tested in your own environment with your own applications to make sure
that the system will behave as advertised, but at least you can insist on a
particular minimum benchmark result to consider the system for an in-house
evaluation that will involve more work and commitment on the part of you
and the vendor.