When occorred split brain, you can see below message (Primary/Unknown) or (Secondary/Unknown).
Node A $ cat /proc/drbd
version: 8.4.5 (api:1/proto:86-101)
GIT-hash: 1d360bde0e095d495786eaeb2a1ac76888e4db96 build by mockbuild@, 2014-08-17 22:54:26
0: cs:StandAlone ro:Primary/Unknown ds:UpToDate/DUnknown r-----
ns:0 nr:0 dw:886517 dr:2106580 al:368 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:5770156
Node B $ cat /proc/drbd
version: 8.4.5 (api:1/proto:86-101)
GIT-hash: 1d360bde0e095d495786eaeb2a1ac76888e4db96 build by mockbuild@, 2014-08-17 22:54:26
0: cs:StandAlone ro:Secondary/Unknown ds:UpToDate/DUnknown 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:16044
Node A (Primary): Assumed your main data in Node A.
First, check your DRBD partition is mounted.
Node A $ sudo drbdadm primary all
Node A $ sudo drbdadm connect all
Synchronizing data.
Node A $ cat /proc/drbd
version: 8.4.5 (api:1/proto:86-101)
GIT-hash: 1d360bde0e095d495786eaeb2a1ac76888e4db96 build by mockbuild@, 2014-08-17 22:54:26
0: cs:SyncSource ro:Primary/Secondary ds:UpToDate/Inconsistent C r-----
ns:379080 nr:0 dw:886698 dr:2495004 al:370 bm:0 lo:2 pe:0 ua:2 ap:0 ep:1 wo:f oos:5391140
[>...................] sync'ed: 6.7% (5264/5632)M
finish: 0:03:04 speed: 29,160 (29,160) K/sec
Synchronization Result.
Node A $ cat /proc/drbd
version: 8.4.5 (api:1/proto:86-101)
GIT-hash: 1d360bde0e095d495786eaeb2a1ac76888e4db96 build by mockbuild@, 2014-08-17 22:54:26
0: cs:Connected ro:Primary/Secondary ds:UpToDate/UpToDate C r-----
ns:233565451 nr:0 dw:233561279 dr:1788854 al:51494 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:0
Node B (Secondary):
First, check your DRBD partition is unmounted.
Node B $ sudo drbdadm disconnect all
Node B $ sudo drbdadm secondary all
Node B $ sudo drbdadm -- --discard-my-data connect all
Synchronizing data.
Node B $ cat /proc/drbd
version: 8.4.5 (api:1/proto:86-101)
GIT-hash: 1d360bde0e095d495786eaeb2a1ac76888e4db96 build by mockbuild@, 2014-08-17 22:54:26
0: cs:SyncTarget ro:Secondary/Primary ds:Inconsistent/UpToDate C r-----
ns:0 nr:4400804 dw:4400804 dr:0 al:0 bm:0 lo:0 pe:6 ua:0 ap:0 ep:1 wo:f oos:1369416
[==============>.....] sync'ed: 76.3% (1336/5632)M
finish: 0:00:37 speed: 36,028 (34,380) want: 102,400 K/sec
Synchronization Result.
Node B $ cat /proc/drbd
version: 8.4.5 (api:1/proto:86-101)
GIT-hash: 1d360bde0e095d495786eaeb2a1ac76888e4db96 build by mockbuild@, 2014-08-17 22:54:26
0: cs:Connected ro:Secondary/Primary ds:UpToDate/UpToDate C r-----
ns:0 nr:233565357 dw:233565357 dr:0 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:0
Reference:https://drbd.linbit.com/users-guide/s-resolve-split-brain.html
