Promise Technology FastTRAK66 User Manual page 50

A combination of both above array types. It can increase performance by reading
and writing data in parallel while protecting data with duplication. A minimum of
four drives needs to be installed. With a four-drive disk array, two pairs of drives
are striped. Each pair mirrors the data on the other pair of striped drives. The data
capacity is similar to a standard Mirroring array with half of total capacity dedicated
for redundancy.
Dual Data Redundancy
One unique (though rarely occurring) feature of RAID 0+1 is dual fault tolerance.
This allows two drives to fail simultaneously and still maintain the integrity of data.
There are six combinations in which two drives can fail; FastTrak66 protects the
data array in four of them for most drives (some drives do not permit the Slave drive
to continue to function if the Master drive fails). Assume the drives are configured
as follows (M = Master, A/B indicates which striped pair the drive belongs to, #
indicates which part of stripe data):
IDE 1
M Drive A1
S Drive B2
Under RAID 0+1, the array maintains data integrity if any 1&2 combination survives.
Event Failed Drives
1 A1/A2
2 B2/B1
3 A1/B2
4 B1/A2
5 A1/B1
6 B2/A2
Spanning (JBOD)
Understanding Disk Array Concepts
Array Status
Working
Working
Working
Working
Offline
Offline
IDE2
Drive B1
Drive A2
Why?
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
6-5