Many organizations 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.
[STORAGE SMACKDOWN: SSDs vs. hard drives]
The SNIA provides a TCO calculation spreadsheet in Microsoft Excel format for download which incorporates all of these factors.
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)
" IOPS: expected I/O operations per second
" %R: percentage of IOPS that are reads
" %W: percentage of IOPS that are writes
" WP: RAID Write Penalty (RAID 1 or RAID 10 = 2)
" 2,000 users x 3 IOPS per user = 6,000 IOPS
The number of 15K 146GB SAS HDDs required is:
[(6,000 x 0.65) + 2(6,000 x 0.35)]/479.7 = 17 (or 18 HDDs for RAID 1 configuration)
The SNIA SSD TCO Calculator can determine and recommend the number of required SSDs based on the following four key inputs from the user:
" Minimum consumed HDD capacity
" IOPS for HDD (4Kbyte block size)
" IOPS for SSS (4Kbyte block size)
" Minimum RAID requirements
When using the above data, the SNIA SSS TCO Calculator recommends the user to replace the 18 HDDs with two SSDs in this Microsoft Exchange Email Server example.