To set up DRBD, MySQL and Heartbeat you need to follow a number of steps that affect the operating system, DRBD and your MySQL installation.

Before starting the installation process, you should be aware of the following information, terms and requirements on using DRBD:

The overview for the installation and configuration sequence is as follows:

$ uname -r
2.6.20-gentoo-r6

root-shell> cat /proc/drbd 
version: 8.0.4 (api:86/proto:86)
SVN Revision: 2947 build by root@drbd-one, 2007-07-30 16:43:05
 0: cs:SyncSource st:Primary/Secondary ds:UpToDate/Inconsistent C r---
    ns:252284 nr:0 dw:0 dr:257280 al:0 bm:15 lo:0 pe:7 ua:157 ap:0
        [==>.................] sync'ed: 12.3% (1845088/2097152)K
        finish: 0:06:06 speed: 4,972 (4,580) K/sec
        resync: used:1/31 hits:15901 misses:16 starving:0 dirty:0 changed:16
        act_log: used:0/257 hits:0 misses:0 starving:0 dirty:0 changed:0

root-shell> cat /proc/drbd 
version: 8.0.4 (api:86/proto:86)
SVN Revision: 2947 build by root@drbd-one, 2007-07-30 16:43:05
 0: cs:Connected st:Primary/Secondary ds:UpToDate/UpToDate C r---
    ns:2175704 nr:0 dw:99192 dr:2076641 al:33 bm:128 lo:0 pe:0 ua:0 ap:0
        resync: used:0/31 hits:134841 misses:135 starving:0 dirty:0 changed:135
        act_log: used:0/257 hits:24765 misses:33 starving:0 dirty:0 changed:33

root-shell> drbdadm primary all

root-shell> drbdadm secondary all

root-shell> drbdadm disconnect all

root-shell> drbdadm connect all

Once you have configured DRBD and have an active DRBD device and filesystem, you can configure MySQL to use the chosen device to store the MySQL data.

When performing a new installation of MySQL, you can either select to install MySQL entirely onto the DRBD device, or just configure the data directory to be located on the new filesystem.

In either case, the files and installation must take place on the primary node, because that is the only DRBD node on which you can mount the DRBD device filesystem as read/write.

You should store the following files and information on your DRBD device:

To set up MySQL to use your new DRBD device and filesystem:

Your MySQL data should now be located on the filesystem running on your DRBD device. The data will be physically stored on the underlying device that you configured for the DRBD device. Meanwhile, the content of your MySQL databases will be copied to the secondary DRBD node.

Note that you cannot access the information on your secondary node, as a DRBD device working in secondary mode is not available for use.

Because of the nature of the DRBD system, the critical requirements are for a very fast exchange of the information between the two hosts. To ensure that your DRBD setup is available to switch over in the event of a failure as quickly as possible, you must transfer the information between the two hosts using the fastest method available.

Typically, a dedicated network circuit should be used for exchanging DRBD data between the two hosts. You should then use a separate, additional, network interface for your standard network connection. For an example of this layout, see Figure 14.2, “DRBD Architecture”.

Figure 14.2. DRBD Architecture

The dedicated DRBD network interfaces should be configured to use a non-routed TCP/IP network configuration. For example, you might want to set the primary to use 192.168.0.1 and the secondary 192.168.0.2. These networks and IP addresses should not be part of normal network subnet.

root-shell> cat /proc/net/bonding/bond0
Bonding Mode: fault-tolerance (active-backup)
Primary Slave: None
Currently Active Slave: eth1
MII Status: up
MII Polling Interval (ms): 100
Up Delay (ms): 200
Down Delay (ms): 200
Slave Interface: eth1
MII Status: up
Link Failure Count: 0
Permanent HW addr: 00:11:22:33:44:55
Slave Interface: eth2
MII Status: up
Link Failure Count: 0
Permanent HW addr: 00:11:22:33:44:56

Heartbeat configuration requires three files located in /etc/ha.d. The ha.cf contains the main heartbeat configuration, including the list of the nodes and times for identifying failures. haresources contains the list of resources to be managed within the cluster. The authkeys file contains the security information for the cluster.

The contents of these files should be identical on each host within the Heartbeat cluster. It's important that you keep these files in sync across all the hosts. Any changes in the information on one host should be copied to the all the others.

For these examples n example of the ha.cf file is shown below:

logfacility local0
keepalive 500ms
deadtime 10
warntime 5
initdead 30
mcast bond0 225.0.0.1 694 2 0
mcast bond1 225.0.0.2 694 1 0
auto_failback off
node drbd1
node drbd2

The individual lines in the file can be identified as follows:

An optional additional set of information provides the configuration for a ping test that will check the connectivity to another host. You should use this to ensure that you have connectivity on the public interface for your servers, so the ping test should be to a reliable host such as a router or switch. The additional lines specify the destination machine for the ping, which should be specified as an IP address, rather than a hostname; the command to run when a failure occurs, the authority for the failure and the timeout before an non-response triggers a failure. A sample configure is shown below:

ping 10.0.0.1
respawn hacluster /usr/lib64/heartbeat/ipfail
apiauth ipfail gid=haclient uid=hacluster
deadping 5

In the above example, the ipfail command, which is part of the Heartbeat solution, is called on a failure and 'fakes' a fault on the currently active server. You need to configure the user and group ID under which the command should be executed (using the apiauth). The failure will be triggered after 5 seconds.

