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From Michael Thomas <tho...@hep.caltech.edu>
Subject Re: why does not hdfs read ahead ?
Date Tue, 24 Nov 2009 23:35:19 GMT
Hey guys,

During the SC09 exercise, our data transfer tool was using the FUSE 
interface to HDFS.  As Brian said, we were also reading 16 files in 
parallel.  This seemed to be the optimal number, beyond which the 
aggregate read rate did not improve.

We have worked scheduled to modify our data transfer tool to use the 
native hadoop java APIs, as well as running some additional tests 
offline to see if the HDFS-FUSE interface is the bottleneck as we suspect.



On 11/24/2009 03:01 PM, Brian Bockelman wrote:
> Hey Raghu,
> There are a few performance issues.  Last week during Supercomputing '09, Caltech was
having issues with getting more than 2.6 Gbps per HDFS client process (I think they were pulling
16 files per process, but Mike knows the details).  I think they'd appreciate any advice you
have about tuning HDFS performance.
> We're starting early R&D for 100Gbps dataflows, and I believe improving our current
HDFS performance is on the TODO list.
> Brian
> (PS - I'm not saying HDFS is at fault here - it always remains a possibility that we're
using it in a sub-optimal manner.  If you have any favorite Java performance instrumentation
to recommend, we'd also be interested in that.)
> On Nov 24, 2009, at 12:35 PM, Raghu Angadi wrote:
>> Sequential read is the simplest case and it is pretty hard to improve upon
>> the current raw performance (HDFS client does take more CPU than one might
>> expect, Todd implemented an improvement for CPU consumed).
>> Just to reiterate what Todd said, there is an implicit read ahead for
>> sequential reads with TCP buffers and kernel read ahead on Datanodes.
>> If you extend the read ahead buffer to be more of a buffer cache for the
>> block, it could have big impact for some read access patterns (e.g. binary
>> search).
>> Raghu.
>> On Mon, Nov 23, 2009 at 11:23 PM, Martin Mituzas<xietao1981@hotmail.com>wrote:
>>> I read the code and find the call
>>> DFSInputStream.read(buf, off, len)
>>> will cause the DataNode read len bytes (or less if encounting the end of
>>> block) , why does not hdfs read ahead to improve performance for sequential
>>> read?
>>> --
>>> View this message in context:
>>> http://old.nabble.com/why-does-not-hdfs-read-ahead---tp26491449p26491449.html
>>> Sent from the Hadoop core-user mailing list archive at Nabble.com.

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