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1) How can I view the history of switching modes (standby/active) in namenode service? 2) After how long of active namenode unavailability, standby becomes active?


1) How can I view the history of switching modes (standby/active) in namenode service?
2) After how long of active namenode unavailability, standby becomes active?


Cloudera Employee



Please find inline answers,


1) How can I view the history of switching modes (standby/active) in namenode service?


--> You can check history of switching modes (standby/active) in namenode service in failover controler's log file.

2) After how long of active namenode unavailability, standby becomes active?


-->  It depends on how large your edits/fsimage files are to be synched.


We recommend you to refer documents[1][2]





Kindly correct if wrong

1) If Active NameNode1 crashes, then after seconds, NameNode2 will try to become Active.

Every 2000ms zookeeper "ticktime" setting to do heartbeats and the minimum session timeout will be twice the tickTime.
The new entry, initLimit is timeouts ZooKeeper uses to limit the length of time the ZooKeeper servers in quorum have to connect to a leader. The entry syncLimit limits how far out of date a server can be from a leader.
Failover controller running on namenode1 and namenode2 check/does  the health monitoing and ZKFC property for monitorHealth RPC timeouts are set by parameter for the actual monitorHealth() calls) parameter means Timeout for the actual monitorHealth() calls. Kindly remember this setting is for timeout of health monitor calls

2) If active node crashes, then after dfs.ha.fencing.ssh.connect-timeout seconds NameNode2 will try to become Active.  
Answer - The above statement is incorrect

dfs.ha.fencing.ssh.connect-timeout is only applicable when dfs.ha.fencing.methods is selected or mentioned as "sshfence"
But in cloudera default dfs.ha.fencing.methods is mentioned as shell(true)

The transition from the active namenode to the standby is managed by a new entity in the system called the failover controller. Failover controllers are pluggable, but the first implementation uses ZooKeeper to ensure that only one namenode is active. Each namenode runs a lightweight failover controller process whose job it is to monitor its namenode for failures (using a simple heartbeating mechanism) and trigger a failover on namenode failure.

Failover may also be initiated manually by an administrator, in the case of routine maintenance, for example. This is known as a graceful failover, since the failover controller arranges an orderly transition for both namenodes to switch roles.

In the case of an ungraceful failover, however, it is impossible to be sure that the failed namenode has stopped running. For example, a slow network or a network partition can trigger a failover transition, even though the previously active namenode is still running, and thinks it is still the active namenode. The HA implementation goes to great lengths to ensure that the previously active namenode is prevented from doing any damage and causing corruption—a method known as fencing. The system employs a range of fencing mechanisms, including killing the namenode’s process, revoking its access to the shared storage directory (typically by using a vendor-specific NFS command), and disabling its network port via a remote management command. As a last resort, the previously active namenode can be fenced with a technique rather graphi- cally known as STONITH, or “shoot the other node in the head”, which uses a specialized power distribution unit to forcibly power down the host machine.

Client failover is handled transparently by the client library. The simplest implementation uses client-side configuration to control failover. The HDFS URI uses a logical hostname which is mapped to a pair of namenode addresses (in the configuration file), and the client library tries each namenode address until the operation succeeds.

Let's take an example -
I have configured Hadoop HA cluster. If I kill Namenode process with command "kill -9 NameNodeProcessId" my standby node changes its state to active. But if I power off active node then standby node can't change its state to active because it trys to connect to the crashed node by using SSH.

This parameter doesn't work:
dfs.ha.fencing.ssh.connect-timeout 3000

It is 5 second by default. But even after 5 minutes standby node continue try to connect to crashed node. I set it manually for 3 second but it still doesn't work. So, if we just kill namenode process our cluster works but if we crash active node our cluster become unavailable.

Since you powered off the Active NN machine, during fail-over SNN(Standby Namenode)  timed out to connect to this machine and fencing is failed. Typically fencing methods should be configured to not to allow multiple writers to same shared storage. It looks like you are using 'QJM' and it supports the fencing feature on its own. i.e. it wont allow multiple writers at a time. So I think external fencing methods can be skipped. AFAIK, to improve the availability of the system in the event the fencing mechanisms fail, it is advisable to configure a fencing method which is guaranteed to return success. You can remove the SSH fencing method from both machines configuration. Please try the below shell based fence method just to skip SSH fence and restart the cluster. Then fail over will happen successfully.