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11.945 bytes |
Service Hints & Tips |
Document ID: YAST-3R4LYC |
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This document is provided to IBM and our Business Partners to help sell and/or service IBM products. It is not to be distributed beyond that audience or used for any other purpose. |
Questions and answers concerning Mean Time Between Failure (MTBF)
Applicable to: World-Wide
Question:
Why is CMOS/BIOS and FDISK not reporting the correct size of my hard disk drive?
Answer:
There is a difference between MB and millions of bytes. FDISK, FORMAT, and BIOS will report the drive size in MB = 1024*1024=1048576. CHKDSK will report the drive size in bytes. If you purchase a drive that is advertised as a 527MB HDD, the storage industry will use the definition of millions of bytes and use MB=1,000,000 bytes. Multiplied out, 527MB*1,000,000 is 527 million bytes. FDISK and software use the definition of MB=1048576 bytes. For the 527MB drive that was purchased, FDISK will calculate the size as 503MB where 503MB*1048576 = 527,433,728 bytes.
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Storage Definition |
Software or Memory Definition |
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1KB = 1000 bytes |
1KB = 1024 bytes |
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1MB= 1,000,000 bytes |
1MB = 1024*1024 = 1,048,576 bytes |
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1GB = 1,000,000,000 bytes |
1GB=1024*1024*1024 = 1,073,741,824 bytes |
The following table provides some examples. Due to rounding and system environments, you may experience slightly different observations.
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Market Specified Drive Size |
Software Reported Drive Size - (FDISK,System Properties,CMOS...) |
Approx Capacity in Bytes |
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(1MB = 1,000,000) 527MB * |
(1MB = 1024*1024 = 1048576) 503MB (=527/(1024*1024)) |
526,433,728 bytes of disk space |
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1.2GB * |
1.12GB |
1,288,490,189 bytes |
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2.1GB * |
1.96GB |
2,104,533,975 bytes |
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4.5GB * |
4.19GB |
4,498,978,242 bytes |
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9.1GB * |
8.48GB |
9,105,330,668 bytes |
Note: Market Specified Drive Size would represent capacity prior to the installation of an operating system. The total user accessible capacity will vary slightly based on hardware and operating system environments. The examples above will vary from customer setup due to rounding and system environments.
Question:
Some Hard Disk Drives publish a Mean-Time Between Failure (MTBF) metric. What does this mean?
Answer:
MTBF is a method to measure the reliability of a small-form-factor hard disk drive. In general terms, MTBF is often estimated by taking the sum of total operating time for a group of drives divided by the number of failures for that group during the testing period. A more technical definition defines MTBF as "the mean of a distribution of product life times, often estimated by dividing the total operating time accumulated by a "defined group" of drives within a given time period, by the total number of failures in that time period. "A"defined group of drives is a group of drives that have not reached end-of-life (typically five to seven years), are operated within a specified reliability temperature range, under specified normal usage conditions, and have not been damaged or abused.
Question:
If a drive has a rating of a MTBF of 300,000 hours, will the warranty of the drive be 300,000 hours (or 34 years)?
Answer:
In most instances, a drive will reach end-of-life before the specified MTBF rating. MTBF has been used with hard disk drives as a way to measure reliability. This measurement has been criticized, as the method to calculate MTBF varies between manufacturers. MTBF is not a condition of a warranty. In many instances, the hard disk drive will not have a published MTBF. "A predicted MTBF provides a reliability indicator for disk drives. It is not a guarantee of product reliability and does not represent a condition of warranty. In general, higher MTBF correlates with fewer drive failures, but an MTBF claim is not a guarantee of product reliability and does not represent a condition of warranty." Look for a statement of warranty when purchasing a hard disk drive. PC Company hard disk drive offerings include extensive compatibility testing on IBM PC Systems, firmware that is optimized for use with IBM PC Systems, comprehensive user guides to assist with installation, and PC Company warranty, service, and support.
Question:
How can I use MTBF?
Answer:
MTBF can be used to calculate an estimate of failures during a specified time interval. For example, if 100 hard disk drives have been purchased with a 3yr warranty, MTBF of 300,000 hours, and all the drives will operate 10 hours a day for 365 days a year for the next 3 years, then the MTBF can be used to project an estimate of failures = [(# of drives) (hours)] / MTBF = [(100)(10*365*3)] / 300,000 = 4
Note that this number is subject to statistical variation.
Question:
Is there an IBM statement on MTBF?
