LAMP

This is the second part in in the series of posts about scaling Drupal stack. The first part can be found here.

GLB has been fixed to support unlimited connections and now I can benchmark Drupal stack cluster on large EC2 instances. What I'm looking for here mainly is how much Galera synchronization overhead affects performance here. With small instances everything was pretty much clear: single core hindered by Xen was an easy traget, and scalability was predictably linear. Large instance is dual core, and Xen interference is minimal, Galera synchronization and serialization effect must be more pronounced. How desperately bad is it?

Drupal is a widely used content management system written in PHP that uses SQL server as a storage backend. Although Drupal can work with PostgreSQL, Apache/Drupal/MySQL stack is the usual production configuration.

A number of strategies were developed to scale Drupal performance through clustering, see excellent series of articles by John Quinn. Naturally all of them were capped by the SQL server component. Perhaps Galera is the "holy grail" of Drupal scaling-out?

MySQL 5.1 introduces new system variables (auto_increment_increment, auto_increment_offset) for managing auto increment 'sequences'. Using these variables, it is possible to set up a multi master replication environment, where auto increment sequences in each master node interleave, and no conflicts should happen in the replication. No matter which master(s) get the INSERTs.

From Seppo's post you already know that writeset replication isn't particularly suited for table level locking. I would not consider it as a show-stopper though.

As one dude would have surely put it had he been a database developer: "To those who cling to table locking in the age of universally available transactional engines, know that you are on the wrong side of history..."

We were running Drupal benchmarks to measure the performance of Drupal/Galera cluster and were surprised to find locking sessions (LOCK TABLES...UNLOCK) in the SQL profile. Locking sessions were left out of Galera supported feature set, but now we need to re-consider our policy a bit.