Installation and Administration Guide for the Cisco TelePresence Exchange System Release 1.1
Split Brain Recovery
Downloads: This chapterpdf (PDF - 104.0KB) The complete bookPDF (PDF - 3.66MB) | Feedback

Split Brain Recovery

Table Of Contents

Split Brain Recovery

Diagnosing Split Brain Mode

Recovering from Split Brain Mode

Verifying Synchronization of the Database Servers

Diagnosing Corrupted DRBD Metadata

Recovering from Corrupted DRBD Metadata


Split Brain Recovery


Split brain mode refers to a state in which each database server does not know the high availability (HA) role of its redundant peer, and cannot determine which server currently has the primary HA role. In split brain mode, data modifications may have been made on either node, and those changes may not be replicated to the peer. Also, neither or both nodes may be functioning in the primary HA role.

Split brain mode occurs when there is a temporary failure of the network connections between the two database servers, for example, due to one of the following occurrences:

Restart of either database server during synchronization.

Physical disconnection of the Ethernet cables from a database server.

Loss of power to one or both database servers.


Note If three or more servers failed, see the "Recovering from a Situation Where Three or More Servers Failed" section.


This chapter includes the following topics:

Diagnosing Split Brain Mode

Recovering from Split Brain Mode

Verifying Synchronization of the Database Servers

Diagnosing Corrupted DRBD Metadata

Recovering from Corrupted DRBD Metadata

Diagnosing Split Brain Mode

Use this procedure to determine whether your database servers are in split brain mode.

Before You Begin

Make sure that the database servers are correctly cabled. See the "Cabling Requirements for the Database Servers" section.

Procedure


Step 1 Log in to the CLI of each database server.

Step 2 On each database server, enter the utils service database status command.

Step 3 If the output indicates the role values as Primary/Secondary and Secondary/Primary with only one server having the current role value as Primary, the database servers are not in split brain mode.

The following split brain conditions are listed:

The role values display one of the following combinations:

The Connection Sync Status field is "StandAlone."

"Primary/Unknown" on one server and "Secondary/Unknown" on the other server.

"Secondary/Secondary" on both servers—In this particular case, if the connection sync status on both servers is "Connected," then the MySQL database is corrupted, and the split brain recovery procedure will not help. Instead, see the "Corrupted MySQL Database Recovery" chapter.

"Secondary/Unknown" on both servers—In this particular case, if you know that one of the database servers had a reboot during the initial synchronization process, then your database system is functioning in a mode for which the split brain recovery procedure will not help. To recover, you need to reinstall both database servers. See the "Installing and Synchronizing the Cisco TelePresence Exchange System Database Servers" section.

Step 4 To recover from split brain mode, proceed to the "Recovering from Split Brain Mode" section.


Example

In the following example, one of the database servers is StandAlone, which indicates that the nodes are in split brain mode:

admin: utils service database status 
--------------------------------------------------------------------------------
The initial configured HA role of this node      : secondary
The current HA role of this node                 : primary
The database vip address                         : 10.22.130.54
Node name                                        : ctx-db-1
Node IP address                                  : 10.22.130.49
Corosync status                                  : Running PID <18613>
Current Designated Controller (DC)               : ctx-db-2 - partition with quorum
MySQL status                                     : Running pid  2810
Connection Sync Status                           : StandAlone
Role (this-node/peer-node)                       : Primary/Unknown
Disk Status (this-node/peer-node)                : UpToDate/DUnknown
--------------------------------------------------------------------------------

Related Topics

Command Reference

Recovering from Split Brain Mode

Use this procedure to recover your database servers from split brain mode.

Before You Begin

Complete the "Diagnosing Split Brain Mode" section to confirm that your system is in split brain mode.

Decide which node has the data that you want to keep. In this procedure, you will give this node the primary HA role. All data on the other node will be lost during this procedure and will not be recoverable.

If you do not know which node has the most recent or most valuable data, follow these recommendations:

If the utils service database status command output on both nodes indicates that one node currently has the primary HA role while the other node currently has the secondary HA role, you should choose the current primary node to keep as the primary database server.

admin: utils service database status 
--------------------------------------------------------------------------------
The initial configured HA role of this node      : primary
The current HA role of this node                 : primary 
The database vip address                         : 10.22.130.54
...
 
