About dual data redundancy – Promise Technology FastTrak SX Series Version 4.4 User Manual
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Appendix A: RAID Concepts
87
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
SX4000 series and S150 SX4 protect the data array in four of those cases.
Assume the drives are configured as follows:
•
CH indicates channels on the FastTrak Controller 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