The auth_keys file holds the authorization information for the Heartbeat cluster. The authorization relies on a single unique 'key' that is used to verify the two machines in the Heartbeat cluster. It is used only to confirm that the two machines are in the same cluster and is used to ensure that the

To use Heartbeat in combination with MySQL you should be using DRBD (see Section 14.1, “Using MySQL with DRBD for High Availability”) or another solution that allows for sharing of the MySQL database files in event of a system failure. In these examples, DRBD is used as the data sharing solution.

Heartbeat manages the configuration of different resources to manage the switching between two servers in the event of a failure. The resource configuration defines the individual services that should be brought up (or taken down) in the event of a failure.

The haresources file within /etc/ha.d defines the resources that should be managed, and the individual resource mentioned in this file in turn relates to scripts located within /etc/ha.d/resource.d. The resource definition is defined all on one line:

drbd1 drbddisk Filesystem::/dev/drbd0::/drbd::ext3 mysql 10.0.0.100

The line is notionally split by whitespace. The first entry (drbd1) is the name of the preferred host, i.e. the server that is normally responsible for handling the service. The last field is virtual IP address or name that should be used to share the service. This is the IP address that should be used to connect to the MySQL server. It will automatically be allocated to the server that is active when Heartbeat starts.

The remaining fields between these two fields define the resources that should be managed. Each Field should contain the name of the resource (and each name should refer to a script within /etc/ha.d/resource.d). In the event of a failure, these resources are started on the backup server by calling the corresponding script (with a single argument, start), in order from left to right. If there are additional arguments to the script, you can use a double colon to separate each additional argument.

In the above example, we manage the following resources:

If you want to be notified of the failure by email, you can add another line to the haresources file with the address for warnings and the warning text:

MailTo::youremail@address.com::DRBDFailure

With the Heartbeat configuration in place, copy the haresources, authkeys and ha.cf files from your primary and secondary servers to make sure that the configuration is identical. Then start the Heartbeat service, either by calling /etc/init.d/heartbeat start or by rebooting both primary and secondary servers.

You can test the configuration by running a manual failover, connect to the primary node and run:

root-shell> /usr/lib64/heartbeat/hb_standby

This will cause the current node to relinquish its resources cleanly to the other node.

As a further extension to using DRBD and Heartbeat together, you can enable dopd. The dopd daemon handles the situation where a DRBD node is out of date compared to the master and prevents the slave from being promoted to master in the event of a failure. This stops a situation where you have two machines that have been masters ending up different data on the underlying device.

For example, imagine that you have a two server DRBD setup, master and slave. If the DRBD connectivity between master and slave fails then the slave would be out of the sync with the master. If Heartbeat identifies a connectivity issue for master and then switches over to the slave, the slave DRBD device will be promoted to the primary device, even though the data on the slave and the master is not in synchronization.

In this situation, with dopd enabled, the connectivity failure between the master and slave would be identified and the metadata on the slave wold be set to Outdated. Heartbeat will then refuse to switch over to the slave even if the master failed. In a dual-host solution this would effectively render the cluster out of action, as there is no additional fail over server. In an HA cluster with three or more servers, control would be passed to the slave that has an up to date version of the DRBD device data.

To enable dopd, you need to modify the Heartbeat configuration and specify dopd as part of the commands executed during the monitoring process. Add the following lines to your ha.cf file:

respawn hacluster /usr/lib/heartbeat/dopd  
apiauth dopd gid=haclient uid=hacluster

Make sure you make the same modification on both your primary and secondary nodes.

You will need to reload the Heartbeat configuration:

root-shell> /etc/init.d/heartbeat reload

You will also need to modify your DRBD configuration by configuration the outdate-peer option. You will need to add the configuration line into the common section of /etc/drbd.conf on both hosts. An example of the full block is shown below:

common {  
  handlers {  
    outdate-peer "/usr/lib/heartbeat/drbd-peer-outdater";  
  }  
}

Finally, set the fencing option on your DRBD configured resources:

resource my-resource {  
  disk {  
    fencing    resource-only;  
  }
}

Now reload your DRBD configuration:

root-shell> drbdadmin adjust all

You can test the system by unplugging your DRBD link and monitoring the output from /proc/drbd.

Because a kernel panic or oops may indicate potential problem with your server, you should configure your server to remove itself from the cluster in the event of a problem. Typically on a kernel panic your system will automatically trigger a hard reboot. For a kernel oops a reboot may not happen automatically, but the issue that caused that oops may still lead to potential problems.

You can force a reboot by setting the kernel.panic and kernel.panic_on_oops parameters of the kernel control file /etc/sysctl.conf. For example:

 kernel.panic_on_oops = 1
 kernel.panic = 1
  

You can also set these parameters during runtime by using the sysctl command. You can either specify the parameters on the command line:

$ sysctl -w kernel.panic=1
  

Or you can edit your sysctl.conf file and then reload the configuration information:

$ sysctl -p
 

By setting both these parameters to a positive value (actually the number of seconds to wait before triggering the reboot), the system will reboot. Your second heartbeat node should then detect that the server is down and then switch over to the failover host.