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Re: Big Numbers

SteveJ Aug 28, 2012 5:29 PM
Posted in group: Guerrilla Capacity Planning
After I wrote this, I realised I have a real gap in my knowledge of stats:

 - for single or small numbers of machines, how do you calculate useful numbers from MTBF's?

If I buy a new PC every 3 years, only ever owning one at time, and the disk drives have are rated at "1M hours MTBF", what's the probability in a lifetime of ownership (50 yrs) of having a failure?

Is it just 1% every year and 50*1% for 50 years?

Or, what proportion of single PC owners will experience a disk drive failure in 50 years of ownership?

For small numbers, many small businesses have 4-5 servers with a few disks each, and tend to keep each server 4-5 years.
What's the likelihood of having to replace a disk in a server?
Comes to a prosaic purchase decision:

steve jenkin wrote on 28/08/12 8:40 AM:
DrQ wrote on 28/08/12 12:59 AM:

"1.5 CPU-millennia every 3 days"

Manufacturers like to quote 1M-hrs MTBF, or 100 years (a 1%/yr failure rate)
The 2007 paper by Google talks of failures as %/yr.
They found 1.7% in 1st year rising to ~8.5% in 3rd year.
Guess 4% = 8,000 drives/yr = ~20 drives/day

Steve Jenkin, Info Tech, Systems and Design Specialist.
0412 786 915 (+61 412 786 915)
PO Box 48, Kippax ACT 2615, AUSTRALIA