Error When Promoting DRBD Primary

I was having some issues today setting up DRBD on some new SANs. When I would try and promote one of the SANs to primary using this command:

sudo drbdadm -f primary r0

I would get this error:

0: State change failed: (-2) Need access to UpToDate data
Command 'drbdsetup 0 primary' terminated with exit code 17

Checking the status reveals:

cat /proc/drbd

version: 8.3.11 (api:88/proto:86-96)
srcversion: DA5A13F16DE6553FC7CE9B2
 0: cs:Connected ro:Secondary/Secondary ds:Inconsistent/Inconsistent C r-----
    ns:0 nr:0 dw:0 dr:0 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:1953460984

The reason for this is because neither server can tell if it has the correct data, so we need to invalidate one of them, so the other one thinks it is up-to-date. This is pretty simple. On the secondary server, simple use this command (replacing r0 with your resource):

sudo drbdadm invalidate r0

That's it. You can now successfully set your primary server with:

sudo drbdadm -f primary r0

Now check your status:

cat /proc/drbd

and you should see something similar to:

version: 8.3.11 (api:88/proto:86-96)
srcversion: DA5A13F16DE6553FC7CE9B2
 0: cs:SyncSource ro:Primary/Secondary ds:UpToDate/Inconsistent C r-----
    ns:1818624 nr:0 dw:0 dr:1819288 al:0 bm:110 lo:0 pe:48 ua:0 ap:0 ep:1 wo:f oos:1951645432
        [>....................] sync'ed:  0.1% (1905900/1907676)Mfinish: 16:47:51 speed: 32,260 (30,772) K/sec

Huzzah, success!