Comment 6 for bug 570805

I've release-noted this as follows (feel free to tweak from here):

== Default file system; package manager performance ==

The default file system for installations of Ubuntu 10.04 LTS is `ext4`, the latest version in the popular series of Linux extended file systems. `ext4` includes a number of performance tuning changes relative to previous versions such as `ext3`, the file system used by default up to Ubuntu 9.04. These generally produce improvements, but some particular workloads are known to be significantly slower when using `ext4` than when using `ext3`. If you have performance-sensitive applications, we recommend that you run benchmarks using multiple file systems in your environment and select the most appropriate.

In particular, the `dpkg` package manager is known to run significantly slower on `ext4` (causing installations using the server or alternate install CD to take on the order of twice as long as before). `ext4` does not guarantee atomic renames of new files over existing files in the event of a power failure shortly after the rename, and so `dpkg` needs to force the contents of the new file out to disk before renaming it in order to avoid leaving corrupt zero-length files after power failures. This operation involves waiting for the disk significantly more than it strictly needs to, and so degrades performance. If fast package management operations are most important to you, then you should use `ext3` instead. (Bug:570805)

The simplest way to select a different file system such as `ext3` at installation time is to add the `partman/default_filesystem=ext3` boot parameter when starting the installer. If you are deploying Ubuntu automatically using Kickstart or preseeding, then you can set a different file system in the partitioning recipe instead.