First of all you need to get the ISO image file for VIOS 3.1 installation.

You should be able to get it from the ESS site (you need a valid SWMA contract):

https://www.ibm.com/servers/eserver/ess/OpenServlet.wss

There's 2 different versions of the installation image :
$ ls -l
total 20441672
-rwxr-xr-x 1 root system 4488343552 Jan 04 05:30 Virtual_IO_Server_Base_Install_3.1.0.0_DVD_1_of_2_112018.iso
-rwxr-xr-x 1 root system 1366530048 Jan 04 05:49 Virtual_IO_Server_Base_Install_3.1.0.0_DVD_2_of_2_112018.iso
-rwxr-xr-x 1 root system 4611252224 Jan 04 06:52 Virtual_IO_Server_Base_Install_3.1.0.10_Flash_112018.iso

The 2 first files are the "standard" ISO image, with splitted mksysb file, we'll see later how to build the final mksysb image using those files.

The third file, we'll call it "Flash" version, was built so that the mksysb will be on a single image.

The following command were performed on a NIM master, but you can use the same procedure on a VIOS.

If done on VIOS? you'll have to use the "oem_setup_env" command to get super user privilege.

NOTE : This technote was written in the idea to build resources on a NIM master, and thus we create the files in "/export" filesystem. But if you plan to create the file on the VIOS itself, the "/export" filesystem does not exist, in that case you can use "/home/padmin" instead, but make sure before starting operation that you'll have enough free space in the "/home" filesystem. (Do not use /export. If there's not such filesystem on the VIOS and you create directory, this will fullfill the "/" filesystem, and may cause hang or crash of the VIOS)

Build the mksysb file using "Flash" version :

We'll have to mount the ISO image, and extract the correct mksysb file.

Note : Pay attention that in this process you must specify the filesystem type as "udfs". Otherwise, AIX will fail to read the file correctly.

Build the mksysb file using the "standard" version :

Here, we'll have to mount the 2 ISO image concurrently, and extract all "mksysb files" to build the final one.

Note : For VIOS 4.1 image, the non-Flash version now contains one single DVD media, so the appropriate command to build the mksysb image is :
$ cat /mnt/dvd1/usr/sys/inst.images/mksysb_image /mnt/dvd1/usr/sys/inst.images/mksysb_image2 > /export/mksysb/VIO31/VIOS_3.1_standard.mksysb

How to check the mksysb image built is correct ?

Before using the created mksysb to migrate or install VIOS we need to check file consistency and readiness, to do so we'll use the restore command to list the content of the file generated :

This command should not prompt for a volume and the return code must be 0. If either of those conditions exist, do not use this mksysb image for anything, there's something wrong with it.

A "failed" mksysb image may provide the following error message :

<-- snip -->
36688 ./opt/IBM/ldap/V6.4/lib64/idsldap_plugin_sasl_digest-md5.a
38 ./opt/IBM/ldap/V6.4/lib64/libibmldap.a
37680 ./opt/IBM/ldap/V6.4/lib64/libibmldapdbg.a
772487 ./opt/IBM/ldap/V6.4/lib64/libibmldapn.a
Mount volume 2 on /export/mksysb/VIO31/VIOS_3.1_standard.mksysb.
Press the Enter key to continue.