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, ... ???]
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