IBM SG24-4576-00 - Manual (Page 39)

1.6.6.1 RAID-0 - Block Interleave Data Striping without Parity
Striping of data across multiple disk drives without parity protection is a disk
data organization technique sometimes employed to maximize DASD subsystem
performance (for example, Novell NetWare
′
s data scatter option).
An additional benefit of this data organization is drive spanning. With data
striped across multiple drives in an array, the logical drive size is the sum of the
individual drive capacities. The maximum file size may be limited by the
operating system.
│
┌───────┴───────┐
│
Disk │
│ Controller │
└───────┬───────┘
│
│
┌─────────────┬────────────┼────────────┬────────────┐
│
│ │ │ │
┌────┴─────┐ ┌────┴─────┐ ┌────┴─────┐ ┌────┴─────┐ ┌────┴─────┐
│ xxxxx │ │ xxxxx │ │ xxxxx │ │ xxxxx │ │ xxxxx │ Block 0
├──────────┤ ├──────────┤ ├──────────┤ ├──────────┤ ├──────────┤
│ yyyyy │ │ zzzzz │ │ │ │ │ │ │
├──────────┤ ├──────────┤ ├──────────┤ ├──────────┤ ├──────────┤
│ │ │ │ │ │ │ │ │ │ Block n
└──────────┘ └──────────┘ └──────────┘ └──────────┘ └──────────┘
Disk 1 Disk 2 Disk 3 Disk 4 Disk 5
xxxxx = Blocks belonging to a long file
yyyyy and zzzzz = Blocks belonging to short files
Figure 11. RAID-0 (Block Interleave Data Striping without Parity)
Data striping improves the performance with large files since reads/writes are
overlapped across all disks. However, reliability is decreased as the failure of
one disk will result in a complete failure of the disk subsystem according to the
formula:
Mean Time to Failure of a single disk
Mean Time to Failure = ─────────────────────────────────────
Number of Disks in the array
1.6.6.2 RAID-1 - Disk Mirroring/Duplexing
This approach keeps two complete copies of all data. Whenever the system
makes an update to a disk, it duplicates that update to a second disk, thus
mirroring the original. Either disk can fail, and the data is still accessible.
Additionally, because there are two disks, a read request can be satisfied from
either device, thus leading to improved performance and throughput. Some
implementations optimize this by keeping the two disks 180 degrees out of phase
with each other, thus minimizing latency.
However, mirroring is an expensive way of providing protection against data
loss, because it doubles the amount of disk storage needed (as only 50% of the
installed disk capacity is available for data storage).
24
NetWare Integration Guide