SSDs Might Not Solve Database Performance Issues

Hadoop

You might have heard, or even experienced, how much faster Solid State Drives (SSD) are when compared to traditional Hard Disk Drives (HDD). You may also have heard that SSDs will make your database instance so much faster because it will reduce the amount of wait times on your disk drives. In this article from Yanpei Chen, he gives us some information that shows this may not always be the case.

It is well-known that solid-state drives (SSDs) are fast and expensive. But exactly how much faster — and more expensive — are they than the hard disk drives (HDDs) they’re supposed to replace? And does anything change for big data?

I work on the performance engineering team at Cloudera, a data management vendor. It is my job to understand performance implications across customers and across evolving technology trends. The convergence of SSDs and big data does have the potential to broadly impact future data center architectures. When one of our hardware partners loaned us a number of SSDs with the mandate to “find something interesting,” we jumped on the opportunity. This post shares our findings.

As a starting point, we decided to focus on MapReduce. We chose MapReduce because it enjoys wide deployment across many industry verticals — even as other big data frameworks such as SQL-on-Hadoop, free text search, machine learning, and NoSQL gain prominence.

We considered two scenarios: first, when setting up a new cluster, we explored whether SSDs or HDDs, of equal aggregate bandwidth, are superior; second, we explored how cluster operators should configure SSDs, when upgrading an HDDs-only cluster.

A closer look at HDFS and MapReduce implementations reveals a critical insight: both the HDFS DataNode and the MapReduce NodeManager write to local directories in a round-robin fashion. A typical setup would mount each piece of storage hardware as a separate directory, e.g., /mnt/disk-1, /mnt/disk-2, /mnt/ssd-1. With each of these directories mounted as a HDFS and MapReduce local directory, they each receive the same amount of data. Faster progress on the SSD does not accelerate slower progress on the HDDs.

So, to fully utilize the SSD, we need to split the SSD into multiple directories to maintain equal bandwidth per local directory. In our case, SSDs should be split into 10 directories. The SSDs would then receive 10x the data directed at each HDD, written at 10x the speed, and complete in the same amount of time. When the SSD capacity accommodates the 10x data size written, performance is much better than the default setup.

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