HP StorageWorks EVA Controller HSG V8.8 Software - Manual (Page 138)

Glossary
138 Command Console V2.5 User Guide
RAID 5
RAID 5 virtual disks use parity for data redundancy. A RAID 5
virtual disk is a type of striped parity virtual disk.
In a RAID 5 virtual disk, host data is written in its entirety as a
strip representing one I/O request into a much larger stripe of
I/O requests stored across the virtual disk membership. An
additional strip for parity information is written into each stripe.
This technique offers the same read performance as reading
from a single device. However, in a RAID 5 virtual disk,
multiple read I/O requests can occur resulting in a very high
overall subsystem read performance.
Write performance, however, is much worse. Because writing a
small strip of data into a larger amount of data on a device is
really a read-modify-write operation, writing becomes
time-consuming. In addition, because each data stripe is
protected by parity information, there is a significant level of
data redundancy for high availability.
RAID 5 virtual disks provide high read performance and high
availability at reasonable cost. They are optimal for use in
applications that have relatively high I/O read request rates and
require relatively low data transfer rates.
read cache
A block of high-speed memory used to buffer data being read
from storage devices by a host. A read cache responds to host
read requests from local cache memory if possible rather than
from external storage devices. Therefore, it increases the
controller's effective device access speed.
The controller maintains copies of data recently requested by
the host in cache, and it may fetch blocks of data ahead of a
request in anticipation that the controller will access the next
sequential blocks. In a typical read cache, host write requests
are handled without involving caching.