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AIX TL upgrade using multibos utility

Updated: Oct 20, 2023

Starting with AIX* 5.3 Technology Level (TL) 3, the new multibos utility has been provided that allows an AIX administrator to create and maintain two separate, bootable instances of the AIX OS within the same root volume group (rootvg). This second instance of rootvg is known as a standby Base Operating System (BOS) and is an extremely handy tool for performing AIX TL and Service Pack (SP) updates.

Multibos lets you install, update and customize a standby instance of the AIX OS without impacting the running and active production instance of the AIX OS. This is valuable in environments with tight maintenance windows. Instead of requiring an outage window of several hours to apply a new TL or SP, you’ll only need a small outage at a convenient time to reboot the system. This may help to reduce

the size of the after-hours effort often required when performing AIX updates, as all the maintenance activities can be performed during business hours. After hours you could log in from home and reboot the system.

Backing out from an AIX TL update is also made easier with multibos. To go back to a previous TL, Reboot the system on the original AIX instance boot logical volume (BLV). It’s also possible to update several AIX systems at once using multibos, which again reduces the amount of after-hours effort required when performing AIX maintenance activities.

Multibos is similar to an alternate disk installation. However, there are several differences between the two methods, one of which is that there’s no need for an additional disk to clone the rootvg. Both utilities can be used to achieve the same goal. Choose the one that’s the best fit for your AIX environment.

Pre – requisites:

  1. System must be running AIX 5.3 with TL3 or higher.

  2. Check installed filesets consistency using the below commands.

instfix –i |grep –i ML

instfix –I |grep –i SP

lppchk –v

  1. Commit all the applied filesets using “smitty commit”.

  2. ensure that there’s enough free space in rootvg for a copy of each BOS logical volume (LV).

  3. By default, the BOS file systems in rootvg(/, /usr, /var, and /opt) and the BLV are copied. All other file systems and LVs are shared between BOS instances.

  4. Check the number of free physical partitions in rootvg (i.e., # lsvg rootvg | grep FREE).

  5. Ensure that you document the system and perform an mksysb before performing the actual TL upgrade.


  1. Remove any pervious stanby BOS instances, so we can update a clean environment. “multibos –R”

  2. Create a new standby BOS instance by running the multibos command.

  3. Preview of multibos standby BOS creation “multibos –sXp”

  4. Multibos standby BOS creation “multibos –sX”

  5. It will take a few minutes to copy all the contents, and after it’s completed all new LVs will be prefixed by “bos_“.

  6. Multibos standby BOS created and looks like something below

# lsvg –l rootvg
hd5     boot       1     1     1    closed/syncd  N/A
hd6     paging     192   192   1    open/syncd    N/A
hd8     jfs2log    1     1     1    open/syncd    N/A
hd4     jfs2       1     1     1    open/syncd    /
hd2     jfs2       17    17    1    open/syncd    /usr
hd9var  jfs2       8     8     1    open/syncd    /var
hd3     jfs2       4     4     1    open/syncd    /tmp
hd1     jfs2       1     1     1    open/syncd    /home
hd10opt jfs2       1     1     1    open/syncd    /opt
lvdump1 sysdump    22    22    1    open/syncd    N/A
lvdump2 sysdump    22    22    1    open/syncd    N/A
bos_hd5 boot       1     1     1    closed/syncd  N/A
bos_hd4 jfs2       1     1     1    open/syncd    /bos_inst
bos_hd2 jfs2       17    17    1    open/syncd    /bos_inst/usr
bos_hd9var jfs2       8     8     1    open/syncd    /bos_inst/var
bos_hd10opt jfs2       1     1     1    open/syncd    /bos_inst/opt
  1. Entering the multibos shell and checking AIX version             “multibos –S”              “oslevel –s”

  2. Mount the NFS filesystem where we have the filesets (Please the filesets on a NIM server). Mount them on /mnt

  3. Update the Standby BOS instance. (preview)            “multibos -Xacp -l /mnt” Summery of output will be looks like below….

Multibos preview
  1. Update the Standby BOS instance. (without preview)      “multibos -Xac -l /mnt” Summery of output will be looks like below….

bosinstallation filesets
  1. After upgrade is successful, enter multibos prompt again and check the TL versions whether it’s upgraded.              “multibos –S”              “oslevel –s”

  2. Configure and ensure the boot list is pointing to the standby BOS as the first boot device.             “multibos –B”              “bootlist –m normal –o”

  3. Ensure that the first boot device is “bos_hd5” bootlist -m normal -o” output:              “hdisk18 blv=bos_hd5 pathid=8”

  4. reboot the server with “shutdown –Fr” and check the “oslevel –s”after system is up.

  5. The multibos LVs and file systems are now open and mounted. The original file systems are renamed with a bos_ prefix but their original LV names are preserved.

  6. If no problems were found and the standby BOS is not necessary any longer, it can be removed by issuing the command                “multibos –R”

Backout Plan:

If the update procedure failed and a fallback is needed, set and verify the boot list back to the previous boot LV and reboot—this will bring back the older AIX version.                     # bootlist -m normal hdisk0 blv=hd5 hdisk0 blv=bos_hd5                     # bootlist -m normal -o output:                     hdisk0 blv=hd5                     hdisk0 blv=bos_hd5

Note: When you upgrade to AIX 6.1 TL9 SP5, please install the below APAR (ifix) as there is a Bug with the csh shell while starting SAP due to which SAP will not start.


Here is the command to install ifix on a bos standby instance.

multibos -Xcl /home/t01ixkg  -b /home/t01ixkg/ifixes.bnd


/home/user  = the source directory of this ifix

ifixes.bnd = The multibos command requires a bundle file that contains a list of the ifixes to be installed in geninstall bundle format.

This is how it looks like.

host (root):/home/user> cat ifixes.bnd



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