Appendix A: RAID Concepts
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 |
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Drive A1 | Drive A2 | Drive B1 | Drive B2 |
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Under RAID 0+1, the array maintains data integrity if any 1, 2 combination survives.
| Failed | Array |
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Event | Drives | Status | Why? |
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1 | A1/A2 | Functional | B1/B2 retain array integrity |
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2 | B1/B2 | Functional | A1/A2 retain array integrity |
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3 | A1/B2 | Functional | B1/A2 retain array integrity |
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4 | B1/A2 | Functional | A1/B2 retain array integrity |
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5 | A1/B1 | Offline | B2/A2 contain only half of array data |
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6 | B2/A2 | Offline | A1/B1 contain only half of array data |
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