A High-Availability Cluster for Linux
Spreading the workload evenly across the nodes in a cluster is preferable to having one node do all the work until it fails, then having another node take over. The term for this is a hot standby system.
Load balancing can take many forms, depending on the service in question. For web servers, which provide simple static pages and are often read-only, a round-robin DNS solution can be quite effective. Unfortunately, with the read/write or transactional type of services such as e-mail or database access, unless the connection and session information from the service on one node can be shared and used by other nodes, it would be very difficult to provide seamless load balancing over the cluster. It would also require the disk mirroring to be near-instantaneous and use lots of distributed locking techniques, which most daemons will not support without complex modifications. To avoid these drawbacks, I decided to use a simpler approach which stands between the hot standby and the network-level load balancing.
In my two-node cluster, I put half of the required services on node “A” (serv1) and the other half on node “B” (serv2). A mutual failover configuration was employed so that if node A failed, node B would take over all of its services and vice versa.
We had to decide which services needed to be running on the overall cluster. This involved comparatively rating how much computing resource each service would consume. For our previous Linux server, it was found that Samba and Cyrus IMAP4 were the most resource-intensive services with FTP, httpd and Sendmail following close behind. Careful consideration had to be given to which services were suited to running on two or more nodes concurrently. Examples of such services included Sendmail (for sending mail only or as a relay), bind, httpd, ftpd (downloading only) and radius. Examples of services which cannot be run in such a way are Cyrus IMAP4, ftpd (uploading) and Samba. Samba cannot be run on two servers at once, as that would result in two servers broadcasting the same netbios name on the same LAN. It is not yet possible to have PDC/BDC (primary and backup domain controller) arrangements with the current stable versions of Samba. The pages on a simple web site, on the other hand, do not change often. Therefore, mirroring is quite effective and parallel web servers can run quite happily without major problems. The servers were configured so that each one took primary care of a specific group of services. I put Samba on serv1 and Cyrus IMAP4 on serv2. The other services are shared in a similar way; httpd, bind and radius run on both nodes concurrently.
In the event of a node failure, the other node takes over all the services of the failed one in such a way as to minimize disruption to the network users. This was best achieved by using IP (Internet protocol) and MAC (mandatory access control) address takeover from the failed node onto an unused Ethernet card on the takeover node. In effect, the node would appear to be both serv1 and serv2 to the network users.
The use of MAC address takeover was preferred in order to avoid potential problems with the clients' ARP (address resolution protocol) cache still associating the old MAC address with the IP address. MAC address takeover, in my opinion, is neater and more seamless than IP takeover alone, but unfortunately has some scalability limitations.
While considering the HA network setup, it was very important to eliminate all possible single points of failure. Our previous server had many single points of failure; the machine itself, the network cable, the Ethernet hub, the UPS, etc. The list was endless. The network has been designed to be inexpensive and reliable as shown in Figure 1.
The network diagram shows the three network interface cards (NICs) in each server. The first NIC in each server is used for the main LAN access to clients. Each node is plugged in to a separate Ethernet switch or hub to give redundancy in case of a switch lockup or failure. (This actually happened to us not so long ago.) The second NIC is used for creating a private inter-node network using a simple cross-over 100BaseTX full-duplex Ethernet cable. A cross-over cable is far less likely to fail than two cables plugged into an Ethernet hub or switch. This link is primarily used for disk mirroring traffic and the cluster heartbeat. It also helps take the network traffic load off the main interface and provides a redundant network path between the nodes. The third NIC is the redundant LAN access card used for MAC and IP address takeover in the event of a remote node failure in the cluster. Again, they are plugged in to different Ethernet switches or hubs for greater network availability.