Saturday, February 05, 2011

Greenplum at Big Data Camp

I was at the Big Data Camp at the Strata Big Data conference the other day and one of the breakout sessions was with Greenplum. They had several interesting things to say on performance in the cloud, map-reduce and performance. Greenplum is a parallel database system that runs on a distributed set of servers. To the user, Greenplum looks like a conventional database server except that it should be faster and able to handle large data because it farms out the data and the workload over all the hosts in the system. Greenplum also has a map-reduce engine in the server and distributed Hadoop file system. Thus the user can use Greenplum both as a big data relational database and as a big data NoSQL database.

Map-reduce is good for taking semi-structured data and reducing it to more structured data. The example of map-reduce that I gave some time ago does exactly that. Thus a good use of Map Reduce is to do the Transformation part of ETL (Extract-Transform-Load), which is the way data gets into a data warehouse. The Greenplum people confirmed that this is a common use pattern for map-reduce in their system.

Next was a discussion of performance. Greenplum has compared performance and asserted that their relational database is 100 times faster than their map-reduce engine for doing the same query. I was somewhat surprised by the magnitude of this number, however I know that at the low end of system and data size, a relational database can be much faster than map-reduce and at the high end there places you can go with map-reduce that conventional database servers will not go, so it is never a level comparison. I will write more on this in another post.

Finally we got to performance on Virtual Machines (VMs) and in the cloud. Again Greenplum had measured their performance and offered the following. In a place where where conditions are well controlled like a private cloud, they expect to see a 30% performance reduction from running on VMs. In a public cloud like the Amazon EC2 server cloud, they see a 4 times performance reduction. The problem in a public cloud is inconsistent speed for data access and networks. They see both an overall speed reduction and inconsistent speeds when the same query is run over and over again.

It is worth remembering that Greenplum and other distributed database systems are designed to run on a set of servers with the same performance. In practice this means that the whole database system tends to run at the speed of the slowest instance. On the other hand, map-reduce is designed to run on distributed systems with inconsistent performance. The workload is dynamically balanced as the map-reduce job progresses, so map-reduce will work relatively better in a public cloud than a distributed database server.

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