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MTBF

Hardware- or Software-Based RAID
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Mean Time Between Failure (MTBF)

The theoretical MTBF of a disk drive represents the steady state failure rate of a large population of drives in volume manufacture. This is the expected time after the initial burn-in phase that it will take a hardware component to fail due to normal wear and tear.

The Bathtub Curve Reference: Microsoft Corporation


Calculating Theoretical MTBFs

The theoretical MTBF of any hardware configuration can be calculated if you have the MTBFs of each component that make up your configuration. For example, you can calculate the MTBF of your server if you have the MTBFs of main CPU board, disk drives, server packaging, etc - a rather daunting task. Most discussions of server MTBF focus on disk drive MTBFs for a several reasons. First of all, components with moving parts (such as disk drive actuators and motors) typically have significantly lower MTBFs than non-moving components (such as memory chips or main CPU boards). Because a server's theoretical MTBF is most influenced by the MTBF of the least reliable component as well as the sheer number of components, disk drive MTBFs typically dominate the overall server configuration theoretical MTBF. Theoretical MTBF decreases in proportion to the number of components that make up the server, so larger configurations containing many disk drives by definition have a lower MTBF. Add to that the fact that disk drives contain data that may be time-consuming or impossible to recreate, it is easy to see why disk drive reliability dominates server reliability discussions.

The following examples illustrate the impact of disk drive MTBF on overall server MTBF.

A server's theoretical MTBF is calculated from the theoretical MTBFs of the components that make up a the server:

MTBF = 1/(1/N1+1/N2+1/N3+...+1/Nx)
where
N = MTBF of each component
x = the number of components in the configuration

If one component, such as a disk drive, has a significantly lower MTBF than the rest of the population, its MTBF dominates the overall server MTBF. For example, examine a server containing one CPU main board with an MTBF of 1,000,000 hours, and a single drive with an MTBF of 300,000 hours (we'll ignore the other components for simplicity's sake). The server's MTBF is calculated as follows:

Server MTBF = 1/(1/1,000,000 + 1/300,000)
= 230,769 hours, close to the disk drive's MTBF

Even if all your components have high MTBFs, the overall configuration's overall MTBF is reduced in direct proportion to the number of components in the configuration. For example, the MTBF of a storage subsystem consisting of two disk drives with identical 300,000 hour MTBFs is:

Disk dr. subsys. MTBF=1/(1/300,000+1/300,000)
= 150,000 hours, exactly half the MTBF of each disk drive

Similarly, a 10-drive configuration MTBF is one-tenth the MTBF of a single drive, or 30,000 hours, and a 100-drive configuration is reduced to 3,000 hours.

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Last modified: June 20, 2002