Promise Technology FastTrak TX4000 - Manual (Page 56)

FastTrak TX4000 User Manual
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About Dual Data Redundancy
One unique (though rarely occurring) feature of RAID 0+1 is dual fault tolerance.
In some cases, two drives can fail simultaneously and still maintain the integrity
of data. There are six combinations in which two drives can fail. FastTrak
TX4000 protects the data array in four of those cases depending on drive type
(some drives do not permit the Slave drive to continue to function if the Master
drive fails).
Assume the drives are configured as follows:
• IDE indicates channels on the FastTrak TX4000 card.
• A/B indicates which striped pair the drive belongs to.
• 1/2 indicates which part of stripe data.
IDE 1 IDE2 IDE3 IDE4
Drive A1 Drive B1 Drive A2 Drive B2
Note that FastTrak TX4000 card does not use Master/Slave drive settings.
Under RAID 0+1, the array maintains data integrity if any A, B combination
survives.
Event
Failed
Drives
Array
Status Why?
1 A1/A2 Offline B1/B2 contain only half of array data
2 B1/B2 Offline A1/A2 contain only half of array data
3 A1/B2 Functional B1/A2 retain array integrity
4 B1/A2 Functional A1/B2 retain array integrity
5 A1/B1 Functional B2/A2 retain array integrity
6 B2/A2 Functional A1/B1 retain array integrity