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From Brian Bockelman <bbock...@cse.unl.edu>
Subject Re: why does not hdfs read ahead ?
Date Tue, 24 Nov 2009 18:33:30 GMT

On Nov 24, 2009, at 12:06 PM, Todd Lipcon wrote:

> Also, keep in mind that, when you open a block for reading, the DN
> immediately starts writing the entire block (assuming it's requested via the
> xceiver protocol) - it's TCP backpressure on the send window that does flow
> control there.

Ok, that's a pretty freakin' cool idea.  Is it well-documented how this technique works? 
How does this affect folks (me) who use the pread interface?

> So, although it's not explicitly reading ahead, most of the
> reads on DFSInputStream should be coming from the TCP receive buffer, not
> making round trips.
> At one point a few weeks ago I did hack explicit readahead around
> DFSInputStream and didn't see an appreciable difference. I didn't spend much
> time on it, though, so I may have screwed something up - wasn't a scientific
> test.

Speaking from someone who's worked with storage systems that do an explicit readahead, this
can turn out to be a big giant disaster if it's combined with random reads.

Big disaster as far as application-level throughput goes - but does make for impressive ganglia


> -Todd
> On Tue, Nov 24, 2009 at 10:02 AM, Eli Collins <eli@cloudera.com> wrote:
>> Hey Martin,
>> It would be an interesting experiment but I'm not sure it would
>> improve things as the host (and hardware to some extent) are already
>> reading ahead. A useful exercise would be to evaluate whether the new
>> default host parameters for on-demand readahead are suitable for
>> hadoop.
>> http://lwn.net/Articles/235164
>> http://lwn.net/Articles/235181
>> Thanks,
>> Eli
>> 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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