VERITAS Volume Manager 3.1 Reference Guide

114 Chapter3
Disk Arrays
Disk Array Overview
An array attempts to provide the highest rate of data transfer by
spreading the I/O load as evenly as possible across all the disks in the
array. In RAID-3, the I/O load is spread across the data disks, as shown
in Figure 3-3, “Data Writes to RAID-3,”, and each write is executed on all
the disks in the array. The data in the data disk is XORed and the parity
is written to the parity disk.
RAID-4 maps data and uses parity in the same manner as RAID-3, by
striping the data across all the data disks and XORing the data for the
information on the parity disk. The difference between RAID-3 and
RAID-4 is that RAID-3 accesses all the disks at one time and RAID-4
accesses each disk independently. This allows the RAID-4 array to
execute multiple I/O requests simultaneously (provided they map to
different member disks), while RAID-3 can only execute one I/O request
at a time.
RAID-4 read performance is much higher than its write performance. It
performs well with applications requiring high read I/O rates. RAID-4
performance is not as high in small, write-intensive applications.
The parity disk can cause a bottleneck in the performance of RAID-4.
This is because all the writes that are taking place simultaneously on the
data disks must each wait its turn to write to the parity disk. The
transfer rate of the entire RAID-4 array in a write-intensive application
is limited to the transfer rate of the parity disk.
Since RAID-4 is limited to parity on one disk only, it is less useful than
RAID-5.
RAID-5
RAID-5 is similar to RAID-4, using striping to spread the data over all
the disks in the array and using independent access. However, RAID-5
differs from RAID-4 in that the parity is striped across all the disks in
the array, rather than being concentrated on a single parity disk. This
breaks the write bottleneck caused by the single parity disk write in the
RAID-4 model.
Figure 3-4, “Parity Locations in a RAID-5 Model,” shows parity locations
in a RAID-5 array configuration. Every stripe has a column containing a
parity stripe unit and columns containing data. The parity is spread over
all of the disks in the array, reducing the write time for large
independent writes because the writes do not have to wait until a single
parity disk can accept the data.