Compaq RAID ARRAY 3000 EK-SMCPO-UG. C01 - Manual (Page 52)

2-18 RAID Array 3000 Pedestal Storage Subsystem Hardware User’s Guide
Compaq Confidential – Need to Know Required
Writer: Bob Young Project: RAID Array 3000 Pedestal Storage Subsystem Hardware User’s Guide Comments:
Part Number: EK-SMCPO-UG. C01 File Name: c-ch2 RAID Array Controller.doc Last Saved On: 12/4/00 1:51 PM
In general, RAID 4 is best suited for applications such as graphics, imaging, or
video that call for reading and writing large, sequential blocks of data.
However, you may find that RAID 4 is preferable to RAID 5 even for
applications characterized by many small I/O operations, such as transaction
processing. This is due to the controller’s intelligent caching, which efficiently
handles small I/O reads and writes, and to the relatively less complex
algorithms needed to implement RAID 4.
The benefits of RAID 4 disappear when you have many, small I/O operations
scattered randomly and widely across the disks in the array. RAID 4’s fixed
parity disk becomes a bottleneck in such applications, as the following
example illustrates. Let’s say the host instructs the controller to make two
small writes. The writes are widely scattered, involving two different stripes
and different disk drives. Ideally, you would like both writes to take place at
the same time, but RAID 4 makes this impossible, since the writes must take
turns accessing the fixed parity drive. For this reason, RAID 5 is the better
choice for widely scattered, small write operations.
CAUTION: RAID 4 can withstand a single failure and handle I/O activity without
interruption in degraded mode until the failed drive is rebuilt. If a second drive
fails while the RAID set is in degraded mode, the entire RAID set will fail.