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I have to update a server this weekend and the process goes with yum update. I have made tests in virtual machines but I am not sure what can go wrong in a real production environment.

The server itself is Centos 6.2 with Apache 2.2.15, PHP 5.3.3 and MySQL 14.14.

I get that those specific program will be updated but to what version, is there a way for me to control it? Or will they be uploaded to whatever Centos 6.6 Final has.

closed as too broad by EEAA, Ward, Tim Brigham, Aaron Copley, masegaloeh Jun 23 '15 at 0:01

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    Worst case? You lose your entire system and all data even on NFS shares that are simply mounted to it. Is that likely? No. Everything else is way too broad - the answer to this question could become a doctoral dissertation in the right (wrong?) hands. – John Jun 22 '15 at 16:32
  • It is possible to only update to specific versions using yum update but by default it'll apply the latest possible updates for the OS family. By far the biggest thing that happens with large updates is that the config files don't match between versions. You also run in to related software that is version dependent but that's a lot less common. – Tim Brigham Jun 22 '15 at 16:43
  • How did you structure your tests in virtual machines? Your test environment should mirror production and then you can have confidence in what will happen. The best test is to P2V your existing server and run the update on the clone. Updating from 6.2 to 6.6 is non-trivial, although it should be safe. – Aaron Copley Jun 22 '15 at 20:01
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You can always check what updates will be updated exactly using yum list updates which would return you a list of packages that have available updates.

Next you could list available updates using yum list --showduplicates <package>, i.e., yum list --showduplicates httpd which would return you a list of available versions for that particular package.

And lastly just update packages one by one using yum update <package-version>, i.e., yum update httpd-2.2.15-39 (or just omit the version and update the package to the latest available version like yum update httpd

As for what might go wrong.. Well there is a few things worth taking into consideration. Most notable is probably configuration files might change and so you would need to adjust your config files accordingly so just to be on the safe side, I'd backup config files if you haven't automated your server configuration. Other than that, since yum uses transactions no real threat to your data should occur but if possible I'd still suggest to put your server into maintenance mode (stop websites or software from writing into database or changing files). Other than that most of the time simply running yum update won't do major version updates and therefore you should have no problems with that.

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If you are using only CentOS repositories, changes that something can go wrong are very low. Inside the same major release (in this case, CentOS 6), configurations are almost always guaranteed to be compatible between updates (this is the very reason because, unless otherwise specified, major software versions remain the same inside all CentOS point releases).

A notable is when a security update disables a buggy or dangerous features: for example, updating a CentOS server from 6.5 to 6.6 release, SSLv3 was disabled by default in Dovecot (but it can be re-enabled editing the config file).

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