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From Todd Lipcon <t...@cloudera.com>
Subject Re: Exponential performance decay when inserting large number of blocks
Date Wed, 13 Jan 2010 21:31:53 GMT
Hi Zlatin,

This is a very interesting test you've run, and certainly not expected
results. I know of many clusters happily chugging along with millions of
blocks, so problems at 400K are very strange. By any chance were you able to
collect profiling information from the NameNode while running this test?

That said, I hope you've set the block size to 1KB for the purpose of this
test and not because you expect to run that in production. Recommended block
sizes are at least 64MB and often 128MB or 256MB for larger clusters.


On Wed, Jan 13, 2010 at 1:21 PM, <Zlatin.Balevsky@barclayscapital.com>wrote:

> Greetings,
> I am testing how HDFS scales with very large number of blocks.  I did
> the following setup:
> Set the default blocks size to 1KB
> Started 8 insert processes, each inserting a 16MB file
> Repeated the insert 3 times, keeping the already inserted files in HDFS
> Repeated the entire experiment on one cluster with 4 and another with 11
> identical datanodes (allocated through HOD)
> Results:
> The first 128MB (2^18 blocks) insert finished in 5 minutes.  The second
> in 12 minutes.  The third didn't finish within 1 hour.  The 11-node
> cluster was marginally faster.
> Throughout this I was storing all available metrics.  There were no
> signs of insufficient memory on any of the nodes; CPU usage and garbage
> collections were constant throughout.  If anyone is interested I can
> provide the recorded metrics.  I've attached a chart that looks clearly
> logarithmic.
> Can anyone please point to what could be the bottleneck here?  I'm
> evaluating HDFS for usage scenarios requiring 2^(a lot more than 18)
> blocks.
> Bes <<insertion_rate_4_and_11_datanodes.JPG>> t Regards,
> Zlatin Balevsky
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