We sometimes get asked, “Why not simply use a mirrored SAN instead of DRBD®”? This post shows some crucial differences between mirrored SAN and DRBD.
Basically, the first setup is having two servers, one of them actively driving a DM-mirror (RAID1) over (e.g.) two iSCSI volumes that two SANs export; the alternative is using a standard DRBD setup. Please note that both configurations need some cluster manager (like Pacemaker).
Here are the two setups visualized, showcasing the main differences:
|1.||High-cost, single supplier.||Lower-cost, commercial-off-the-shelf parts.|
|2.||At least four boxes (2 application servers, 2 SANs).||Two servers are sufficient.|
|3.||DM-Mirror has only recently gotten a ||Optimized Activity Log.|
|4.||Maintenance needs multiple commands.||Single userspace command: |
|5.||Split-Brain is not automatically handled.||Automatical Split-Brain detection – policies via DRBD configuration.|
|6.||Data Verification needs to get all data over the network – twice||Online-Verify transports (optionally) only checksums over the wire.|
|7.||Asynchronous mode (via WAN), not in a standard product.||Protocol A available – optional proxy for compression and buffering.|
|8.||Black Box.||GPL solution, integrated in standard Linux Kernel since 2.6.33.|