Consolidated disk storage

Achieving economies of scale

An industry-wide trend towards external disk storage has been underway for many years now. Until the advent of Storage Area Networks, best practice in enterprise disk storage was use of large, external enterprise disk arrays to which each host was directly connected by SCSI or ESCON.

Consolidated Disk Storage

Advantages

• Improved performance - Consolidated arrays support performance-enhancing technologies such as cache management and load balancing across drives. While the final performance benefits are application dependant, most commercial enterprises will achieve major performance increases with the use of consolidated disk arrays over distributed storage.
• Improved system uptime - With all servers using the same disk platform, it becomes cost-effective to use enterprise-class storage with all the redundancy and availability features expected of such a product. Common storage also makes server clustering practical.
• Cost savings from consolidation - Some purchase cost savings may be achieved through the volume purchase of storage from a single vendor, but the big savings come from reduced management, maintenance and downtime.

Consolidated, direct attached storage still has its limitations.

Disadvantages

• Greater, though still limited, scalability - Once the consolidated array is fully utilised, either in disk capacity or host connections, it's time for either a second array or a forklift upgrade. In either case, the process is far from smooth and there is no scope for cost advantages from building on the existing infrastructure.
• Limited advantage for tape backup - This architecture offers little benefit for tape backup: while the storage is physically consolidated, it's still partitioned and assigned to individual servers that must complete backups in the same manner as before.
• If basic filesystem-level backups are acceptable in place of application-aware backups, then hardware snapshot capabilities within the array can be used to achieve backup consolidation. However, when used with databases or applications this approach forces the use of full backups daily, with greatly increased backup times and tape media consumption.
• Finite performance - A single, consolidated disk array must meet the storage performance requirements of all of the connected servers. In some cases it will be sufficient, but when it cannot an entire new array must be purchased to deliver additional performance.
• Clustering limitations - Consolidated arrays usually support host-based failover or clustering to some extent, by enabling reassignment of storage to a different server upon request. However, many organisations are going beyond simple two-way active/passive failover to reap the cost savings from N-way clusters. The internal architecture of many consolidated arrays limit storage reassignment to only one or a couple of other servers, putting a cap on the size of clusters that are supported.

From this point it's a much smaller next step to the optimal data storage architecture: SAN-based disk sharing.

SAN Based File Sharing