Block device for building high-availability clusters
'drbd' is a block device for building high-availability clusters. DRBD takes over the data, writes it to the local disk and sends it to the other host. On the other host, it takes it to the disk there. The other components needed are a cluster membership service, which is supposed to be heartbeat, and an application that works on top of a block device (ie, a filesystem and fsck). Each device has either a 'primary' or 'secondary' state. The application should run and access the device on the node with the primary device. Every write is sent to the local 'lower level block device' and to the node with the device in 'secondary' state. The secondary device then writes the data to its lower level block device. If the primary node fails, heartbeat is switching the secondary device into primary state and starts the application there. If the failed node comes up again, it is a new secondary node and will synchronise its content to the primary node whithout interruption of service in the background. Only those parts of the device that actually have been changed will resync.
2 February 2005
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