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From "David B. Ritch" <david.ri...@gmail.com>
Subject RAID vs. JBOD
Date Sun, 11 Jan 2009 21:23:01 GMT
How well does Hadoop handle multiple independent disks per node?

I have a cluster with 4 identical disks per node.  I plan to use one
disk for OS and temporary storage, and dedicate the other three to
HDFS.  Our IT folks have some disagreement as to whether the three disks
should be striped, or treated by HDFS as three independent disks.  Could
someone with more HDFS experience comment on the relative advantages and
disadvantages to each approach?

Here are some of my thoughts.  It's a bit easier to manage a 3-disk
striped partition, and we wouldn't have to worry about balancing files
between them.  Single-file I/O should be considerably faster.  On the
other hand, I would expect typical use to require multiple files reads
or write simultaneously.  I would expect Hadoop to be able to manage
read/write to/from the disks independently.  Managing 3 streams to 3
independent devices would likely result in less disk head movement, and
therefore better performance.  I would expect Hadoop to be able to
balance load between the disks fairly well.  Availability doesn't really
differentiate between the two approaches - if a single disk dies, the
striped array would go down, but all its data should be replicated on
another datanode, anyway.  And besides, I understand that datanode will
shut down a node, even if only one of 3 independent disks crashes.

So - any one want to agree or disagree with these thoughts?  Anyone have
any other ideas, or - better - benchmarks and experience with layouts
like these two?



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