Compex Systems Advanced SerialRAID Adapters SA33-3285-02 - Manual (Page 310)

SSA Disk Fencing
SSA disk fencing is a facility that is provided in the SSA subsystem. It allows multiple
using systems to control access to a common set of disks.
Using the fencing commands that are provided by the hardware, you can prevent
particular using systems from accessing a particular disk drive. Each disk drive has an
access list that is independent of the access lists for the other disk drives.
Fencing is a function that is provided by the hardware and manipulated by hardware
commands. The device driver also has some effect.
The SSA disk device driver provides support for fencing by allowing the
SSADISK_ISALCMD ioctl operation to issue the FN_ISAL_FENCE command to SSA
logical disk drives.
The FN_ISAL_FENCE command is defined in the Technical Reference for the adapter.
To use fencing, set the node_number attribute of the ssar router to a different value on
each using system that is included in fencing. Use the chdev command to do this. For
example, to set the node_number to a value of 3, type:
chdev -l ssar -a node_number=3
Note: The command fails if any target-mode SSA devices are active.
By default, the value of node_number is 0. This value has particular importance,
because it is not possible to exclude a using system with node number 0 from access
to the disk drive. Therefore, if a disk drive is moved from a machine that has been
using fencing to a machine that has not been using fencing, the new machine can
communicate with the disk drive.
If a using system attempts to use the open subroutine to open a disk drive to which it is
not allowed access, the return code is -1 and the global variable errno is set to the
value ENOCONNECT. Similarly, if an application already has a SSA logical disk open
but that logical disk has been fenced out since the open, calls to the read or write
subroutine fail, with errno set to ENOCONNECT.
The hardware fencing commands provide a method by which you can break through a
fence. You can use the SSADISK_ISALCMD ioctl operation to give the command, but
you must first open the disk drive. To open a disk drive from which the using system
has been excluded, use the openx subroutine, and specify SSADISK_FENCEMODE
extension flag as described in the section on SSA disk device driver device-dependent
subroutines. While the disk drive is open in this mode, no read or write operations are
permitted.
If fencing has excluded a using system from access to a disk drive, but that disk drive
is also reserved to another using system, the reservation takes priority. The return code
from the open subroutine is -1, and the global variable errno is set to EBUSY. If the
using system attempts to break through the reservation by passing the ext parameter
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