Chapter 1. DRBD Fundamentals

DRBD is a software-based, shared-nothing, replicated storage solution mirroring the content of block devices (hard disks, partitions, logical volumes etc.) between hosts.

DRBD mirrors data

  • in real time. Replication occurs continuously while applications modify the data on the device.
  • transparently. Applications need not be aware that the data is stored on multiple hosts.
  • synchronously or asynchronously. With synchronous mirroring, applications are notified of write completions after the writes have been carried out on all (connected) hosts. With asynchronous mirroring, applications are notified of write completions when the writes have completed locally, which usually is before they have propagated to the other hosts.

1.1. Kernel module

DRBD’s core functionality is implemented by way of a Linux kernel module. Specifically, DRBD constitutes a driver for a virtual block device, so DRBD is situated right near the bottom of a system’s I/O stack. Because of this, DRBD is extremely flexible and versatile, which makes it a replication solution suitable for adding high availability to just about any application.

DRBD is, by definition and as mandated by the Linux kernel architecture, agnostic of the layers above it. Thus, it is impossible for DRBD to miraculously add features to upper layers that these do not possess. For example, DRBD cannot auto-detect file system corruption or add active-active clustering capability to file systems like ext3 or XFS.

Figure 1.1. DRBD’s position within the Linux I/O stack