Page 232 -

6.2 / RAID 199

Physical Physical Physical Physical
Logical disk disk 0 disk 1 disk 2 disk 3

strip 0 strip 0 strip 1 strip 2 strip 3
strip 1 strip 4 strip 5 strip 6 strip 7
strip 2 strip 8 strip 9 strip 10 strip 11
strip 3 strip 12 strip 13 strip 14 strip 15
strip 4
strip 5
strip 6
strip 7 Array
strip 8 management
software
strip 9
strip 10
strip 11
strip 12
strip 13
strip 14
strip 15

Figure 6.9 Data Mapping for a RAID Level 0 Array

strips on each disk; and so on. The advantage of this layout is that if a single I/O re-
quest consists of multiple logically contiguous strips, then up to n strips for that re-
quest can be handled in parallel, greatly reducing the I/O transfer time.
Figure 6.9 indicates the use of array management software to map between
logical and physical disk space.This software may execute either in the disk subsys-
tem or in a host computer.

RAID 0 FOR HIGH DATA TRANSFER CAPACITY The performance of any of the
RAID levels depends critically on the request patterns of the host system and
on the layout of the data. These issues can be most clearly addressed in RAID 0,
where the impact of redundancy does not interfere with the analysis. First, let us
consider the use of RAID 0 to achieve a high data transfer rate. For applications to
experience a high transfer rate, two requirements must be met. First, a high transfer
capacity must exist along the entire path between host memory and the individual
disk drives. This includes internal controller buses, host system I/O buses, I/O
adapters, and host memory buses.
The second requirement is that the application must make I/O requests that
drive the disk array efficiently. This requirement is met if the typical request is for
large amounts of logically contiguous data, compared to the size of a strip. In this
case, a single I/O request involves the parallel transfer of data from multiple disks,
increasing the effective transfer rate compared to a single-disk transfer.