IBM Midrange System DS4000 Series Hardware Manual page 444

Missing paths in a cluster environment
LUN bouncing can be caused by an incorrect configuration in which one cluster node can
access the shared logical drives only through one controller and another node can only
access the same shared logical drives through the alternate controller.
The missing paths can be identified using the same method described in "Missing path(s) to
host" on page 421.
Persistent reservations
A host may issue a SCSI persistent reservation command to restrict which other HBA ports
can access a particular LUN. These commands can be used by a server to prevent HBA
ports in other servers from accessing the LUN and thereby prevent accidental data corruption
caused by one server overwriting another server's data. "Reserve" and "Persistent Reserve"
are often used by clustering software to control access to shared logical disks.
If a server is not shut down or removed from the server cluster in a controlled way, its reserves
and persistent reserves can sometimes be left in place, preventing other servers from
accessing data that is no longer in use by the server holding the reservation. In this situation,
a storage administrator or server administrator might want to break the reservation and allow
a new server to access the virtual disk.
In this situation, the safest thing to do is to have the server that owns the reservation explicitly
release the reservation, as this ensures that the server concerned has flushed its caches and
its software is aware that it has lost access to the disk. In circumstances where this is not
possible, then most operating systems provide operating system specific tools to allow
reservations to be removed. Consult the operating system documentation for details.
It is possible to view the Logical Drive Reservations and Logical Drive Registrations from the
Storage Manager Subsystem Management window by selecting Advanced 
Maintenance  Persistent Reservations. The dialog shows any logical drives in the
storage subsystem that have registrations, with the first logical drive in the list highlighted by
default. This table describes the information and buttons shown in the Persistent Reservation
dialog:
Logical drive name Shows the user label of the logical drive with persistent reservations.
LUN
Registrations
Reservation type
None
WE
EA
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IBM Midrange System Storage Hardware Guide
Logical drives are listed in alphabetical order. If a logical drive user
label is not available, then its World Wide Identifier (WWID) appears.
Shows the assigned LUN number for the particular logical drive.
Shows the number of registrations for the particular logical drive.
Shows an abbreviated form of the associated reservation type for the
particular logical drive. Each addressable logical drive can have one
reservation. Each reservation can grant access rights to one or more
registrants, depending on the reservation type. You can reserve a
logical drive for a specific access level by a group of registrants. All of
the registrants within a group are restricted to the access level defined
by the reservation type. This list shows reservation types as follows:
The logical drive has registrants, but it currently has no reservation.
Write exclusive: Only the host port that reserved the logical drive
may write to the logical drive. Reads shared: Any host port may
read from the logical drive.
Exclusive Access: Only the host port that reserved the logical drive
may read from or write to the logical drive.