[CentOS] how long to reboot server ?

Ross Walker rswwalker at gmail.com
Fri Sep 3 17:15:22 EDT 2010

On Sep 3, 2010, at 4:10 PM, Marko Vojinovic <vvmarko at gmail.com> wrote:

> 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.

This is good info!

What I am wondering is if there is a way to prevent new kernels from becoming the default by... default?

That way one won't be "pleasantly" surprised that after a long uptime and several updates, that on the next reboot their applications stop working because of a kernel update that hadn't been tested yet.

A way where the admin must manually choose the default kernel.


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