Drbd is a block device which is designed to build high availability clusters by providing a virtual shared device which keeps disks in nodes synchronised using TCP/IP.
This simulates RAID 1 but avoiding the use of uncommon hardware (shared SCSI buses or Fibre Channel). It is currently limited to fail-over HA clusters.
This package contains the programs that will control the drbd kernel module.
You will need a clustering service (such as heartbeat) to fully implement it.