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Caution

This is an advanced topic. Use at your own risk and ALWAYS backup your data before.

Useful Commands

View DRBD Status - DRBD 7


Translations Ignore


Code Block
cat /proc/drbd


View DRBD Status - DRBD 9

Translations Ignore


Code Block
drbdadm status


Reload all parameters

Translations Ignore


Code Block
drbdadm adjust jtelshared


Disconnect the share (useful for planned maintenance)

Translations Ignore


Code Block
drbdadm disconnect jtelshared


Down the share (useful for planned maintenance)

Translations Ignore


Code Block
drbdadm down jtelshared


Up the share

Translations Ignore


Code Block
drbdadm up jtelshared


Set the node to primary

Translations Ignore


Code Block
drbdadm primary jtelshared


Connect the share

Translations Ignore


Code Block
drbdadm connect jtelshared


PCS Cluster Commands (CentOS 8)

Code Block
pcs cluster stop acd-store2
pcs cluster start acd-store2

pcs node standby acd-store2
pcs node unstandby acd-store2

Split Brain

Background

See also:

https://docs.linbit.com/doc/users-guide-84/s-resolve-split-brain/

Symptoms - CentOS 7 and earlier


Translations Ignore


Code Block
titlecat /proc/drbd
cat /proc/drbd

-->

GIT-hash: a4d5de01fffd7e4cde48a080e2c686f9e8cebf4c build by mockbuild@, 2017-09-15 14:23:22
1: cs:StandAlone ro:Primary/Unknown ds:UpToDate/DUnknown r-----
 ns:0 nr:119823323 dw:119823323 dr:2128 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:0


cs:StandAlone means the node is not connected. 

This should be visible on both sides.

Symptoms - CentOS 8 and earlier


Translations Ignore


Code Block
titledrbdadm status
drbdadm status

-->

jtelshared role:Primary
  disk:UpToDate
  acd-store1 connection:Connecting

drbdadm status

-->

# No currently configured DRBD found.


The first command shows that DRBD is active on the first node, but not active on the second node.

Note: this can be due to the second node being stopped or in standby.

Find out which node is active in the PCS cluster - CentOS 7

Translations Ignore


Code Block
titlepcs status
pcs status

-->

Cluster name: portal

Stack: corosync
Current DC: acd-store1 (version 1.1.16-12.el7_4.7-94ff4df) - partition with quorum
Last updated: Sun Mar 18 18:05:32 2018
Last change: Fri Feb 16 00:07:51 2018 by root via cibadmin on acd-store2
2 nodes configured
3 resources configured
Node acd-store1: standby
Online: [ acd-store2 ]
Full list of resources:
Resource Group: haproxy_group
 ClusterDataJTELSharedMount (ocf::heartbeat:Filesystem): Started acd-store2
 ClusterIP (ocf::heartbeat:IPaddr2): Started acd-store2
 samba (systemd:smb): Started acd-store2
Daemon Status:
 corosync: active/enabled
 pacemaker: active/enabled
 pcsd: active/enabled


In the example above, the first node is in standby. The most important thing to check, is on which server the resources are started. 

In this case, the resources are started on acd-store2.

This will therefore be defined as the NON BROKEN node.

Find out which node is active in the PCS cluster - CentOS 8

Translations Ignore


Code Block
titlepcs status
pcs status

-->

Cluster name: jtel_cluster
Cluster Summary:
  * Stack: corosync
  * Current DC: acd-lb1 (version 2.0.3-5.el8_2.1-4b1f869f0f) - partition with quorum
  * Last updated: Sat Oct  3 12:39:22 2020
  * Last change:  Sat Oct  3 12:31:22 2020 by root via cibadmin on acd-lb2
  * 2 nodes configured
  * 5 resource instances configured

Node List:
  * Online: [ acd-lb1 ]
  * OFFLINE: [ acd-lb2 ]

Full List of Resources:
  * Clone Set: DRBDClusterMount-clone [DRBDClusterMount] (promotable):
    * Masters: [ acd-lb1 ]
    * Stopped: [ acd-lb2 ]
  * DRBDClusterFilesystem       (ocf::heartbeat:Filesystem):    Started acd-lb1
  * Samba       (systemd:smb):  Started acd-lb1
  * ClusterIP   (ocf::heartbeat:IPaddr2):       Started acd-lb1

Daemon Status:
  corosync: active/enabled
  pacemaker: active/enabled
  pcsd: active/enabled


In the example above, the second node is offline. The most important thing to check, is on which server the resources are started. 

In this case, the resources are started on acd-lb1.

This will therefore be defined as the NON BROKEN node.

Standby the broken node in the PCS cluster (if necessary)

This command can be run on either machine.

CentOS 7

Code Block
titleStandby broken node
pcs cluster standby acd-lb-broken
 
--> Verify this with
 
pcs status

CentOS 8

Code Block
titleStandby broken node
pcs node standby acd-lb-broken
 
--> Verify this with
 
pcs status

On broken node

Note: the first command will probably throw an error. Also, the share may not be mounted. This is OK.

