Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

DRAFT -- Revision 2.0
August 22, 1996Page 9
Disk arrays using RAID technology
Solid state disks
Standalone disk drives with LVM mirroring
Standalone disk drives are simple, non-array type disks; i.e., they do not use implement
any RAID level in hardware. A new acronym has emerged to describe these simple
disks: JBODs, which stands for "Just a Bunch Of Disks". JBODs can be single disk
spindles with a controller and power supply or can be combined into towers or racks
with a single power supply. Even in combination, each disk drive retains its own
controller and is addressed individually. However, there is only one physical path from
the computer system to the entire tower or rack; i.e., each disk does not have its own
SCSI connector.
JBODs are available in many capacities. The most popular disk mechanisms have
capacities of 2 GB, and 4 GB and are typically based on the newest, fastest disk
technology. One GB mechanisms, although still available, are declining in popularity.
Due to their simplicity, standalone disk drives are the least expensive solution. In a
High Availability environment however, it makes no sense to discuss standalone disk
drives without data protection. JBODs can be combined with software solutions such
as MirrorDisk/UX to provide higher availability than they would alone provide, although
this significantly increases hardware costs. JBODs also provide the greatest flexibility
in configuration such as data placement, power source, racking, etc.
Figure 1 shows a typical way of configuring standalone mirrored disks in an HA
environment. It is important to notice that each copy of the data is on a different disk
link. This configuration is necessary to minimize Single Points of Failure (SPOFs).
Each group of disks is shown physically connected to multiple computer systems.
Logical Volume Manager (LVM) is necessary to control shared or exclusive access to
the data. However, this does not automatically imply that concurrent access to the
same data is possible or desired. Data corruption can occur unless network-based
software controls the access to the multiply-connected disks.
The High Availability software MC/LockManager in conjunction with Oracle Parallel
Server are required to arbitrate concurrent write access to the same data. The
application runs on both systems and access to the data continues even if one of the
SPUs fails.
This configuration can also be used with the HA software MC/ServiceGuard to provide
exclusive access to the data. MC/ServiceGuard ensures that an application that is
accessing a particular group of disks runs on only one system at a time, thus preventing