Cluster Administration

 

Management Node

A management node can be used to access the consoles of all of the cluster nodes and to monitor their status.� This is particularly useful (even necessary) in the event that a node loses network connectivity or if a node stops responding altogether.

KVM Switches

Keyboard/Video/Mouse (KVM) switches are one method used to simplify access to the consoles of multiple cluster nodes.� With this method, a keyboard, mouse, and video cable is run from the back of each node and connected to a KVM switch.� Then a keyboard, video, and mouse cable is run from a designated port on the KVM switch to the management node.� A special keyboard sequence is used to obtain an on-screen listing of the available systems on the KVM switch from which the administrator can choose.

Advantages of using a KVM switch are that there are usually no modifications needed to the operating system or BIOS.� There is less of need to understand terminal emulation and no complications with properly viewing long messages or scrolling messages (as sometimes can happen with serial consoles).

Disadvantages of using a KVM switch include increased cable management.� Although some KVM switches use an adapter to combine the keyboard/video/mouse cables into one cable.� Also, KVMs may not support remote access.� Usually this capability is found in �enterprise� level KVM switches, which are more expensive.

Serial Port Concentrators

A serial port concentrator, also known as a serial port multi-plexor, is used as a sort of switch for all of the serial connections that are used (either for an EMP or for serial consoles).� The serial port concentrator will allow you to log into it and access any one of the connected systems.� You will need a serial port concentrator if you are planning on using an EMP or serial console for your systems.� Usually, there is not much involved in setting up a serial port concentrator.� The interface and associated commands will vary from vendor to vendor.� Unfortunately, the serial port concentrators will sometimes be used for other things, such as terminal servers, and so there may be a lot of other functionality that you do not need.� This can sometimes be confusing as you read through the documentation on these products or try to find one to purchase.

Serial Consoles

A serial console provides access to a cluster node�s console by using one of its serial ports.� With this method, a cable is run from a serial port on each node to a serial port concentrator.� A cable is then run from a port on the serial port concentrator to a serial port on your management node.� You also have the option to run a cable from the serial port concentrator to a network switch in order toUnlike using a KVM, you will need to make changes to the configuration of your system to use a serial console.

Advantages of this method are the ease of cable management (just one cable per system).� Also, many port concentrators allow remote access.

Disadvantages of using a serial console include the number of changes required to the operating system and the BIOS.

In order to properly interact with your system via a serial console from boot time to login, you will need to address four distinct stages of the startup:

1.      The system loads the BIOS.

2.      The BIOS transfers control to the boot loader.

3.      The boot loader loads the kernel.

4.      The kernel starts user-space processes that provide a login prompt for connections over the serial ports.

As you start to modify your system�s settings to enable serial console support, it will be very helpful to you if keep your keyboard and monitor attached until after you have completed the entire process.� This way you will be able to see how each component (i.e., the BIOS, boot loader, kernel, and OS) is behaving.� For example, once you enable serial port redirection in your BIOS, your BIOS may allow you to interact with it over the serial port in addition to the attached keyboard/monitor.� However, your boot loader, once you have instructed it to use the serial port, may only allow you to interact with it over the serial port (you may not see anything on an attached monitor).� Knowing these behaviors can help you troubleshoot serial console problems and help you avoid confusion when you see some output on your serial console only and some on both your serial console and an attached monitor.

Serial Consoles and the BIOS

In order for a serial connection to effectively replace a directly attached keyboard/monitor, you will need to be able to interact with your cluster node�s BIOS via the serial port.� Unfortunately, many computer systems do not have a feature allowing you to redirect your BIOS interaction through a serial port.� Many �server� computer systems offered by various vendors do indeed provide system BIOSs that support serial consoles; however, this may not be the case.� You will need to check your BIOS� support for serial consoles.� If your BIOS does not support console redirection, you will not be able to interact with your BIOS without using, for instance, an attached monitor (perhaps via a KVM).

If your BIOS supports it, enable support for console redirection in your BIOS.� The procedure for doing this will vary from BIOS to BIOS but should be just like changing any other BIOS setting.� This means that you will need to find the appropriate menu and then select the entries related to serial port redirection.� If you have a choice, pick the type of terminal emulation that is used (e.g., vt102, ansi, etc.).� This must match the terminal emulation used by your terminal application (e.g., minicom).� The emulation you use does not matter much as long as the sending and receiving ends agree on it.� Each component (including the serial port concentrator) will also need to agree on the speed of the serial connection (e.g., 9600, 115200 baud).� Obviously, with higher speeds you will have a better experience; but the limiting factor is the maximum speed allowed by any of the involved components.

At this point, if you reboot the system and monitor the serial console, you should see the BIOS output and be able to enter your BIOS by hitting the appropriate keys.� Each vendor may supply you with special key combinations to substitute for keys that are not supported by your terminal emulation (e.g., F10, F11, F12 for vt100).� Depending on your BIOS, you may also see output on an attached monitor.� Take note of your BIOS� behavior for future reference.� After the BIOS is done loading, however, no more output will be sent to your serial console.� When the BIOS transfers control to the boot loader, you will only be able to see/interact with the boot loader on an attached monitor.� The same goes for the kernel and OS.� We will configure these components to use the serial console in the following sections.