IBM System Storage DS4000, Hardware Manual

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Chapter 4. IBM System Storage DS planning and configuration
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4.10.9 Persistent reservations
Persistent reservations are a SCSI-3 feature for restricting access to storage media, based on
the concept of reservations that the host can establish and manipulate. Earlier versions of
SCSI provide a simple reservation capability through the RESERVE and RELEASE
commands. SCSI-3 persistent reservations provide a significant superset of the earlier
capability. Improvements that come with persistent reservations include:
򐂰 A well-defined model for reserving across multiple host and target ports
򐂰 Levels of access control, for example, shared reads, exclusive writes, exclusive reads, and
writes
򐂰 Ability to query the storage system about registered ports and reservations
򐂰 Provisions for persistence of reservations through power loss at the storage system
A logical drive reservation is a feature of the cluster software (the actual reservation of the
logical drive is handled by the host application) that allows one or more host ports to reserve
a logical drive, thus preventing other host ports from accessing the same logical drive.
Unlike other types of reservations, a persistent reservation reserves across multiple host
ports, provides various levels of access control, offers the ability to query the storage system
about registered ports and reservations, and, optionally, provides for persistence of
reservations in the event of a storage system power loss.
The benefits of the persistent reservations feature is that it allows the DS5000 storage
subsystem to integrate with cluster solutions that use shared logical drives for increased
availability, scalability, and performance.
The Persistent Reservation options provided with the DS Storage Manager enables you to
view and clear logical volumes reservations and associated reservations.
To locate this setting, highlight the logical drive in the Storage Manager menu and select
Advanced > Maintenance > Persistent Reservations. The window shown in Figure 4-117
opens.
Figure 4-117 View of persistent reservations