Cost of Storage - Direct Attached vs SAN

Acronyms

DAS	Direct Attached Storage
SAN	Storage Area Network
FC	Fibre Channel
SATA	Serial ATA

Introduction

When considering whether to use direct attached storage (DAS) or storage area network (SAN) storage, the question of cost is often raised. At first glance DAS seems to be the cheaper option but as the data volume grows to terabytes, this is no longer the case.

The purpose of this document is to explain the cost of DAS storage as opposed to SAN storage.

Cost of DAS, SAN FC and SAN SATA

The document compares the cost of purchasing SUN DAS, SAN fibre channel disk and SAN SATA disk as these are the technologies Monash ITS will be deploying.

Cost of SUN DAS

Monash purchases mainly SUN storage arrays in two configurations, either a Sun Storedge 6120 array with a capacity of 2TB  or the sun Storedge 3510 FC array with a capacity of 876GB. The cost per gigabyte per year for these arrays is $5.98 and $13.32 respectively, equating to an average cost of $9.65 per gigabyte per year. This cost remains the same as the system scales.

Cost of SAN FC Disk

Monash ITS is in the process of implementing an IBM FastT900 storage array and the costs below have been calculated on this infrastructure and software.  This will be our Tier One disk service offering.

As you can see, the cost per gigabyte quickly drops as the system scales to be comparable with DAS at 32TBs. Monash expects to have approximately 16TB installed by mid 2005.

Cost of SAN SATA

SATA is the practice of using ATA disk (same as used in PCs) in a redundant array to provide mass, reliable storage that is relatively inexpensive. SATA is being widely accepted in the industry, with many companies now adopting for production services over the more expensive FC disk. Industry analyst are stating that SATA is suitable for 70% of applications.

Monash ITS are using the existing XSI SAN to provide SATA disk storage to our customers.

As you can see, the cost of SATA disk is cheaper than DAS from day one. Monash will have approximately 24TBs of SATA installed during 2005.

Efficiency of DAS vs SAN - True Cost of Storage

Probably the single greatest selling point for a SAN is the efficient utilisation of disk storage. People are terrified of running out of disk space so they cater for the worst case scenario and purchase far more disk than is required. This is very inefficient practice, as if each system across the University allows a 30% safety factor for their DAS, you have substantial amounts of wasted storage. Industry statistics reflect a worse picture than this with most sites only utilising between 40 to 50% of there DAS storage. 

As storage is centralised, a SAN can run at 80% capacity and still have ample spare space for those emergencies . For example, on a 16TB SAN, running at 80% utilisation, there is 3.2 TBs of spare capacity, far more than DAS could provided cost effectively.

Monash staff report that our DAS utilisation rates are approximately 40  - 50%. This means the true cost to the business for DAS is double, as we purchase more than necessary.

As the table below is shows, the cost difference between DAS and SAN FC is minimal when when utilisation rates are considered.

The above costs are based on 16TB configuration. 

Benefits of SAN vs DAS

Cost is the only factor this document has discussed in detail but it should not be the only consideration. Other benefits a SAN can provide which are not available to DAS are:

•  SAN enable data mirroring across multiple sites. This will be a key feature in business continuity plans for priority one services.
• SAN provides snapshot technology for data replication and protection
• SAN provide tools to effectively manage large volumes of storage. Consider the staff required to manage 50TBs of direct attached storage effectively? The more likely scenario is this the storage would not be proactively managed but more reactive when problems occur.
• Scalability. DAS storage arrays have limited capacity and are difficult to grow. SAN can dynamically allocate storage as required and have larger capacity limits (64 TBs for a FastT900 storage array)

Summary

Taking into considering the utilisation rate of disk storage, the cost of SAN storage is comparable DAS. As Monash's storage requirement grows over the next 5 years, the cost of SAN storage will become significantly cheaper than DAS, be more efficient to manage and provide greater business benefits such as data mirroring across multiple sites.

SATA storage has the potential to dramatically reduce ITS storage costs. All industry reports are very positive, and Monash should investigate and pilot which services this technology is appropriate for.