Promise Management(PAM) FastTrack S150 SX$ and SX4000 Series User Manual

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