What is Pressflow?
Pressflow is a fork of Drupal core with enhancements for performance, scalability, and infrastructure. Pressflow receives the same security patches and updates that Drupal core does, so that it is constantly in sync with Drupal. When Drupal gets updated, so does Pressflow. Pressflow is a "drop in" replacement for Drupal. Unpack the files on top of your existing Drupal installation, and — provided you are using MySQL — everything will be compatible.
Why should I use Pressflow?
If you want better performance and scalability out of your high-traffic Drupal 6 site, Pressflow is ideal for your situation. It offers integration with Varnish and allows for MySQL replication. If you have mostly anonymous traffic, using Varnish as a reverse proxy cache is the most effective change you can make to improve performance. Furthermore, Pressflow is supported by the Drupal community, and is easier and safer than maintaining your own set of Drupal core patches.
If your project is built on Drupal 7 you might not need to use Pressflow at this time because the key enhancements from Pressflow 6 were added to Drupal 7.
Who uses Pressflow?
Pressflow is used by many Drupal 6 websites that have a high-level of traffic and large amount of content to serve. Because of this, it tends to be popular with online publishers and magazines like The Economist. There is a collection of websites powered by Pressflow on the wiki – feel free to add your own!
What's the deal with Pressflow 7?
Pressflow 6 made significant improvements to Drupal core in scalability and performance and most of these features have been contributed back to Drupal 7 core. Until well understood performance patches are created for Drupal 7 Pressflow development will be slow. Additionally, as Drupal 8 matures, significant enhancements will likely be backported to Pressflow 7.
Will Drupal contrib modules work with Pressflow?
Yes! Pressflow is fully API-compatible and retains all the same functionality of Drupal core. All contributed modules and themes found on Drupal.org will work with Pressflow. However, some contrib modules can reduce the effectiveness of the Pressflow stack.
When there is a new version of Drupal, does Pressflow also get updated?
Yes! All patches and security updates provided for both Drupal 6 and 7 are also be applied to Pressflow 6 and 7. We will release updates to Pressflow in a few days after they are available on Drupal.org.