SSD TCO (Total Cost of Ownership)

Back in November, this blog covered TCO (total cost of ownership) for blackout-proof server memory using NAND-Flash-backed AGIGARAM versus battery-backed DRAM. (See “Bulletproof Memory for RAID Servers, Part 2.”) Those numbers showed that NAND Flash provided a clear advantage over batteries in terms of TCO. Unsurprisingly, there’s a similar sort of situation with solid-state drives (SSDs) built from NAND Flash memory versus hard disk drives (HDDs). That NAND Flash memory is just really handy stuff. And speaking of handy, Jim Handy covered the topic of SSD TCO for servers in his Bell Micro seminar on solid-state disks (SSDs). He used an interesting calculation presented by Sun Microsystems at an Open Storage Breakfast that computed the crossover point where it made economic sense to use SSDs mixed with high-capacity HDDs rather than enterprise-class HDDs based on the IOPS required by the server design.

First, take a look at this graphic depicting a “typical” server storage array consisting of 100 enterprise-class HDDs.

Handy HDD Server Array Image

Each short-stroked, enterprise-class HDD has a capacity of 300 Gbytes, for a total array capacity of 30 Tbytes. This enterprise-class HDD array delivers 30K IOPS, costs $55,000, and consumes 1.75 kilowatts of electricity (not to mention an equivalent amount of electricity required for cooling). That’s the baseline.

Now look at this graphic, which compares the previously discussed array of enterprise-class HDDs with a hybrid array consisting of one SSD and 30 high-capacity, 1-Tbyte HDDs.

Handy HDD  Plus SSD Server Array Image

The high-capacity drives are not short-stroked, so they can provide a total storage capacity of 30T bytes with only 30 drives instead of 100 enterprise-class HDDs. However, the one SSD inserted into the drive array provides the same IOPS read performance as the 100 enterprise-class HDDs, so the use of the slower, less expensive, high-capacity HDDs in the second array is not a detriment to the second array’s IOPS performance, as long as the server software is written to make use of the hybrid array’s abilities.

The SSD-enhanced drive array costs $6040 or about 90% less than the array of 100 enterprise-class HDDs. The SSD-enhanced array consumes 0.392 kilowatts, which is nearly 80% less than the enterprise-class array of 100 short-stroked HDDs. Consequently, the second drive array generates substantially less waste heat (that must be cooled) than the full array of enterprise-class HDDs.

As a result, the SSD-enhanced drive array saves the enterprise customer a substantial amount of money when viewed from a systemic perspective. Relative to the enterprise-class HDD array, the SSD-enhanced hybrid drive array costs less to purchase; costs less to provision because fewer drives require less rack space and fewer racks; consume less electricity for operation; need less electricity for cooling because fewer, slower drives generate less heat; and reduce maintenance costs because the high-capacity drives run cooler (increasing MTBF), because there are fewer drives to maintain, and because high-capacity drives are much less expensive than enterprise-class drives. Overall, the TCO calculations favor the SSD-enhanced, hybrid drive array.

Handy took Sun’s numbers a step further by calculating the crossover point where TCO considerations favor an SSD-enhanced drive array over an array of enterprise-class HDDs when the IOPS performance is the main consideration rather than capacity. Here are Handy’s graphs:

Handy HDD and SSD TCO Graphs

The left graph shows price curves for an array of enterprise-class HDDs versus an array of SSDs. The array of SSDs initially costs more than the enterprise-class HDDs, so the crossover point is 1200 IOPS due to the high initial SSD cost. As the IOPS requirement rises, you need to add enterprise-class HDDs to the array to meet the higher IOPS requirements but one SSD gets you a lot of IOPS so there’s no need to add one until the IOPS requirement exceeds around 3000 IOPS. For the enterprise-class hybrid array, which mates one SSD with several high-capacity HDDs, the purchase cost of the SSD-enhanced array is much lower for a given capacity so the crossover point is also lower—just 400 IOPS.

TCO computations such as these are required for storage and for memory subsystems. It’s easy to be myopic and compare component cost to component cost, but system architects are creating systems and should always try to view component costs through a TCO lens.

Wednesday, December 16th, 2009 at 23:33
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