# Linux-HA Hardware Installation Guideline

This document (c) 1999 Volker Wiegand <Volker.Wiegand@suse.de>

This document serves as the starting point to plan, execute, and verify your hardware setup for a High Availability (HA) environment.

## Contents

1. Introduction
2. Hardware Requirements
   1. Minimum Installation
   2. More Advanced Installation
   3. Fully Redundant Installation
3. Hardware Setup and Test
   1. Serial Ports
   2. LAN Interfaces
   3. Other Devices
4. Troubleshooting
5. References

## Introduction

With the high stability Linux has reached, this Operating System is well suited to be used for HA purposes. The Linux-HA project, based upon Harald Milz's HOWTO and Alan Robertson's Heartbeat code, provides the building blocks for a professional solution.

This document provides some advice on the initial planning, the installation and cabling, and the test and verification of the overall setup. We use the word takeover to mean transferring some kind of server functionality from a broken entity to a sane one. In this context, entities can be network adapters, computers, or something else. Our current focus is to provide HA capabilities among PC's which we will call "nodes" from now on.

## Hardware Requirements

Since we want to be able to provide failover capabilities on the machine level, we need at least two computers. Obvious, isn't it? In our current setup, all we require that they are running Linux. No particular distribution is preferred (although most tests have been carried out on RedHat and SuSE systems). The minimum kernel version is [TODO: which one is it ???], although the software makes fairly minimal demands on the OS.

These two nodes have to be connected in some way to exchange status information and to monitor each other. The more channels our nodes have to talk to each other, the better it is. We will use the term "medium" for such a communication channel.

In general we work from the assumption that we use standard hardware where ever possible. This means that we do not modify our PC's other than to expand them with components off the shelf. And we use only cabling that can be bought without "special orders" such as split serial cables or the like. After all we want solutions that can be installed and used by everyone, not just some experts.

### Minimum Installation

In order for the takeover to work, we need at least one medium to exchange messages. Given that we use TCP/IP as the basis for our service, some kind of LAN is certainly available. Of course having only the LAN provides poor monitoring capabilities, but on the other hand this is the minimum chapter anyway :-)

So how will the hardware be planned? Well, straight forward.

        +-------------------+  +-------------------+
        |                   |  |                   |
        |      Node A       |  |      Node B       |
        |                   |  |                   |
        |       eth0        |  |       eth0        |
        +---------+---------+  +---------+---------+
                  |                      |
                  |                      |
        |---------+----------------------+---------| LAN (Ethernet, etc.)

As was mentioned before, this design obviously provides insufficiently reliable monitoring capabilities. In a LAN, there are many different points of failure. Another issue is that the LAN is a public medium and that there are several levels of possible failures. So we would be well-advised to look for more robust options to use in addition to the LAN for heartbeats.

### More Advanced Installation

So let's see what we can do to provide sound monitoring and good takeover capabilities and still not having to purchase excessive hardware or software add-ons.

The main idea is to have a simple private medium like one or more serial cables. We can use the standard serial ports, provided they are not already occupied by modems, mice, or other vermin. If you have a server with a PS/2 mouse, it probably has two such ports available.

So here's what this configuration looks like.

                  +----------------------+           (Nullmodem Cable)
                  |                      |
        +---------+---------+  +---------+---------+
        |       ttyS0       |  |       ttyS0       |
        |                   |  |                   |
        |      Node A       |  |      Node B       |
        |                   |  |                   |
        |       eth0        |  |       eth0        |
        +---------+---------+  +---------+---------+
                  |                      |
                  |                      |
        |---------+----------------------+---------| LAN (Ethernet, etc.)

What do we gain? We have now two media to exchange the heartbeat. This provides greater reliability in the case of failure. Of course the restriction with the LAN still holds true, but now Node A could use the serial line to initiate a takeback of the service. And if just the serial connection should fail, we still have the LAN. Reliable intracluster communications is very important, and this design is a low-cost improvement over the previous one.

### Fully Redundant Installation

The point of the addition of the serial links to the system is that a single failure cannot cause the nodes to become confused about the overall system configuration. This is vitally important for many HA systems, because the cost of this confusion can be scrambled disks, and other problems which are often worse than the cost of an outage. With more resources, the following provides a general guideline to set up things. To illustrate the principle, a third node has been included, but we can install any number of nodes in this way. Well, almost any. Note: The takeover code which is part of the heartbeat package will not yet correctly manage takeovers for more than two nodes.

The serial lines are now arranged in a ring structure. As you will have noticed, this occupies two serial ports on each node as per our discussion in the previous chapter. But on the other hand we do now have a general setup that can easily be extended and also provides a good level of redundancy. We can now send our heartbeat now in both directions over the ring, thus reaching every other node even in case of a (single) cable defect (or down system) anywhere on the ring.

Another facet of our high end design covers the LAN access. Having two adapters connected to the wire allows us to provide intra-node failover capabilities in case of an interface or LAN cable breakdown. Plus it gives us the chance to take over the IP address of Node A, eth0 onto Node B, eth1 and keeping Node B, eth0 as it is. In fact, this is the primary operation mode of several professional systems, including IBM's HACMP or HP's MC/ServiceGuard. Which doesn't imply that we are not professional, of course :-)

So, here is the block diagram for this third design.