Answer:
Yes. The following is dated August 27, 1996:
TODAY THE PSG/PCCO IS ANNOUNCING AN ON-LINE (AEFORMS) FORM AND PROCESS TO PROVIDE MTBF DATA FOR MARKETING SALES OPPORTUNITIES. TO ACCESS THE FORM, SIMPLY ACCESS AEFORMS AND THE FORM 'MTBF'. WHEN YOU'VE COMPLETED THE FORM, USE THE SEND FUNCTION TO SEND THE FORM TO THE RTP FOCAL POINT FOR THE PROCESS (AHLMAN @ RALVM8 or AHLMAN @ IBMUSM20).
THE CRITERIA FOR FULFILLING A REQUEST FOR MTBF DATA IS AS FOLLOWS:
1) A NON-DISCLOSURE AGREEMENT MUST BE IN PLACE WITH THE CUSTOMER.
2) MTBF DATA MUST BE A CUSTOMER REQUIREMENT FROM AT LEAST 1 OTHER MANUFACTURER AS WELL AS FROM IBM. IN OTHER WORDS, WE MUST BE IN A COMPETITIVE MARKETING SITUATION.
THE UNDERSTANDING OF MTBF BY THE MARKETING REP IS CRITICALLY IMPORTANT. THERE ARE AT LEAST 2 DISTINCTLY DIFFERENT APPROACHES TO THE CALCULATION OF MTBF AND THEY RENDER RADICALLY DIFFERENT RESULTS. THESE 2 APPROACHES ARE AS FOLLOWS:
1) THE TRUE ENGINEERING DEFINITION AND APPLICATION: THIS APPROACH TO MTBF TAKES THE FAILURES OF A PRODUCT AND STRIPS OUT THE DEAD-ON-ARRIVALS, EARLY LIFE FAILURES AS THE PRODUCT RUNS IN, AND POTENTIALLY THE END-OF-LIFE WEAROUT ELEMENTS. IT ALSO ELIMINATES SERVICE CALLS THAT DO NOT RESOLVE THE SERVICE ISSUE AS IN THE CASE OF INTERMITTENT PROBLEMS THAT ARE NOT DIAGNOSED ON THE FIRST SERVICER VISIT. FURTHER, MANY APPLICATION RELATED PROBLEMS MAY NOT BE INCLUDED.
THIS IS THE TRUE ENGINEERING METHODOLOGY FOR DETERMINING THE BASIS FOR MTBF CALCULATION.
2) THE CUSTOMER PERCEIVED BASIS AND APPLICATION: THIS APPROACH USES THE FAILURES THAT A CUSTOMER SEES, AND THOSE DIRECTLY ATTRIBUTABLE TO THE PRODUCT AS THE BASIS FOR DETERMINING
MTBF. THIS MEANS THAT DEAD-ON-ARRIVALS, EARLY LIFE, AND END OF LIFE (ALTHOUGH IT'S EXTREMELY RARE TO REACH THIS POINT DURING WARRANTY) ARE INCLUDED. ALSO, LIMITED APPLICATION ELEMENTS ARE INCLUDED, IE. PRELOADED SOFTWARE.
THE IBM PC COMPANY USES THE CUSTOMER PERCEIVED METHOD TO CALCULATE MTBF. THE BELIEF IS THAT ANY LARGE CUSTOMER THAT WOULD TRACK THE PERFORMANCE OF OUR PRODUCTS, SHOULD REASONABLY BE ABLE TO DETERMINE THAT THE MTBF WE PROVIDED WAS REASONABLE. OUR DECISION TO USE THE CUSTOMER PERCEIVED BASIS SHOULD PROVIDE AN OPPORTUNITY FOR THE MARKETING REPS TO PROMPT THE CUSTOMER TO DETERMINE THE OTHER MANUFACTURERS BASIS FOR CALCULATION.
IF YOU HAVE ANY QUESTIONS ABOUT THE /PERSONAL SYSTEMS GROUP/PC COMPANY MTBF RELEASE PROCESS, PLEASE SEND A PROFS NOTE TO AHLMAN @ RALVM8 or AHLMAN @ IBMUSM20).
Keywords: OBI mtbf hard drives
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Hint Category |
General Information, Hard Drives | |
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Date Created |
26-01-98 | |
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Last Updated |
24-11-98 | |
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Revision Date |
19-11-99 | |
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IBM Options | |
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Hard Drives | |
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