How to boot AIX V5.3 into single-user mode in an LPAR

Assuming an hdisk is the first choice in both the AIX normal and service mode boot lists:

p520-sr3:/ # bootlist -m normal -o
hdisk0 blv=hd5
p520-sr3:/ # bootlist -m service -o
hdisk0 blv=hd5
ent0
p520-sr3:/ #

(which, BTW, is a good practice), then getting into single user mode is straight-forward.

First, SSH into the HMC and use the vtmenu command to open the system console on the LPAR about to be booted into single user mode.

Then, on the HMC GUI, activate the LPAR. In the Activate Logical Partition window which opens, there is a Advanced... button. Click that. In the Activate Logical Partition - Advanced window which opens, set Keylock position to manual. Click OK twice to begin booting.

Back in the SSH system console window, when the DIAGNOSTIC OPERATING INSTRUCTIONS prompt appears, press Enter to continue. Then, choose option 5 - Single User Mode. When prompted for a SINGLE-USER MODE password, enter root's password.

That's it.

If an hdisk is not the first choice in AIX's service mode boot list, then a similar approach can be used, except that options in the Activate Logical Partition - Advanced window must be used to boot into SMS mode. SMS mode can then be used to select the AIX boot hdisk in addition to selecting service mode boot.

If root's password has been lost, then AIX bootable media (eg, a mkdvd backup, a mksysb backup, or an AIX installation CD) is required. The media should be at the same Technology Level as the AIX image on hdisk. Boot to SMS mode and use SMS to boot from the media in normal mode. Once the media boots, do not proceed to restore AIX! Instead, choose to enter maintenance mode. Once in maintenance mode, reset root's password, then shut AIX down and boot back up from hdisk.

Please note that if an LPAR has been started with Keylock position set to manual, then when AIX comes back up after a shutdown -Fr, the Keylock position will still be manual and AIX will go into single user mode. The LPAR must be stopped (with shutdown -F) and restarted (with Keylock position set to normal) to bring AIX up in normal mode.

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