Some disk arrays provide multiple ports to access their disk
devices. These ports, coupled with the HBA (Host Bus Adaptor) controller
and any array-local data bus and I/O processor, make up multiple
hardware paths to access the disk devices. Such disk arrays are
called multipathed disk arrays. This type of disk array can be connected to host
systems in many different configurations, (such as multiple ports connected
to different controllers on a single host, chaining of the ports through
a single controller on a host, or ports connected to different hosts simultaneously).
Multipathed disk arrays can be classified as either Active/Active
type, or Active/Passive type.
Active/Passive Type Disk Arrays |
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This type of disk array designates one of the multiple paths
to a disk device as the primary path, and the others as secondary
paths. Access to a disk is enabled through its primary path. If
the primary path fails, one of the secondary paths is made the new
primary path to the disk device, either automatically, or through
administrator intervention. Access to the disk device through a
secondary path may be disabled, or may degrade system performance
severely.
In this type of disk array, a disk device is bound to one
of the redundant hardware components local to the disk array,( for
example, the I/O bus, I/O controller, cache and access port). This
disk is then said to be owned by the port, and the I/O path through
that port is designated as the active path
Some disk arrays have a mode setting, called autotresspass
mode, in which the disk arrays automatically mark a path as primary
path when I/O is attempted through that path. This switch of primary
path is an expensive operation, and degrades the performance of
the disk array severely if different paths are used to access the
disk alternately, by the same or different hosts.
Active/Active Type Disk Arrays |
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This type of disk array allows access to the disk devices
simultaneously (at any time) through all the paths that are available,
without significant performance degradation. Thus, all the paths
are active all the time, except for failed paths.
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