Translations Ignore


Code Block
titledrbd on broken node
umount /srv/jtel/shared
drbdadm disconnect jtelshared
drbdadm secondary jtelshared
drbdadm connect --discard-my-data jtelshared



On the healthy node

Translations Ignore


Code Block
titledrbd on healthy node
drbdadm primary jtelshared
drbdadm connect jtelshared



Check re-sync activity

The re-sync might take a long time. 

Watch the status of this using:

cat /proc/drbd

Example output:

Translations Ignore


Code Block
titlecat /proc/drbd
[root@storage01 ~]# cat /proc/drbd
version: 8.4.10-1 (api:1/proto:86-101)
GIT-hash: a4d5de01fffd7e4cde48a080e2c686f9e8cebf4c build by mockbuild@, 2017-09-15 14:23:22
1: cs:SyncTarget ro:Secondary/Primary ds:Inconsistent/UpToDate C r-----
 ns:0 nr:1411538 dw:121234862 dr:2128 al:0 bm:0 lo:0 pe:0 ua:0 ap:0 ep:1 wo:f oos:184698664
 [>....................] sync'ed: 0.8% (180368/181744)M
 finish: 26:12:15 speed: 1,940 (2,760) want: 2,120 K/sec



Tune the transfer (Second Node) - Only CentOS 7.x

Currently there is no procedure for tuning the transfer on CentOS 8.x

If the transfer is going to take ages, then tune it on the broken node:

Translations Ignore


Code Block
titledrbd Transfer Tuning (on broken node)
drbdadm disk-options --c-plan-ahead=0 --resync-rate=110M jtelshared


Put broken node back to primary - CentOS 7.x ONLY

Translations Ignore

Do not do this on CentOS 8.x installations! Here DRBD is managed by the cluster.

Code Block
titleUnstandby broken node
drbdadm primary jtelshared
 
--> Verify this with
 
cat /proc/drbd



Restart PCS node

Translations Ignore

CentOS 7.x

Code Block
titleUnstandby broken node
pcs cluster unstandby acd-lb-broken
 
--> Verify this with
 
pcs status

...

CentOS 8.x

Code Block
titleUnstandby broken node
drbdadm primary jtelshared
pcs cluster start acd-lb-broken
pcs node unstandby acd-lb-broken
 
--> Verify this with
 
cat /proc/drbd
pcs status



Untune the transfer (Second Node) - CentOS 7x only

If the transfer was tuned, then untune it (on the broken node)

...

.

Note: it won't hurt to run this command anyway.

Translations Ignore


Code Block
titledrbd - Untune Transfer
drbdadm adjust jtelshared

...

Unstandby broken node


Check everything
 Translations Ignore

 Code Block
title
Check everything
pcs 
resource show
status
# CentOS 7.x
cat /proc/drbd
# CentOS 8.x
drbdadm status
# On some other linux machines
ls /home/jtel/shared
# Windows
dir //acd-store/shared


File System Corrupt
Sometimes, when DRBD fails, the file system will also become corrupt.
In this case both nodes might be primary, however neither will have the share mounted. 
The command mount /srv/jtel/shared will fail.
In this case, it may be necessary to repair the file system.
Symptoms
 Translations Ignore

 Code Block
[17354513.483526] XFS (drbd1): log mount/recovery failed: error -22
[17354513.483569] XFS (drbd1): log mount failed
[17355040.104433] XFS (drbd1): Mounting V5 Filesystem
[17355040.122234] XFS (drbd1): Corruption warning: Metadata has LSN (56:112832) ahead of current LSN (56:112733). Please unmount and run xfs_repair (>= v4.3) to resolve.
[17355040.122239] XFS (drbd1): log mount/recovery failed: error -22
[17355040.122322] XFS (drbd1): log mount failed


Repairing
One one of the nodes (need to choose one to become primary):
 Translations Ignore

 Code Block
xfs_repair /dev/drbd/by-res/jtelshared/0
pcs resource cleanup


This should then mount and start the resources on that node.
Then proceed with the other node as "broken" in the split brain situation.
Stalled Resync
If the DRBD resync stalls - the output will be "stalled" when cat /proc/drbd is executed - then it may be necessary to restart the machine. 
This has been observed once, and restarting resolved the situation. However not much more is known about this state, or the cause, at this time.
Failed Connect (Unrelated data, aborting)
When the secondary has been told to discard it's data, and all of the commands to start the sync have been entered on both the healthy and the broken node, sometimes cat /proc/drbd will not report a connection.
Check /var/log/messages
If you can see output like this:
 Code Block
kernel: block drbd0: uuid_compare()=-1000 by rule 100
kernel: block drbd0: Unrelated data, aborting!
Then the metadata has become corrupt.
This requires that the metadata be completely reconstructed on the bad node.
Use the following commands to recreate the data on the broken node:
 Code Block
drbdadm down jtelshared
drbdadm wipe-md jtelshared
drbdadm create-md jtelshared
Then proceed with the re-sync as above (start with the part "On the broken node" with the commands to place this in secondary and discard data.
Failed Connect (Unknown Connection)
This produces errors something like this:
 Code Block
??: Failure: (162) Invalid configuration request
additional info from kernel:
unknown connection
In this case, drbd might not be loaded and enabled.
Execute the following code on the broken node and then proceed as above:
 Code Block
modprobe drbd
systemctl enable drbd
systemctl start drbd


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