Promise Technology FastTrak S150 - Manual (Page 54)

FastTrak S150 TX4 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 yet the array still maintains the
integrity of data. There are six combinations in which two of the four drives can
fail. FastTrak S150 TX4 protects the data array in four of those cases.
Assume the drives are configured as follows:
• CH indicates channels on the FastTrak S150 TX4 card.
• A/B indicates which striped pair the drive belongs to.
• 1/2 indicates which part of stripe data.
CH 1 CH 2 CH 3 CH 4
Drive A1 Drive A2 Drive B1 Drive B2
Under RAID 0+1, the array maintains data integrity if any 1, 2 combination
survives.
Event
Failed
Drives
Array
Status Why?
1 A1/A2 Functional B1/B2 retain array integrity
2 B1/B2 Functional A1/A2 retain array integrity
3 A1/B2 Functional B1/A2 retain array integrity
4 B1/A2 Functional A1/B2 retain array integrity
5 A1/B1 Offline B2/A2 contain only half of array data
6 B2/A2 Offline A1/B1 contain only half of array data