When you start to look at SQL Server 2008, there is a sub-theme of using compression to help drive performance and usefulness. When I hear the word compression, my first thought is "No - I remember Stacker". How many years ago was that obscure reference? Too many... Stacker was a third party product for MS-DOS which basically doubled your disk space. It worked, but it also seemed to increase chances for corruption on those old disks. I can't recall the exact name of the product, but there was also a third-party product which compressed RAM to give you more of that too.
SQL Server 2008 will use compression when transmitting the transaction from the principal to the mirror. Since a majority of DB Servers are not CPU-bound, there are spare cycles to get the compression of this task done quickly. The goal of using this compression technology is to cut down on network latency. We have all seen this when we have a large file to transfer over the network. It is often faster for us to ZIP a file, copy it over the network and unZIP it than it would be to simply copy the file without being zipped. The same logic applies here.
Compression is used in other areas of SQL 2008 as well such as backups! More on that later in a future blog post perhaps.