DRBD and MySQL: Cheap HA

The initials stand for Distributed Replicated Block Device. What does that mean exactly? Well the file system is part of your operating system, and it is a layer between the OS and the disk hardware, either a single disk or a disk I/O subsystem such as a RAID. It is exposed to the OS as a block device, and data is read and written in discrete chunks called blocks. The OS issues requests via the filesystem.

Ok, enough about filesystems, and block devices, what's DRBD then? Well it allows you to have a mirror copy of a block device in real-time.