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Mirrored SAN vs. DRBD

Every now and then we get asked “why not simply use a mirrored SAN instead of DRBD”? This post shows some important differences.

Basically, the first setup is having two servers, one of them being actively driving a DM-mirror (RAID1) over (eg.) two iSCSI volumes that are exported by two SANs; the alternative is using a standard DRBD setup. Please note that both setups need some kind of cluster manager (like Pacemaker).

Here are the two setups visualized:
The main differences are:

1. High cost, single supplier Lower cost, commercial-off-the-shelf parts
2. At least 4 boxes (2 application servers, 2 SANs) 2 servers are sufficient
3. DM-Mirror has only recently got a write-intent-bitmap, and at least had performance problems (needed if active node crashes) Optimized Activity Log
4. Maintenance needs multiple commands Single userspace command: drbdadm
5. Split-Brain 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 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

So the Open-Source solution via DRBD has some clear technical advantages — not just the price.

And, if that’s not enough — with LINBIT you get world-class support, too!

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