IBM Midrange System DS4000 Series Hardware Manual page 158

Figure 4-10 shows the ADT-disabled failover mode phases.
1
Host / Server
3
Host / Server
Figure 4-10 ADT-disabled mode path failover
Note: In ADT-disabled mode, you are required to issue a redistribution command
manually to balance the LUNs across the controllers.
ADT-enabled failover
The multi-path driver starts using the alternate path by sending the I/O down the path it
chooses and lets the ADT react. This is the normal configuration setting for Novell
NetWare, Linux (when using the FC HBA failover driver instead of RDAC), and HP-UX
systems. After the I/O data path problem is corrected, the preferred controller
automatically reestablishes ownership of the logical drive as soon as the multipath driver
detects that the path is normal again.
140
IBM Midrange System Storage Hardware Guide
Application
0
X
1
Port 0 receives an error.
Connection is no longer
valid, error is passed on
to the failover driver.
- 020 failed
- 130 alternate
Host / Server
Application
0
X
1
The failover driver can
now use the alternate
path for any retired or
new I/Os.
- 020 failed
- 130 current
Host / Server
A
0 1 2
[2]
B
[3] 1 2
0
Application
0
X
1
A
0 1 2
[2]
B
[3]
0 1 2
Application
0
X
1
Application is using LUN 0.
3
Failover driver hides the
two physical paths to LUN.
- 020 current (green)
- 130 alternate (white)
3
A
0 1 2
[2]
B
[3] 1 2
0
Failover driver determines
usage of alternate path to
3
continue I/O. Send a Mode
Select SCSI command to the
storage. Volume ownership
changes as a result of SCSI
mode select command.
- 020 failed
3
- 130 pending transfer
A
1
0 2
[2]
B
[3]
0 1 2
3
2
3
3
4
3