   

If the utils service database status command output on both nodes indicates that neither or both nodes have the primary HA role, choose the node that you initially installed as the primary server to keep as the primary database server.

admin: utils service database status
--------------------------------------------------------------------------------
The initial configured HA role of this node      : primary 
The current HA role of this node                 : secondary
The database vip address                         : 10.22.130.54
...

Procedure


Step 1 Log in to the CLI of the database server which has the data that you want to keep.

Step 2 Enter the utils service database drbd keep-node command to reset the server to currently function in the primary HA role.

admin: utils service database drbd keep-node 
This command will make this node as Primary
Trying to assume primary role......... [Done]
Reconnecting to MySQL......... [Done]
 
   

Step 3 Log in to the CLI of the other database server.

Step 4 Enter the utils service database drbd discard-node command to reset the server to currently function in the secondary HA role.

admin: utils service database drbd discard-node 
This command will make this node as Secondary
Trying to assume secondary role......... [Done]
Ensuring DRBD volume unmounted...
Ensuring DRBD role is Secondary...
Discarding local MySQL data..... [Done]
 
   

Synchronization begins between the two database servers.

Step 5 Proceed to the "Verifying Synchronization of the Database Servers" section.


Related Topics

Command Reference

Verifying Synchronization of the Database Servers

Procedure


Step 1 Log in to the CLI of each database server.

Step 2 On each database server, enter the utils service database status command.

The following examples show that synchronization is in progress and proceeding successfully, because each node is aware of the HA role of its redundant peer, and the output displays the percentage of the synchronization progress. Also, the current primary database server identifies itself as the SyncSource, while the current secondary database server identifies itself as the SyncTarget.

Sample output from the current primary database server:

admin: utils service database status 
--------------------------------------------------------------------------------
The initial configured HA role of this node      : primary
The current HA role of this node                 : primary 
The database vip address                         : 10.22.130.54
Node name                                        : ctx-db-1
Node IP address                                  : 10.22.130.49
Corosync status                                  : Running PID <20414>
Current Designated Controller (DC)               : ctx-db-2 - partition with quorum
MySQL status                                     : Running pid 10100
Connection Sync Status                           : SyncSource
Role (this-node/peer-node)                       : Primary/Secondary
Disk Status (this-node/peer-node)                : UpToDate/Inconsistent
 
   

Sample output from the current secondary database server:

admin: utils service database status 
--------------------------------------------------------------------------------
The initial configured HA role of this node      : secondary
The current HA role of this node                 : secondary 
The database vip address                         : 10.22.130.54
Node name                                        : ctx-db-1
Node IP address                                  : 10.22.130.49
Corosync status                                  : Running PID <17842>
Current Designated Controller (DC)               : ctx-db-2 - partition with quorum
MySQL status                                     : Not running (only runs on database 
server with current role primary.)
Connection Sync Status                           : SyncTarget
Role (this-node/peer-node)                       : Secondary/Primary
Disk Status (this-node/peer-node)                : Inconsistent/UpToDate
--------------------------------------------------------------------------------
 
   

Note The synchronization takes approximately 40 minutes. During this time, the disk status value of the current secondary server is shown as inconsistent. An inconsistent disk state indicates that the synchronization between the database servers is not complete.


Step 3 To confirm that the synchronization is complete, enter the utils service database status command on both the primary and secondary database servers.

The following examples show that synchronization is complete, because the disk status of the current secondary server is now up to date.

