[CentOS] how long to reboot server ?

Marko Vojinovic vvmarko at gmail.com
Fri Sep 3 16:10:27 EDT 2010

On Friday, September 03, 2010 18:34:51 Matthew Miller wrote:
> On Fri, Sep 03, 2010 at 12:17:37PM -0500, Les Mikesell wrote:
> > Does anyone know if this is special-cased or some config setting?  I
> It's special-cased.

I remember the discussion on the Fedora-list about this a very long time ago, 
and the bottomline is roughly the following:

* when a yum update installs a new kernel, it checks if the total number of 
installed kernels exceeds the installonly_limit parameter
* if not, everything is ok
* if yes, the oldest *non-running* kernel is removed and the remaining number 
of kernels is checked again against installonly_limit, and the removal step is 
repeated if they still don't match up.

This was done precisely because it was understood that a currently running 
kernel can be assumed to be stable and bootable. So if you have several 
kernels, run a yum update while the oldest one is running, get a new kernel, 
the extra kernels that will get removed are those "in between". This ensures 
that with any multiple-kernel configuration of yum, there will be at least one 
kernel known to work, as a failsafe.

I believe CentOS just inherited this behavior of yum. Though I might be wrong, 
it seems unlikely that anyone would remove this feature from yum on purpose.

So all in all, you should never be afraid that yum will leave you only with 
untested kernels while updating.

HTH, :-)

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