Hadoop, Hadoop hardware, Uncategorized

A Question of Balance

When you add nodes to a cluster, they start out empty.  They work, but the data for them to work on isn’t co-located, so it’s not very efficient. Therefore, you want to tell HDFS to rebalance.


After adding new racks to our 70 node cluster, we noticed that it was taking several hours per terabyte to rebalance the nodes. You can copy a terabyte of data across a 10GbE network in under half an hour with SCP, so why should HDFS take several hours?

Continue reading


What’s So Important About Compression?

HDFS storage is cheap—about 1% of the cost of storage on a data appliance such as Teradata.  It takes some doing to use up all the disk space on even a small cluster of say, 30 nodes. Such a cluster may have anywhere from 12 to 24 TB per node, so a cluster of that size has from 720 to 1440 TB of storage space.  If there’s no shortage of space, why bother wasting cycles on compression and decompression?

clampWith Hadoop, that’s the wrong way to look at it because saving space is not the main reason to use compression in Hadoop clusters—minimizing disk and network I/O is usually more important. In a fully-used cluster, MapReduce and Tez, which do most of the work, tend to saturate disk-I/O capacity, while jobs that transform bulk data, such as ETL or sorting, can easily  consume all available network I/O.

Continue reading

Hadoop, YARN

The YARN Revolution

YARN—the data operating system for Hadoop.  Bored yet? They should call it YAWN, right?


Not really—YARN is turning out be the biggest thing to hit big-data since Hadoop itself, despite the fact that it runs down in the plumbing of somewhere, and even some Hadoop users aren’t 100% clear on exactly what it does. In some ways, the technical improvements it enables aren’t even the most important part. YARN is changing the very economics of Hadoop.

Continue reading

Hadoop Hive

Shifting to Hive Part I: Origins

SQL is the lingua-franca of data big and small, but SQL is a language, not a platform—it serves as the conceptual framework for data tasks on many platforms, ranging from blog content management with MySQL, to high-frequency online transaction processing (OLTP) systems, to heavy-duty batch processing on Hadoop and other big-data platforms.


I hope this page will help people who are experienced with conventional RDBMS’s and OLTP systems make the jump to working with big data using Apache Hive, the most important of the SQL big-data platforms.

Continue reading