User guide

Compaq StorageWorks RAID Array 4000 User Guide
■ Disk failure will result in data loss for all RAID 0 logical drives.
■ You cannot assign an online spare to an array containing a RAID 0
logical drive.
During configuration, consider assigning an online spare to further improve
your system’s fault tolerance. An online spare (sometimes called a hot-spare)
is a drive the controller uses when a drive failure occurs. If a drive fails, the
controller rebuilds the data that was on the failed drive onto the online spare.
The controller also sends data that it would normally store on the failed drive
directly to the online spare.
The online spare improves the overall system fault tolerance by automatically
replacing a failed drive and quickly restoring the system to full RAID fault
tolerance protection. During the time the system is operating in interim
recovery mode with the failed drive, the system is vulnerable to an additional
drive failure.
You can install up to four online spares for each array controller. To provide
an online spare, you must have at least one installed but unassigned physical
drive and the system must be running some type of fault-tolerance method
(RAID 1, RAID 4, or RAID 5).
Drive arrays allow data access performance to be dramatically enhanced over
non-arrayed drives. In addition, several other performance enhancement
features are discussed here.
Distributing data makes it possible to access data concurrently from multiple
drives in an array. This yields I/O rates many times faster than non-arrayed
drives. Distributing data allows data striping.
Data striping is generated automatically on an array to store user data. A stripe
is a collection of contiguous data that is distributed evenly across all physical
drives in a logical drive. The width of the stripe (the amount stored on each
physical drive) can be selected when the logical drive is created. The
following table shows the valid stripe sizes for each RAID level supported.
Default values are shown in bold.