Tuesday, March 15, 2011

Installing Red Hat Enterprise Linux 5.5 on a Multipath Device

Note: Most, if not all, of the tips I'm providing in this post are available in Red Hat or IBM documentation.  It took me awhile to find it though, so I thought it may be nice to save for a quick reference.

This week, our team was tasked with installing Red Hat Enterprise Linux (RHEL) 5.5 on an IBM server.  This server connects to two different drives provided by a DS8100 storage system, and has two paths to each drive.  In our first attempt to install RHEL, we had to use the text-based installer since this server does not have a video card, which made this entire process a little more fun.

In going through the installer, it detected the two paths to each of our two drives, so instead of showing just the two drives it showed four (counting each path as an individual drive).  We went through the motions and targeted one of the drives it detected for the install.  Unfortunately, when we rebooted the machine, it didn't see that drive as a bootable device, marking our first defeat.

To make things a little easier on ourselves, we decided to go with the recommended graphical installer.  How did we do this without a video card, you may ask?  With our friend, VNC.  If you're interested in using VNC to connect to your install, the documentation mentions the TigerVNC client, so that's what we went with.  In order to get the installer to start a VNC server, you need to add the following option at boot when prompted:
boot: linux vnc
You'll be prompted to configure the IP address for the server to run VNC on the machine, but once it's up and running you will be presented with the display to connect to.

Now that we had a graphical installer to work with, we were feeling much better and went through another install.  It failed.  So, we tried again testing various disk layouts.  They failed.  We made a few more attempts before the end of the day, but just couldn't get this installer to lay down a bootable OS on the drive.

In the back of my mind, I was thinking it had to do with multipath aspect.  If we could somehow get this installer to only see two drives, instead of four, maybe our luck would change.  After doing a little research through the Red Hat documentation, I discovered that if you try to install to a multipath device, it's probable that it will fail and/or it won't be bootable.  The key to solving this problem: adding just one additional boot parameter.  So, in the end, our boot parameters looked like:
boot: linux vnc mpath
Then, in the installer, it detected two multipath devices, we installed the OS to the first multipath device, and like magic, we had a working installation of RHEL 5.5!

The following section only applies if you are running an IBM Storage System, specifically something in the DS6000 or DS8000 series.  So, if that doesn't apply to you, feel free to move along :)

After the install, there were a couple additional steps to make sure multipath was configured correctly for our DS8100 system.  You will need to make sure the following packages are installed:

Next, you'll need the multipath config file from the IBM website (currently located on this page in the section labeled "Device Mapper Multipath Configuration File for ESS DS6000 DS8000 and SVC".  Make sure you grab the one for the RHEL 5 Platform).  After obtaining multipath.conf, back up your existing one under /etc/multipath.conf and then simply copy this one into the /etc directory.  In order to get my system to recognize the new config, I had to run the following command:
multipath -r

You can then confirm that the drives are discovered with the following command:
multipath -l

Hopefully this post will save you a little bit of time and heartache!

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