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FOCUS ASHRAE


Issue 16, June/July


ASHRAE TABLE C-1. (MODIFIED) TEMPERATURE IMPACT ON VOLUME SERVER HARDWARE FAILURE RATE


Dry Bulb Temp (°C) A


15.0 17.5 20.0 22.5 25.0 27.5 30.0 32.5 35.0 37.5 40.0 42.5 45.0


Failure Rate X-Factor Lower Bound


B


0.72 0.80 0.88 0.96 1.04 1.12 1.19 1.27 1.35 1.43 1.51 1.59 1.67


Average Bound C


0.72 0.87 1.00 1.13 1.24 1.34 1.42 1.48 1.55 1.61 1.66 1.71 1.76


Note: Relative Hardware Failure Rate X-Factor For Volume Servers As A Function Of Continuous Operation © ASHRAE Table reformatted by DLB Associates


ASHRAE TABLE 7 (MODIFIED) TIME-AT-TEMPERATURE WEIGHTED FAILURE RATE CALCULATOIN FOR IT EQUIPMENT IN CHICAGO


Location Chicago


% Bin Hours and Associated X Factors for Chicago at Various Temperature Bins 15-20 °C


20-25 °C


% of Hours 67.60%


X Factor 0.865


% of Hours 17.20%


X Factor 1.13


% of Hours 10.60%


25-30 °C


X Factor 1.335


30-35 °C


% of Hours 4.60%


X Factor 1.482


Net X-Factor 0.99 © ASHRAE Table reformatted by DLB Associates


Because there are so many different variables and scenarios, the approach taken by TC 9.9 to provide some reliability data was to use the X-Factor Approach. This approach establishes a baseline failure rate of 1.0 for a data center running continuously at 20 °C. An X-Factor below 1 means fewer failures than the baseline and an X-Factor above 1 means more failures.


The key is to focus on the X-Factor being a relative failure rate compared to the baseline. Whitepaper Table C-1 (over page top left) identifies the impact of temperature on volume server hardware failure rates in terms of a Failure Rate X- Factor. It provides values for the Lower, Average, and Upper Bounds.


The way to interpret this table is as follows: • Assume 1,000 servers for a particular data center operating environment have


48 www.datacenterdynamics.com


a failure rate of four servers across a one year period if the operating environment ran continuously at 20°C


• If the data center continuously operated at 15°C over the entire year, an average X-Factor of 0.72 applies. This would mean four normal failures x 0.72 equals approximately three server failures or a reduction of one server failure per year


• Conversely, if the data center operated at 45°C over the entire year, an average X-Factor


of 1.76 applies and would


mean four normal failures x 1.76 equals approximately seven server failures or an increase of three server failures per year per 1,000 servers.


The X-Factors used in this example are the Average Bound X-Factors from Table C-1 but the Upper and Lower Bound X-Factors may


be more suited depending on the level of risk tolerance for a given application.


The whitepaper Table 7 (above) provides a good example of the impact on the Relative Hardware Failure Rate X-Factor of allowing the outdoor temperature to also directly vary the operating temperature of your data center (eg when operating fulltime in an economizer mode).


The example in Table 7 is for the city of Chicago.


Although Chicago would not


be classified as a moderate climate, the accumulative effect of “time at temperature” demonstrates a potentially attractive condition. In Table 7, it is assumed that the economizer system has the ability to always deliver a minimum temperature of 15°C (through mixing, etc). Therefore the lower temperature bin considered is 15°C to 20°C.From there, a simple proportional analysis of the percentage


Upper Bound D


0.72 0.95 1.14 1.31 1.43 1.54 1.63 1.69 1.74 1.78 1.81 1.83 1.84


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