Promise Technology FastTrak SX4000 Lite User Manual download pdf (Page 52)

FastTrak S150 SX4 / SX4000 / SX4000 Lite 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 and still maintain the integrity

of data. There are six combinations in which two drives can fail. FastTrak S150

SX4 and SX4000 / SX4000 Lite 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

1 2 ... 47 48 49 50 51 52 53 54 55 56 57 ... 77 78

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Promise Technology FastTrak SX4000 Lite User Manual Page 52