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How to calculate the costs of solid state drives

John Scaramuzzo | Sept. 19, 2013
Many organisations are considering swapping out some of their HDD for SSD to increase I/O throughput and power density. But how do you compare cost vs. performance to determine the best technology mix for specific application environments?

New drive comparison rules for SSD and HDD
Calculating a full TCO is an important discipline. But for immediate comparison between SSD types and designs, some simple metrics are useful. In the HDD era, the rule of thumb measures were cost per GB and I/O throughput. The characteristics of SSDs weaken the relevance of cost per GB and mean that tests of I/O throughput need to be appropriate to SSDs to ensure accurate real-world performance comparisons can be made between drive types.

This leads to two new rules. One, adopt cost per terabyte written ($/TBW) as a key metric for comparison. And two, compare performance on the basis of standardized tests, suited to SSDs.

Why do you need the $/TBW figure? The various NAND technologies used to create SSDs all have finite lives. The logic gates degrade after a certain number of writes, after which they can no longer reliably store data. The critical factor is that the warranty always incorporates a limit on the number of writes in that warranty period. The impact of this is significant, and clarity about its impact is essential when making a purchasing decision.

The management of NAND flash memory devices within an SSD to optimize the drive's overall performance is complex. The effectiveness of flash management, leading to the drive's measured performance, is affected by several factors: the state of the drive prior to the test, the workload pattern (such as the read/write mix and block size being written) and the data pattern.

SNIA has evaluated the impact of these different factors and developed a performance test specification for enterprise solid state storage.

Realistic comparisons between HDDs and the best performing SSDs show that for the majority of enterprise applications the performance benefits of SSDs are now available at a price that is becoming competitive with HDDs. Think TCO and ask for $/TBW and SNIA performance figures. Then simply run the numbers; they speak for themselves.

TCO calculation example
This TCO calculation example provided by SNIA is based on a random write intensive Microsoft Exchange email application designed for 2,000 heavy BlackBerry users. The TCO results indicate that a SSD is a better choice than a HDD for this application.


" Workload Read/Write ratio: 65% Read / 35% Write

" Block Size: 4Kbyte random small block I/O

" A short stroked 3.5 inch 146GB 15K SAS HDD can sustain 479.7 IOPS (4Kbyte block size)

" An enterprise-grade 2.5 inch 256GB SATA SSD can sustain 30,000 IOPS (4Kbyte block size)

The formula below is used to calculate the required number of HDD drives:

(IOPS x %R) + WP (IOPS x %W)


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