HP Raid Software for OpenVMS, Version 3.0A
HP RAID Software for OpenVMS, SPD 46.49.12
Version 3.0A
• more I/O requests serviced per unit time due to prob-
abilistic load balancing, or
• higher data transfer rate due to concurrent transfer of
data to or from more than one disk to satisfy a single
request.
Features Applicable to RAID Level 5 Arrays
The usable capacity of RAID Level 5 array is approxi-
mately given by:
Usable Capacity = (N-1) * 0.99 * CS
Where N is the number of physical disks in the array.
CS is the capacity of the smallest disk in the array.
RAID Software protects against loss of data and loss of
data accessibility due to the failure of any single disk in
a RAID Level 5 array. If a disk in a RAID Level 5 ar-
ray fails, RAID Software provides continued service to
applications by regenerating the failed disk’s data using
information from the array’s remaining disks. (For full
protection against loss of data availability due to single
hardware failures, RAID Software can be used in con-
junction with redundant hardware and supporting soft-
ware throughout the system.)
While a RAID Level 5 array is reduced by a failed disk,
data can be read and written, but no redundancy is pro-
vided. A second disk failure while an array is reduced
prevents application access to the data stored on the
array, and may result in data loss.
RAID Software reconstructs the contents of a failed disk
if a replacement disk is assigned to it. Reconstruction
does not interrupt application access to data on the ar-
ray, although performance may be affected. Replace-
ment disks may either be assigned to RAID Software
by the storage administrator (using a DCL command) or
they may be placed in a spareset associated with one
or more RAID Level 5 arrays. If a disk in a RAID Level
5 array with an associated spareset fails, RAID Soft-
ware automatically acquires a replacement disk from the
spareset and performs reconstruction without storage
administrator intervention.
Using RAID Software for OpenVMS
To use RAID Software for OpenVMS, the storage ad-
ministrator first creates an array using DCL functions
supplied with the software. This destroys any data previ-
ously stored on the disks and creates the data structures
required to manage the array. The member disks of a
RAID Software array are ODS-2 structured volumes, so
OpenVMS mechanisms protect against inadvertent mis-
use of members for the life of the array. However, the
virtual disk units created by RAID Software can be initial-
ized as FILES-11 ODS2 or ODS5 or any other volume
structure.
Each virtual disk created by RAID Software is a single
management entity. The storage administrator should
use appropriate storage management procedures (such
as backups) with virtual disks, which can be consider-
ably larger than typical physical disks. Once a collection
of physical disks is bound into an array, it is not possi-
ble to retrieve data directly from the array’s individual
member disks.
Performance of RAID Software for OpenVMS
The primary purpose of RAID for OpenVMS RAID Level
0 arrays is to enhance application performance by im-
proving I/O request processing and/or data transfer rate.
RAID Level 0 technology normally implies a reduction in
data reliability. The storage administrator can improve
data reliability by increasing backup frequency or using
RAID Software in conjunction with Volume Shadowing
for OpenVMS.
The primary purpose of RAID Software Level 5 arrays is
to improve data reliability. It may provide the secondary
benefit of improved performance (due to load balanc-
ing) for applications whose I/O workload consists largely
of reading data. For applications with mostly write I/O
workloads, RAID Software Level 5 arrays may provide
lower I/O performance than conventional disks because
it must update redundant information each time an ap-
plication writes data.
The storage administrator should understand applica-
tion I/O characteristics and weigh the relative priorities
of performance, equipment cost, and data reliability to
determine whether RAID Software is appropriate for use
with a given application.
HARDWARE REQUIREMENTS
RAID Software is supported when used with the VAX, Al-
pha, and I64 processors and supported OpenVMS clus-
ter configurations. See the Software Requirements sec-
tion for the quali fied OpenVMS versions. RAID Software
may be installed in an OpenVMS cluster configuration
of any size supported by OpenVMS Software, but the
RAID Software has been qualified to run on a maximum
of 20 nodes within a single OpenVMS cluster configu-
ration.
RAID Software requires a minimum of 1 (for RAID Level
0) or 3 (for RAID Level 5) and a maximum of 32 physical
disks for each array. Up to 50 arrays may be created
in a single OpenVMS cluster. Disks, storage elements,
(and the subsystem configurations that contain them)
are supported by the OpenVMS Operating System Ver-
sions (see Software Requirements) through the follow-
ing device drivers shown in Table 1.
2