About Dual Data Redundancy - Promise Technology FastTrak S150 User Manual

FastTrak S150 TX4 User Manual

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.
Under RAID 0+1, the array maintains data integrity if any 1, 2 combination
survives.
Event
1
2
3
4
5
6
CH 1 CH 2
Drive A1 Drive A2
Failed Array
Drives Status
A1/A2 Functional
B1/B2 Functional
A1/B2 Functional
B1/A2 Functional
A1/B1 Offline
B2/A2 Offline
CH 3
Drive B1
B1/B2 retain array integrity
A1/A2 retain array integrity
B1/A2 retain array integrity
A1/B2 retain array integrity
B2/A2 contain only half of array data
A1/B1 contain only half of array data
50
CH 4
Drive B2
Why?