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 ||Optimized Activity Log|
|4.||Maintenance needs multiple commands||Single userspace command: |
|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!