
                   Linux-HA Hardware Installation Guideline

   This document (c) 1999 Volker Wiegand [1]<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. [2]Introduction
    2. [3]Hardware Requirements
         1. [4]Minimum Installation
         2. [5]More Advanced Installation
         3. [6]Fully Redundant Installation
    3. [7]Hardware Setup and Test
         1. [8]Serial Ports
         2. [9]LAN Interfaces
         3. [10]Other Devices
    4. [11]Troubleshooting
    5. [12]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 [13]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 [14]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
       [15]Ethernet HOWTO.
     * For more information on diagnosing serial port problems, consult the
       [16]Serial Port HOWTO.

References

   The Linux-HA homepage on the internet is: [17]http://linux-ha.org/

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

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

   $Id: HardwareGuide.html,v 1.3 2001/03/08 14:34:55 alan Exp $

References

   1. mailto:Volker.Wiegand@suse.de
   2. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#Introduction
   3. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#Hardware Requirements
   4. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#Minimum Installation
   5. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#More Advanced Installation
   6. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#Fully Redundant Installation
   7. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#Hardware Setup and Test
   8. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#Serial Ports
   9. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#LAN Interfaces
  10. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#Other Devices
  11. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#Troubleshooting
  12. file://localhost/tmp/tmp.lvO22249/linux-ha/doc/HardwareGuide.html#References
  13. http://metalab.unc.edu/pub/Linux/ALPHA/linux-ha/High-Availability-HOWTO.html
  14. http://metalab.unc.edu/LDP/HOWTO/Ethernet-HOWTO.html
  15. http://metalab.unc.edu/LDP/HOWTO/Ethernet-HOWTO.html
  16. http://metalab.unc.edu/LDP/HOWTO/Serial-HOWTO.html
  17. http://linux-ha.org/
  18. http://metalab.unc.edu/pub/Linux/ALPHA/linux-ha/High-Availability-HOWTO.html
  19. http://www.sun.com/clusters/dh.brown.pdf
