Ultra Monkey: High Availability and Load Balancing Solution for Linux [Monkey]
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Ultra Monkey 3 : Download | Installation | Configuration || Linux Virtual Server | Linux-HA | Ldirectord


The Linux Virtual Server

The Linux Virtual Server [offsite] (LVS) enables TCP/IP and UDP/IP connections to be load balanced This mechanism of connection control is often referred to as Layer 4 Switching as information available at layer 3 of the ISO seven layer protocol stack is used to make load balancing decisions. That is IP address and port information is used.

Linux-Director

The host that LVS runs on is refered to as the Linux-Director. And the service that connections are recieved for is refered to as the virtual service, as it is a service that may be provided by many physical servers. The Linux Director essentially acts as a specialised router that forwards packets from end-users to real-servers - hosts running the applications that actually process the requests.

When packets are received for a virtual service by the linux-director, the scheduling algorithm decides which which real-server to send the packet to. Once this decision is made subsequent packets to for the same connection will be sent to the same real server. Thus the integrity of the connection is maintained. Optionally, a service can be marked as persistent. When this is done all subsequent connections from a given end-user will be forwarded to the same real-server until a timeout elapses. This is useful for applications such as FTP where end-users should consistently be directed to the same real-server to ensure integrity of data connections.

Virtual Services

On the Linux Director a virtual service is defined by either an IP Address, port and protocol, or a Firewall-Mark.

Scheduling Algorithm

The virtual service is assigned a scheduling algorithm to use to allocate incoming connections to the real-servers. There are many different sheduling algorithms available, and new ones are added from time to time. For an up to date list, and more detailed description, please see the ipvsadm(8) man page.

Packet Forwarding

The Linux Virtual Server has three different ways of forwarding packets: Network Address Translation (NAT), IP-IP encapsulation or tunnelling and Direct Routing.

Connection Synchronisation

In order for LVS to forward all packets for the same connection to the same real server a small ammount of state is maintained for each connection. In a nuthsell, the source and destination ip address and port of the connection, and the real-server that is being used. If persistance is in use then the information to allow an end-user to reconnect to the same real-server is kept in a similar fashion. If two linux-directors are being used in an active-standby set-up, such as the any of the High Availability and Load Balancing topologies, then this connection infromation needs to be synchronised, so existing connections can continue if the active linux-director fails and the standby becomes active. This is done by an in-kernel synchronisation daemon that is part of LVS. The daemon on the active linux-director periodically sends out the neccessary connection information for the active connections using multicast UDP. A corresponding daemon on the standby linux director uses this information to seed the local connection table.

Real Servers

Real servers are the ultimate destination of a connection made to a virtual-service. They run the applications that handle requests from end-users. A virtual service's real-servers and are defined by an IP address, a port and optionally a weight if supported by the scheduling algorithm that is in effect. In general, the port of the real-servers must be the same as that of the virtual service. If firewall-mark virtual services are used then the port that an incoming connection is received on will be the port that the connection is forward to on the real-server, that is the port specification for the real-erver is not used. The health of the real-servers may be monitored using ldirectord.

Version

The version of LVS used by Ultra Monkey 3 depends on the kernel in use. The 2.4.27-8 kernel for Debian Sarge and the 2.4.21-27.0.2.EL kernel for Red Hat Enterprise Linux 3 have been tested and work with the configurations described.


Copyright © 2000-2005, Horms
Last Updated: Sat Mar 4 16:33:58 2006 +0900

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