Expanding Capacity with External Disk Arrays

When an application demands more storage than the server's internal disks can supply, the simplest approach is to add capacity by connecting a dedicated, external disk array.

External Arrays

Advantages

• Extends useful life of servers - For the first time, servers need not be sized and retired based upon the storage requirements of the applications they run. Capacity planning for CPUs can be separated from capacity planning for storage, meaning both become more efficient.
• Minimal disruption - Requires very little change in approach from a strategy based upon server internal disks.
• Almost all the limitations of server internal disks apply equally to external arrays.

Disadvantages

• Greater, though still limited scalability - When an external disk array reaches its capacity limit, a second unit must be purchased and connected to the server when additional capacity is needed.
• Limited availability - Clustering is still difficult if not impractical to implement. SCSI technology requires server downtime in order to safely connect additional storage.
• Poor utilisation - Storage is still distributed and usually under utilised.
• Higher TCO - Cost of ownership is little changed, compared to server internal disk.

Some of these limitations are solved with the next architecture: Consolidated Disk Storage.