single_path, no_reserve and more

no_reserve Does not apply a reservation methodology for the device. The device might be accessed by other initiators, and these initiators might be on other host systems.

single_path Applies a SCSI2 reserve methodology for the device, which means the device can be accessed only by the initiator that issued the reserve. This policy prevents other initiators in the same host or on other hosts from accessing the device. This policy uses the SCSI2 reserve to lock the device to a single initiator (path), and commands routed through any other path result in a reservation conflict.

From http://www.aixmind.com/?p=757

Digests:

1)　Any attempt to set the algorithm to round_robin, load_balance, or load_balance_port with single_path reserve policy will fail. When using single_path reservation methodology with fail_over path selection algorithm only one active path is used for I/O at a time. The active path actually selected for I/O can be configured using the chpath command by setting the path priority attribute accordingly.

2) In order to use multiple paths with native AIX MPIO it is required to set the hdisk’s algorithm attribute to round_robin and the reserve_policy attribute to no_reserve (no_reserve will be fine for most situations but is, of course, depending on your requirements and specific SCSI reservation policy) using:

# chdev -l <hdisk#> -a algorithm=round_robin -a reserve_policy=no_reserve -P

This change requires a reboot. Alternatively one can stop using the hdisks (varyoff any VG with the disks) and eliminate the -P flag to go into effect without a reboot.

3) Note, that a Virtual SCSI Disk only offers fail_over as path selection algorithm, round_robin is not available here!

4) Here a varyoffvg of the AIX volume group is required or with a chdev -l <hdisk#> -a reserve_policy=single_path -P command the change will be applied after the next reboot.

5)[??]  # lspath -l hdisk1 -F "name path_id connection parent path_status status"|while read hd pth cnt fc rst; do echo "$pth => $(lspath -AE -l hdisk1 -p $fc -w $cnt -a priority)" ; done