When the NameNode doesn't see a heartbeat for 10 minutes, it then recognizes that the node is down.
Per the Hadoop online documentation:
"Each DataNode sends a Heartbeat message to the NameNode periodically. A network partition can cause a
subset of DataNodes to lose connectivity with the NameNode. The NameNode detects this condition by the
absence of a Heartbeat message. The NameNode marks DataNodes without recent Heartbeats as dead and
does not forward any new IO requests to them. Any data that was
registered to a dead DataNode is not available to HDFS any more. DataNode death may cause the replication
factor of some blocks to fall below their specified value. The NameNode constantly tracks which blocks need
to be replicated and initiates replication whenever necessary. The necessity for re-replication may arise due
to many reasons: a DataNode may become unavailable, a replica may become corrupted, a hard disk on a
DataNode may fail, or the replication factor of a file may be increased.
I was trying to find out if there's an hdfs-site parameter that could be set to decrease this time period, but wasn't successful.
> Date: Tue, 29 Mar 2011 08:13:43 -0400
> Subject: live/dead node problem
> From: [EMAIL PROTECTED]
> To: [EMAIL PROTECTED]
> Hello All,
> Is there a parameter or procedure to check more aggressively for a live/dead
> node? Despite me killing the hadoop process, I see the node active for more
> than 10+ minutes in the "Live Nodes" page. Fortunately, the last contact
> Using, branch-0.21, 0985326
> --- Get your facts first, then you can distort them as you please.--