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SuperTrak66™ User's Manual
Chapter 6
Mirroring (RAID 1)
Mirroring is a widely-used method of fault tolerance. Mirroring exactly duplicates
the content of one drive on to the other drive for every write operation. If either drive
fails, the array continues to function using the remaining working drive. This also
allows for time to "hot" swap the failed drive and rebuild data on to a replacement
drive without downtime (the RAID system can also invoke a "hot" spare drive, if
available).
Generally, mirrored RAID arrays provide little-to-no performance enhancement. The
SuperTrak66 system provides some performance enhancement on mirrored arrays
by using elevator seek and load-balancing to achieve the fastest response possible
from each access. Still, mirroring is not a performance replacement for striping.
Use a mirrored array in environments which are not performance intensive, but
rather must act as secure, reliable data/file servers.
Striping/Mirroring (RAID 0+1)
Striping/mirroring gives the best of both worlds for RAID 0 and 1. You've got all the
performance gains and concerns of striping, coupled with all the advantages of data
protection, elevator seek, and load balancing of mirroring. This type of array should
be used for those cases which must match the application requirements of both
striping and mirroring.
An added gain for RAID 0+1 is that, in certain situations, such an array offers
double fault tolerance. Double fault tolerance allows your data array to continue to
operate should two drives fail. We'll describe the four scenarios in which two drives
can fail and still operate the array below. Under such an array structure, you would
have a striped Array A consisting of two drives 1 and 2 (we'll call the drives, A1 and
A2, for our purposes here). This striped pair of drives is duplicated by a second
striped array B consisting of two drives 1 and 2 (we'll call them B1 and B2). The
array will continue to operate if two drives fail as follows:
Scenario 1: Drives A1 and B2 fail, drives B1 and A2 operate the array.
Scenario 2: Drives A2 and B1 fail, drives B2 and A1 operate the array.
Scenario 3: Drives A1 and A2 fail, drives B1 and B2 operate the array.
Scenario 4: Drives B1 and B2 fail, drives A1 and A2 operate the array.
So long as there is a "1" drive from either "A" or "B" and a "2" drive from either
"A" or "B", the array will remain functional. There are two double fault drive failure
scenarios that will take the array offline. Should both drives A1 and B1 go down, or
drives A2 and B2 go down simultaneously, the array will be offline.
Data striping with dedicated parity drive (RAID 3)
RAID Level 3 stripes data across several drives. Parity data is calculated and
stored on a single dedicated parity drive. The parity information allows recovery if
any single drive fails. Performance of a RAID 3 array is nearly the same as RAID 0
during data reads. However, during data writes, parity data must be calculated and
updated to the dedicated drive each time. This slows performance during smaller
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