Redhat / Centos update

When issuing "yum update", will it update kernel as well. If not, how can I upgrade the kenerl and fall back in case there is any problem ?

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yum update updates everything, including the kernel. The new kernel gets added to your GRUB configuration menu, so if you want to boot to an older version, you just have to select the older entry when GRUB shows it's menu at bootup. As far as I know, CentOS keeps the last 3 kernels and deletes the older ones.

As CentOS uses on tested software, you should always be safe using yum update, and I'd run it regularly to keep the system properly patched.

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Shiju JacobInfo Security ManagerCommented:
You can change how yum handles kernel packages with some simple changes to your /etc/yum.conf. The installonly_limit option controls how many old packages are kept:

    installonly_limit Number of packages listed in installonlypkgs to keep installed at the same time. Setting to 0 disables this feature. Default is ‘0’.

I disabled the functionality altogether by setting installonly_limit to 0:


It’s important to keep in mind that you will need to purge these packages from your system yourself now. Kernel packages can occupy a fair amount of disk space, so make a note to go back and clean them up when you no longer need them.
If it fails, you would need to be in front of the system to choose the former kernel version to boot from.

The only possible cause for a failure is if you have "non-supported" peripherals/devices i.e. network..

integrating DKMS would help auto transition between kernel as DKMS will handle the recompile/reincorporate the module/during the bootup.

Leaving the above referenced old kernel packages is fine.
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"yum update" is a bit short-toothed as it will not handle package deprecations/renames

yum check-update -> show needed updates
yum upgrade -> install them

yum-protectbase and yum-versionlock somehow can be made to emulate appearance of Debian apt-get upgrade  vs dist-upgrade.

IMPORTANT: Please do not follow advice of Shiju to keep full history of installed kernels. 10 upgrades later your /boot will be full and you will have no chance to use YUM to clean oldest kernel.
Shiju JacobInfo Security ManagerCommented:

You can rather have the value set to 3 or 2 to keep only 3 or 2 packages  my eg said 0 to keep all as per the request from the user

I had clearly told that it will eat up space4

Shiju Jacob
AXISHKAuthor Commented:
RHEL has it at 5, oracle linux (that loads 2 kernels by default) has value of 3
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