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From <Zlatin.Balev...@barclayscapital.com>
Subject Exponential performance decay when inserting large number of blocks
Date Wed, 13 Jan 2010 21:21:23 GMT
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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