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HDFS, mail # user - TestDFSIO on Lustre vs HDFS


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Re: TestDFSIO on Lustre vs HDFS
Allen Wittenauer 2011-01-29, 06:35

On Jan 28, 2011, at 10:39 AM, Nathan Rutman wrote:
> Your storage type should depend on the kind of data your storing, the quantity, the reliability, scalabilty, heterogenicity (sic), data access pattern, applications you're using, performance requirements, and system cost.   My point in posting this stuff is not to say the Lustre should be your choice for Hadoop backend in all situations.  It was really to show that HDFS was designed for a particular usage pattern and scale, and using it outside of that realm may not be the best choice.  I was looking to the HDFS community to poke holes in my arguments.

People who approach HDFS from a pure filesystem perspective are often disappointed because they miss out on the fact that it is written primarily to support Hadoop's MapReduce framework.  In particular, this means having access to data locality information so that the network hit is mostly immaterial when reading or writing.   It is going to make a huge difference if you are reading a single TB file from one node for processing (which in turn will likely require many many block fetches from across the network) vs. being able to distribute that read to multiple hosts (such that there are is little-to-no network activity at all).

>  Also, to get improved Hadoop performance, the network needs to be more expensive than 1gigE.  

Hardly, especially when trunking is thrown into the mix.  

> And Lustre requires more sysadmin care and understanding, which adds to total cost of ownership.
> But all of that is a "fixed" cost -- it does not scale linearly with your storage size. If you double your storage requirement, you'll pay ~1.2x for RAID parity and spare space with Lustre, but you'll pay 3x for HDFS disks.  The Lustre initial costs are higher.  So at some scale there will necessarily be a cost crossover.

As nodes are added, the network costs will also go up, regardless of setup.  The only time they don't is if the original design had significantly over provisioned network vs. node count.  Only using 8 nodes hides this fact.