previous kernel purpose

[Dhananjaya Amarakoon]

Hi frinds

I have done following functions to our HP-UX server (9000/800/N4000-36) that running OS is HP-UX 11.23.

I have installed few patches to the server. Due to some testing purpose I have remove one of that patch. Before that I have generate a new kernel using running system and take it as backup kernel.
When I list the install patch, now it shows, there is no any such patch available in the system as expected.
So then I restart the machine using previous kernel by expecting previouly install patch also available. But it didn’t show in the system.
Same procedure I have done for kernel parameter also. But it also didn’t revert back when after reboot using previous kernel.

So I want to know what the exact purpose of the previous kernel? If something goes wrong to system is it possible to revert back the system using previous kernel? If so what extend?


Can you please explain above incidents, because I plan to do some critical changes to our live system in very soon day


Thanks & Reg’s
Dhananjaya Amarakoon.

[Matti_Kurkela · Accepted solution]

Patches may affect the entire OS, not only the kernel.

Kernel patches (PHKL) will affect the files that are used to create a new kernel (the "raw materials"): they will *not* affect the kernel that is running when you swinstall the patch. This is why a new kernel must be generated after installing a kernel patch.

If you install a kernel patch, create a new kernel, then boot using a previous kernel, the raw materials are still updated and this is what swlist shows. Swlist will not look inside your kernel to determine if your active kernel is really up to date or not - your responsibility as a sysadmin is to make sure of that.

The purpose of the previous kernel:
if a failed kernel patch or a bad parameter change makes the new kernel unbootable, it allows you to reboot using a known good kernel so that the problem can be fixed.

This helps in _kernel problems only_: if, for example, a damaged/failed patch makes /sbin/init unusable, the kernel will still boot but will probably panic immediately because it cannot start the "init" process. In this case, booting with a previous kernel will not help: you must boot from some other media to fix the problem. (In this case, you could boot from CD into a recovery shell, mount the filesystems, find the old version of /sbin/init in /var/adm/sw/save, copy it to /sbin, restart the system and then *remove the failed patch*.

If you install a large set of patches, it is not absolutely guaranteed that you can restore the system to exactly original state by removing the patches. This is why it's recommended to take an Ignite backup before any major patch operations.

MK

[Dhananjaya Amarakoon]

Hi Matti Kurkela
Thanks for your reply and guide me. I did patch update for my live system and system works without problem.

Dhananjaya Amarakoon