Category Archives: Impala

Databricks to commercialize Spark and Shark in-memory processing

Shark utilizes in-memory SQL queries for complex analytics, and is Apache Hive compatible. The name “Shark” is supposed to be short hand for “Hive on Spark”. This seems to be a competitor to Cloudera Impala or the Hortonworks implementation of Hive.

Apache Spark utilizes APIs (Python, Scala, Java) for in-memory processing with very fast reads and writes, claiming to be 100x faster than disk-based MapReduce. Spark is the engine behind Shark. Spark can be considered as an alternative to MapReduce, not an alternative to Hadoop.

Scala is an interesting language being used by companies such as Twitter as both higher performance and easier to write than Java. Some companies that had originally developed using Rails or C++ are migrating to Scala rather than to Java.

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HortonWorks trying to make Hive faster, contrasting it to Impala

Hive was invented by Facebook as a data warehouse layer on top of Hadoop, and has been adopted by HortonWorks. The benefit of Hive is that it enables programmers, with years of experience in relational databases, to write MapReduce jobs using SQL. The problem is that MapReduce is slow, and Hive slows it down even further.

HortonWorks is pushing for optimization (via project Stinger) of the developer friendly toolset provided by Hive. Cloudera has abandoned Hive in favor of Impala. Rather than translate SQL queries into MapReduce, Impala implements a massively parallel relational database on top of HDFS.

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Selecting the right hardware for a Hadoop cluster

I’m summarizing this article. For specifics (such as how to configure split machines across racks to better configure the network switches) see the article. None of this content is operating system or hardware vendor specific, but generally the discussions assume Linux.

Goal is to minimize data movement and process on the same machine that stores the data. Therefore each machine in the cluster needs appropriate CPU and disk. Problem is that when building the cluster the nature of the queries and the resulting bottlenecks may not yet be known. If a business is building its first Hadoop cluster, it may not yet fully understand the types of business problems that will eventually be solved by it. That’s in contrast to a business deploying it’s Nth Oracle server.

Types of bottlenecks:

  1. IO: reading from disk (or a network location)
    • indexing
    • data import/export
    • data transformation
  2. CPU: processing the Map query
    • clustering
    • text mining
    • natural language processing

Other issues, since a cluster could eventually scale to hundreds or thousands off machines

  1. Power
  2. Cooling

The Cloudera Manager can provide realtime statistics about how a currently running MapReduce job impacts the CPU, disk, and network load.

Roles of the components within a Hadoop cluster:

  1. Name Node (and Standby Name Node): coordinating data storage on the cluster
  2. Job Tracker: coordinating data processing
  3. Task Tracker
  4. Data Node

Data Node and Task Tracker

  • The vast majority of the machines in a cluster will only peform the roles of Data Node and Task Tracker, which should not be run on the same nodes as Name and Job.
  • Other components (such as HBase) should only be run on the Data Nodes if they operate on data. You want to keep data local as much as possible. HBase needs about 16 GB Heap to avoid garbarge collection timeouts. Impala will consume up to 80% of available RAM.
  • Assumed to be lower performance machines than the Name Node and Job Tracker

Name Node and Job Tracker

  • Standby Name Node should (obviously) not be on the same machine as the Name Node.
  • Name Node (and Standby Name Node) and Job Tracker should be enterprise class machines (redundant power supplies, enterprise class raid’ed disks)
  • Name Node should have RAM in proportion to number of data blocks in the cluster. 1GB RAM for every 1 million blocks in HDFS. With 100 Data Node cluster, 64 GB RAM is fine. Since the machine’s tasks will be disk intensive, you’ll want enough RAM to minimize virtual memory swapping to disk.
  • 4 – 6 TB of disk, not raid’ed (JBOD configuration)
  • 2 CPUs (at least quad code). Recommend more CPUs and/or cores as opposed to faster CPU speed, since in a large cluster the higher speed will draw more power and generate more heat, yet not scale as well as if there were simply more CPUs or better yet nodes.

Cloudera has defined four standard configurations

  1. Light Processing (I’m not sure what the use case is for this. Prototype? Sandbox?)
  2. Balanced Compute (recommeded for your 1st cluster, since it’s not likely you’ll properly identify which configuration is best suited for your use case)
  3. Storage Heavy
  4. Compute Heavy

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