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Majority Vote and Minority Polling Recovery Procedures for Files

IP.com Disclosure Number: IPCOM000038924D
Original Publication Date: 1987-Mar-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 14K

Publishing Venue

IBM

Related People

Cunningham, EA: AUTHOR

Abstract

Most data bits from a storage file are correct most of the time, some may be wrong a small fraction of the time, and a very few might be wrong more than half the time, but bits are almost never totally wrong. Rereads can recover those bits that are wrong a very small fraction of the time, but are nearly useless for rates which produce several or more errors per sector. Majority vote is the process of voting on a bit by bit basis of the values read on several different revolutions. The process greatly reduces the effect of random noise and allows the recovery of signals having very low signal-to-noise ratio. This allows recovery of failure rates up to 100 times higher than practical by a reread process.

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Majority Vote and Minority Polling Recovery Procedures for Files

Most data bits from a storage file are correct most of the time, some may be wrong a small fraction of the time, and a very few might be wrong more than half the time, but bits are almost never totally wrong. Rereads can recover those bits that are wrong a very small fraction of the time, but are nearly useless for rates which produce several or more errors per sector. Majority vote is the process of voting on a bit by bit basis of the values read on several different revolutions. The process greatly reduces the effect of random noise and allows the recovery of signals having very low signal-to-noise ratio. This allows recovery of failure rates up to 100 times higher than practical by a reread process. Minority polling tests the bits that are varying significantly between reads, which can recover bits that are wrong more than half the time (including use of error checking codes). Error correction codes (ECC) can be reserved for errors that are totally wrong. Minority polling adds to the majority vote in cases where the written transitions of the disk are distorted by defects, erasure or write faults such that the average of many reads gives the wrong answer. Minority polling is a trial and error flipping of bits combined with testing with the ECC for bits that show about comparable odds of changing between one and zero with each reread. These bits are thus identified as probable errors. The use of ECC can handle one or two bits per sector, but the minority polling has the capability to handle around 12 per sector in any position without requiring large amounts of time. The process of majority vote involves multiple reads in which all the data of the sector is stored. Each bit position has data from several different reads. The data can either be a one or a zero. Thus if there is an odd number of reads, it can be determined whether the bit was more often a one or a zero. Votes such as 2 out of 3 for a simple case or 3 out of 5 for better results can be used. The assumption for this procedure, is that the recording basically has the correct value. In terms of margins, the value is still of the correct polarity, but may be quite low. The noise level adds and subtracts from the nominal margin level causing it at times to be of either polarity. Given enough averaging, the number of times it is of the correct polarity will be larger than the number of times it is of the wrong polarity. The averaging achieved in the "voting" process greatly reduces the effect of noise. The use of ECC can be used on the result to correct some of the remaining...