Here is a light document, in the form of a presentation, to help IT architects or sales people to understand High Availability Clusters with Linux:
- How it works
- Components needed to build HA clusters
- SCSI and Fiber Channel considerations
- Reference architectures
- Clustering of popular products
- Sizing guides
Start here and follow the links to browse it, download PDF or the original (ODF) file.
There are also excelent presentations and tutorials in the Linux-HA Project website.