Sample output from the primary database server:

admin: utils service database status 
--------------------------------------------------------------------------------
The initial configured HA role of this node      : primary
The current HA role of this node                 : primary 
The database vip address                         : 10.22.130.54
Node name                                        : ctx-db-1
Node IP address                                  : 10.22.130.49
Corosync status                                  : Running PID <20414>
Current Designated Controller (DC)               : ctx-db-2 - partition with quorum
MySQL status                                     : Running pid 10100
Connection Sync Status                           : Connected
Role (this-node/peer-node)                       : Primary/Secondary
Disk Status (this-node/peer-node)                : UpToDate/UpToDate
--------------------------------------------------------------------------------
 
   

Sample output from the secondary database server:

admin: utils service database status 
--------------------------------------------------------------------------------
The initial configured HA role of this node      : secondary
The current HA role of this node                 : secondary 
The database vip address                         : 10.22.130.54
Node name                                        : ctx-db-1
Node IP address                                  : 10.22.130.49
Corosync status                                  : Running PID <17842>
Current Designated Controller (DC)               : ctx-db-2 - partition with quorum
MySQL status                                     : Not running (only runs on database 
server with current role primary.)
Connection Sync Status                           : Connected
Role (this-node/peer-node)                       : Secondary/Primary
Disk Status (this-node/peer-node)                : UpToDate/UpToDate
--------------------------------------------------------------------------------
 
   


Tip If this verification procedure shows that the split brain recovery procedure did not work for either or both servers, proceed to the "Diagnosing Corrupted DRBD Metadata" section.


Diagnosing Corrupted DRBD Metadata

If, after you complete the split brain recovery procedure, the database servers still cannot connect to each other and complete synchronization, the metadata for the Distributed Replicated Block Device (DRBD) may be corrupted. The DRBD is what synchronizes the secondary database with changes that are made on the primary database.

Before You Begin

This procedure applies only after you attempt split brain recovery. (See the "Recovering from Split Brain Mode" section.)

Procedure


Step 1 Log in to the CLI of each database server.

Step 2 On each database server, enter the utils service database status command.

Step 3 The DRBD metadata is corrupted if the disk status value is "Inconsistent/Inconsistent" while the connection sync status is "StandAlone" or "WFConnection" on one or both servers.

Step 4 To recover from corrupted DRBD metadata, proceed to the "Recovering from Corrupted DRBD Metadata" section.


Example

In the following example, the status of one database server indicates that the nodes have corrupted DRBD metadata:

admin: utils service database status 
--------------------------------------------------------------------------------
The initial configured HA role of this node      : secondary
The current HA role of this node                 : secondary
The database vip address                         : 10.22.130.54
Node name                                        : ctx-db-1
Node IP address                                  : 10.22.130.49
Corosync status                                  : Running PID <11459>
Current Designated Controller (DC)               : ctx-db-2 - partition with quorum
MySQL status                                     : Not running (only runs on database 
server with current role primary.)
Connection Sync Status                           : WFConnection
Role (this-node/peer-node)                       : Secondary/Unknown
Disk Status (this-node/peer-node)                : Inconsistent/Inconsistent
--------------------------------------------------------------------------------

Related Topics

Command Reference

Recovering from Corrupted DRBD Metadata

Before You Begin

Make sure that the database servers are correctly cabled. See the "Cabling Requirements for the Database Servers" section.

Complete the "Diagnosing Corrupted DRBD Metadata" section to confirm that your system has corrupted DRBD metadata.

Procedure


Step 1 Log in to the CLI of the database server which has the data that you want to keep.

This should be the same node whose data you decided to keep when you completed the procedure in the "Recovering from Split Brain Mode" section.

Step 2 Enter the utils service database drbd force-keep-node command to reset the DRBD metadata and set the server to currently function in the primary HA role.

admin: utils service database drbd force-keep-node 
This command will make this node as Primary
Trying to assume primary role......... [Done]
Overwriting peer data... [Done]
 
   

Step 3 Log in to the CLI of the other database server.

Step 4 Enter the utils service database drbd force-discard-node command to reset the DRBD metadata and set the server to currently function in the secondary HA role.

admin: utils service database drbd force-discard-node 
Shutting down Heartbeat...
Stopping High-Availability services: 
[  OK  ]
Ensuring DRBD volume unmounted...
umount: /dev/drbd0: not mounted
Taking down DRBD Resource...
Recreating DRBD meta-data...
NOT initialized bitmap
Bringing up DRBD...
Starting Heartbeat...
Starting High-Availability services: 
[  OK  ]
 [Done]
 
   

Synchronization begins between the two database servers.

Step 5 Proceed to the "Verifying Synchronization of the Database Servers" section.


Related Topics

Command Reference