                                                      (Nullmodem Cables)
              +-----------------------------------------------------+
              |       +--------------+        +-------------+       |
              |       |              |        |             |       |
        +-----+-------+-----+  +-----+--------+----+  +-----+-------+-----+
        |   ttyS0   ttyS1   |  |   ttyS0   ttyS1   |  |   ttyS0   ttyS1   |
        |                   |  |                   |  |                   |
        |      Node A       |  |      Node B       |  |      Node C       |
        |                   |  |                   |  |                   |
        |   eth0     eth1   |  |   eth0     eth1   |  |   eth0     eth1   |
        +-----+--------+----+  +-----+--------+----+  +-----+--------+----+
              |        |             |        |             |        |
              |        |             |        |             |        |
        |-----+--------+-------------+--------+-------------+--------+----|
                                                      LAN (Ethernet, etc.)

Future releases of the heartbeat software will support such a configuration, but current takeover code restricts the configuration to a single interface and two nodes in the network. Of course we could also use other media for the heartbeat exchange. Recent suggestions include SCSI buses in target mode and IrDA ports "connected" with a mirror. Another candidate that comes to mind is the USB found in many modern PC's. As I said before, the more (and more different) the better.

## Hardware Setup and Test

The following chapter deals with the installation and verification of the various components within the nodes.

### Serial Ports

First of all, let's recap how a Nullmodem Cable is wired. The pain is that you certainly possess the pin assignment a thousand times, but you don't have it handy when you need it. So here it is ...

        25-pin        9-pin                          9-pin        25-pin

          2     TxD     3  --------------------------  2     RxD     3
          3     RxD     2  --------------------------  3     TxD     2
          4     RTS     7  --------------------------  8     CTS     5
          5     CTS     8  --------------------------  7     RTS     4
          7     GND     5  --------------------------  5     GND     7
          6     DSR     6  ---+----------------------  4     DTR    20
          8     DCD     1  ---+                  +---  1     DCD     8
         20     DTR     4  ----------------------+---  6     DSR     6

Once you have these cable(s) in place you will want to test them. This is fairly easy since the serial ports are usually configured with decent default values. On a freshly booted Linux system we can assume the ports to be in a "sane" state, with the speed set to 9600 baud. If not, you can do a "`stty sane 9600 </dev/ttyS0`" with ttyS0 replaced by the actual device. Please note the input redirection which selects the device.

Then you can set up one node as receiver ("`cat </dev/ttyS0`") and the other one as transmitter ("`echo hello >/dev/ttyS0`"). Voila! What you expect is that the "hello" is printed out at the receiver. Pressing Ctrl-C on the receiver's keyboard will return you to the prompt. Then do the same test with mutually exchanged roles.

### LAN Interfaces

Rumor has it that there is work in progress to provide some level of diagnostic capabilities for Ethernet adapters and wiring. I don't know the actual status, and can only suggest to use a shabby "ping" provided that the interfaces are set up correctly with "ifconfig". For more information on Linux ethernet, please check the Ethernet HOWTO.

If you are planning to use more than one adapter per node (usually called "Standby Adapters"), please make sure to connect them to the same physical medium as the primary adapters. Otherwise you will of course not be able to takeover the IP address. Having them in a different subnet is perfectly okay. More than that: it's preferred. [TODO: this is what I learned with HACMP. Can anyone please give the *correct* rationale --- or rephrase the whole paragraph?]

Note: The heartbeat software does not yet support this kind of configuration.

### Other Devices

[TODO: well, to do]

## Troubleshooting

If things don't work in the first place -- don't panic! Usually it's just a trifle. Things to check include:

- Check the startup messages of the kernel, e.g. using "dmesg". Is the serial driver (either the standard one or the special one for your hardware) compiled in or available as a module?
- Check the serial port(s) and cable(s). Do your modem and mouse still work? Using a battery, a light bulb or buzzer and some wire you can easily verify that all pins are connected and there are no short circuits. Inexpensive breakout boxes are available for diagnosing such conditions as well. They contain the light bulbs, the connectors and the wire in one handy little unit.
- For serial ports, the file `/proc/tty/driver/serial` can be very helpful for diagnosing serial port problems in Linux. It contains lines of this form in it:

      1: uart:16550A port:2F8 irq:3 baud:19200 tx:24423 rx:4680 RTS|CTS|DTR|DSR|CD

  This particular line corresponds to a working "raw" serial port, /dev/ttyS1 with both sides cabled up correctly, and heartbeat active on both sides. The first number on the line is the port number. The built-in serial ports on PCs are numbered 0 and 1. With heartbeat only active on the local side (and not the far side), it looks like this instead:

      1: uart:16550A port:2F8 irq:3 baud:19200 tx:43558 rx:12277 RTS|DTR

  Note the lack of the CTS (Clear To Send), DSR (Data Set Ready), and CD (Carrier Detect) bits on the interface. When heartbeat is only running on the far side interface, it looks like this:

      1: uart:16550A port:2F8 irq:3 baud:19200 tx:55039 rx:12277 CTS|DSR|CD

  Note that when the local port isn't active, the RTS (Request To Send), and DTR (Data Terminal Ready) bits aren't active. When heartbeat isn't running on either interface, the line looks like this:

      1: uart:16550A port:2F8 irq:3 baud:19200 tx:55039 rx:12277

  This is essentially a software breakout box.
- Check that the cables are properly plugged into their sockets. For a production High-Availability system, it is a very good idea to fasten the screws in order to avoid loose contacts.
- For more information on diagnosing ethernet problems, consult the Ethernet HOWTO.
- For more information on diagnosing serial port problems, consult the Serial Port HOWTO.

## References

The Linux-HA homepage on the internet is: http://linux-ha.org/

Harald Milz' Linux-HA HOWTO that started the whole thing can be found at: http://metalab.unc.edu/pub/Linux/ALPHA/linux-ha/High-Availability-HOWTO.html

A comprehensive survey on professional HA solutions is here: http://www.sun.com/clusters/dh.brown.pdf
[TODO: should we include links to HACMP, Veritas, Wizard, ... ???]