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 A smidgen of pigeon is just enough to give your kids a treat |
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Menu: :: Home :: Fascinations :: Goat Labs :: Miscellany :: RANT! :: Silly stuff :: Useful stuff :: Cars & Driving |
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:: Printable version | |
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Drive array MTBF calculatorIntroductonRight then - drive arrays. Before we start, bear in mind that the best indication of a drive's useful life span is its guarantee period. If a manufacturer is prepared to guarantee a drive for five years, it's a fair bet that the drive will perform reliably throughout that time. If it doesn't (and you're making regular backups and / or using a fault tolerant drive setup) then it's gonna cost you as much as the postage and packing to return the drive under warranty, rather than buying a new one.Drive failures tend to adhere to the "bathtub curve" when it comes to failures: As a rule of thumb, they have a high 'infant mortality rate' which falls off sharply after the first couple of weeks to an insignificant amount. It then stays low until it approaches the end of the guarantee period, where failures become more likely rather quickly, hence making a graph in the shape of a bathtub :-) So, you have a bunch of drives arranged in some form of array and you need to get an idea as to how often you can expect a drive to keel over. The astronomical MTBF (Mean Time Before Failure) figures that manufacturers provide are totally pointless in a single drive scenario (best to go by the warranty period as mentioned above), but come in to their own in an array calculation, as this is how they were worked out in the first place! Calculating failure ratesIn order to make life easy, I've written this little calculator to allow you to enter the number of drives in your array and the manufacturers MTBF figure. It's not the best idea to "mix and match" drive types in any form of array, so it (at present) assumes that each drive has the same MTBF value. My little web swerver (sic) will then work out how often you'll be woken up in the middle of the night by a pager message saying your main database server has gone offline :-)Note that the MTBF of any array is directly proportional to the number of drives it contains: An array with three drives (the minimum needed for a RAID-5 array) will generally last longer than an array of six drives. Remember, however, that an array with only three drives may be slower than an array with six as it can only get data from three drives simultaneously... These calculations will provide you figures for RAID levels 0 and 5 - I need to check my theories behind the calcs for levels 1, 10 and 50. Tell me about your shelf...To use this gadget, just fill in the boxes below!This page was last updated: 14th June 2005 at 12:47am BST |
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