Many organisations are considering swapping out some of their Hard Disk Drives (HDD) for Solid State Drives (SSD) to increase I/O throughput and power density (IOPS/Watt). But how do you compare cost vs. performance to determine the best technology mix for specific application environments?
The Storage Networking Industry Association (SNIA) has completed an in-depth analysis of the factors that need to be considered in comparing HDDs and SSDs for any given application. It concluded that assessing Total Cost of Ownership offered the most realistic basis for comparison - assessing both the direct and indirect cost of deploying a storage system over its life cycle.
Direct costs, labor and capital costs are familiar and relatively easy to measure. However, indirect costing becomes more complex to expose. Industry studies have shown it can cost more to operate a storage device over three years than to buy it. To get a clean TCO, it is essential to objectively include all relevant data including cost for:
* Acquisition. Analysis of acquisition costs needs to include cost per drive, software licenses and differing architecture options. Consider that, in a high random I/O transaction application (i.e. Exchange email, banking transactions, etc.), a single SSD could replace an array of 10 or more HDDs, resulting in a smaller footprint, higher performance and lower costs for supporting hardware and software licensing.
* Maintenance and Repair. HDDs have an annual failure rate of 2% to 8%. So as many as one in 12 HDDs deployed will fail every year. Factor in the cost of a drive, the personnel to replace it and any system downtime to get a clear picture of the true cost of these replacements. In addition, SSDs need to be treated as a consumable with endurance dependent on multiple factors, which vary by manufacturer and design.
* Power and Cooling. In tier 0 and 1 storage systems, choosing SSDs can save more than 80% in total storage system energy requirements. There are many elements to this calculation. Ultimately, it is the greater power density (IOPS/Watt) of SSDs that make the difference; fewer drives can deliver the same throughput for less power, with the added benefit of requiring less space and cooling.
* RAID Configuration. It is standard practice to use RAID configurations to improve performance and reliability. Conventional RAID configurations mask the high I/O latency inherent in HDDs. New SSD-friendly RAID implementations both exploit and enhance the performance and reliability of SSDs. The tradeoffs between levels of RAID can significantly shift the performance, cost and reliability equations so the relative benefits need to be factored into a complete TCO exercise.
The SNIA provides a TCO calculation spreadsheet in Microsoft Excel format for download which incorporates all of these factors.
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