MILLE-XTERM and LTSP
Linux-based X terminals are well known for making computing affordable, for giving a second life to old hardware and for lightening administrative burdens. If you ever have toyed with the idea of using Linux X terminals, you are probably familiar with the Linux Terminal Server Project (LTSP), described in Figure 1. An LTSP server is perfectly suited for small workgroups or classrooms. However, in order to deploy a greater number of terminals, say thousands of them, the current LTSP model encounters scalability problems.
The main goals of the MILLE-XTERM Project are:
Centralized X-terminal management.
Enhanced user experience.
The MILLE-XTERM Project applied clustering concepts to the X-terminal infrastructure to achieve these goals.
The MILLE Project is funded by Canadian public agencies and school districts in the province of Quebec. MILLE means “Free Software Infrastructure Model for Education” and is targeted at educational institutions. It is composed of four subprojects: a portal (based on uportal), an open-source middleware stack, a CD with free software for Windows/Mac and, finally, MILLE-XTERM (the object of the present article).
The solution entails centralizing servers in a secure, air-conditioned computer room to form a cluster of terminal servers. The cluster has four major components, as shown in Figure 2. The first is the boot server, which provides DHCP and TFTP services and serves as a base system optimized for the terminals via NFS. Next comes the configurator, which generates the lts.cfg configuration file from an SQL database. The terminal then queries the load balancer, which in turn seeks out the cluster's least-loaded application server. The chosen application server login screen then appears, and after a successful authentication, the user can start using the desktop, browser, office suite and other applications.
MILLE-XTERM relies on central file and authentication services that provide users with the same account and file on every application server. The open-source choice is NFS for users' home directories and OpenLDAP for the directory service. It also can be integrated into a Novell or Windows environment with additional configuration.
Unlike LTSP, there is no need for a separate network dedicated to terminals. They can share a LAN with other PCs. However, a reliable network infrastructure is crucial. With usual usage, each terminal generates an average of 1Mb/sec of X11 traffic. Low-end hubs should be avoided; managed switches with full-duplex capabilities really make a difference.
Unlike LTSP, each component is built from RPMs; system administrators easily can add features and local applications with standard package manager tools. The init scripts from the distribution are replaced by standard LTSP scripts. We are currently using Mandriva 2006 as the base distribution, though other distributions can be supported.
The boot server is mainly a read-only NFS server. Optionally, it can provide DHCP, TFTP and NBD swap services. The xtermroot contains a base system and an X server. With more than one boot server, it is easy to rsync the xtermroot periodically on each boot server. The terminals then boot in a uniform way, whichever boot server they use.
MILLE-XTERM supports different booting methods: CD-ROM, local hard drive, Flash disk, Etherboot or PXE. Each boot method has its advantages and drawbacks.
PXE and Etherboot rely on TFTP to transmit the initial file used for the boot process. It simplifies the deployment, as no configuration is stored on the terminal itself. However, simultaneously booting up hundreds of clients via TFTP can result in transmission errors and, consequently, boot problems.
An alternative is to use a 16MB IDE Flash disk that holds the kernel, the initrd and grub. The Flash disk is updated automatically as the terminal boots up. The disk is used only at startup and contains no moving parts.