Our research shows that SANs of all sizes are being built. The most common size appears to be between 100G bytes and 500G bytes, which is not a lot of capacity by today's standards. If an average Windows NT server were using 50G bytes or so, it would only take two to 10 servers to fall into this range. Organizations choosing to delay SAN implementations because of their storage capacity often already have data storage capacity well in excess of this range. The other problem I have with basing a SAN decision on storage capacity is that those implementing SANs seldom, if ever, report that their driving motivation for a SAN is based on capacity alone.
Scalability is the only capacity-related reason that is usually mentioned.
And scalability may be an issue regardless of storage capacity, since growth rate is usually more of a problem than total capacity. Scalability may also be related more to connectivity issues and limitations. I will concede that, in some instances, scalability may simply mean the need to exceed the capacity restrictions of more traditional forms of storage. Other frequently mentioned reasons for SANs include faster data access, higher availability, and better storage resource sharing and management. While capacity may be a factor in determining the severity of these problems, it seems the business applications' requirements for performance, availability, and reliability are far more significant.