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The ROI you get from a SAN can be different from company to company.
Although DAS may cost less in upfront capital outlay, SAN attached storage provides technical, availability and management benefits that can make the investment in SAN storage pay for itself in very short order. A buddy of mine, David Merrill, wrote a great white paper called "Developing return on investment and business case support for storage area networks".
In general, there are a few basic areas where employing a SAN can reduce overall storage costs significantly:
Availability:
SAN storage is in general more reliable than DAS attached disk. This can save your company money by avoiding application outages. The cost of an hour of application downtime varies from company to company but can exceed millions in some cases.
Disk utilization:
When buying server-attached storage most people buy more than they currently need so they can "grow into" the storage. The space that is unutilized is wasted until it is needed. In SANs, that space can be "assigned" to any server that needs more storage, thus deferring new storage purchases. Using DAS, I find that a 40% utilization rate is high whereas in a SAN, you can get to the ultimate 80/20 (used/unused) ratio without a problem.
Management:
Using DAS you need to manually install new disks to add storage. In a SAN you can remotely assign it to a server. No downtime and perhaps not even a reboot is required if the OS can handle it. You can manage ALL your storage GLOBALLY from a single console.
Backup:
Using snapshots and data replication can save your backside when disasters happen and using a SAN for centralizing data backup can improve recovery time dramatically while reducing overall costs by sharing tape resources and eliminating backup windows.
In David's white paper, he lists a 29 case scenario where SAN gives better TCO and ROI on a dollar per MB basis including:
Case 1-Increased disk utilization
Case 2-Deferring disk procurement
Case 3-Terabytes (TB)-per-administrator improvement
Case 4-Reduce data center rack/floor space
Case 5-Deferring tape library procurement
Case 6-New disaster recovery (DR) capabilities
Case 7-On-line recoverability options
Case 8-Improved data path availability
Case 9-Reduction of general-purpose UNIX and Microsoft Windows NT servers
Case 10-Improve LAN/WAN performance, avoid upgrades
Case 11-Reduce/eliminate backup servers
Case 12-Reduce/eliminate batch, backup windows
Case 13-Storage on demand
The remaining case models
Chris
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