This article explains some often misunderstood concepts about RAID that need to be correctly understood in order to make good judgements about which type of RAID scheme is best.
The redundant check values c0, c1, c2, and c3 are sums of products. The message values are multiplied by constants from the encoding matrix and summed to form the check values. The c’s shown above in red are all constants, but not the same constant. With that scheme, records from individual devices can be read independently. Many reading operations can occur simultaneously when everything is operating without error.
To update this one check value for a new message value, the old message value and old check value must first be read, the new check value computed and the new message value and check value written. There are usually thousands of bytes in each record, so the above computation must be done thousands of times for each record and that process must be repeated R times where R is the number of redundant bytes in each horizontal codeword. This method becomes impractical when R > 2 and is highly inefficient even when R = 2. It is important to understand that the above-mentioned mathematical relationship between the message and check values must always be maintained in order to maintain fault tolerance. When writing new data values, some companies are recomputing check values over a potentially very long period of time - maybe microseconds, milliseconds, seconds, minutes, hours or even days - it all depends upon how busy the operating system is, the current state of the storage system and what priority recomputing check value tasks have compared to other tasks in a multitasking operating system. During that recomputing time, fault-tolerance is temporarily lost, and, if anything goes wrong during that time period, fault-tolerance is permanently lost. ECC Tek’s Proposal ECC Tek is proposing a RS RAID system whereby the relationship between the message and check values is continuously maintained because all of the check values are always computed simultaneously in one tick of a very high frequency clock in hardware as illustrated below. |
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