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Be map slot or not to be: that is the question!

| Posted in Monitoring |

0

A couple months ago, we got an email from Chris:

Hi!

The Hadoop cluster has been a bit slow the past few days and I noticed that the bottleneck seems to be coming from the map tasks. We have separate map and reduce task capacities and it continuously looks like the mapper slots are all taken while there’s a surplus of open reduce slots. Is there any reason that we can’t open any of the free reduce slots to map tasks?

Regards,
Chris

Ganglia configuration for a small Hadoop cluster and some troubleshooting

| Posted in Monitoring, Software, Troubleshooting |

1

Ganglia is an open-source, scalable and distributed monitoring system for large clusters. It collects, aggregates and provides time-series views of tens of machine-related metrics such as CPU, memory, storage, network usage. You can see Ganglia in action at UC Berkeley Grid.

Ganglia is also a popular solution for monitoring Hadoop and HBase clusters, since Hadoop (and HBase) has built-in support for publishing its metrics to Ganglia. With Ganglia you may easily see the number of bytes written by a particular HDSF datanode over time, the block cache hit ratio for a given HBase region server, the total number of requests to the HBase cluster, time spent in garbage collection and many, many others.

Basic Ganglia overview

Ganglia consists of three components:

  • Ganglia monitoring daemon (gmond) – a daemon which needs to run on every single node that is monitored. It collects local monitoring metrics and announce them, and (if configured) receives and aggregates metrics sent to it from other gmonds (and even from itself).
  • Ganglia meta daemon (gmetad) – a daemon that polls from one or more data sources (a data source can be a gmond or other gmetad) periodically to receive and aggregate the current metrics. The aggregated results are stored in database and can be exported as XML to other clients – for example, the web frontend.
  • Ganglia PHP web frontend – it retrieves the combined metrics from the meta daemon and displays them in form of nice, dynamic HTML pages containing various real-time graphs.

If you want to learn more about gmond, gmetad and the web frontend, a very good description is available at Ganglia’s wikipedia page. Hope, that following picture (showing an exemplary configuration) helps to understand the idea: