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From "Ellis H. Wilson III" <el...@cse.psu.edu>
Subject Re: fs.local.block.size vs file.blocksize
Date Sun, 12 Aug 2012 17:40:29 GMT
Many thanks to Eli and Harsh for their responses!  Comments in-line:

On 08/12/2012 09:48 AM, Harsh J wrote:
> Hi Ellis,
>
> Note that when in Hadoop-land, a "block size" term generally means the
> chunking size of HDFS writers and readers, and that is not the same as
> the FS term "block size" in any way.

Yes, I do know that, but I was confused about something else.  More on 
that later in #2.

> On Thu, Aug 9, 2012 at 6:40 PM, Ellis H. Wilson III<ellis@cse.psu.edu>  wrote:
>> Can someone please briefly explain the difference?  I do not see deprecated
>> warnings for fs.local.block.size when I run with them set and I see two
>> copies of RawLocalFileSystem.java (the other is local/RawLocalFs.java).
>
> The right param still seems to be "fs.local.block.size", when it comes
> to using "getDefaultBlocksize" calls via the file:/// filesystems or
> other filesystems that have not over-riden the default behavior.

This question was more out of curiosity than anything.  My experiments 
agree that "fs.local.blocksize" is the right parameter for controlling 
the blocksize of file:///, but I'm still quite perplexed as to where 
file.blocksize actually is used.  I chased it around for a while in 
Eclipse last night, but have yet to see where it is directly resourced 
(keyconfigs sets it and suggests FileSystem, RawLocalFileSystem and 
CheckSumFileSystem all use it, but I don't see it being used in any 
practical way).

>> The things I really need to get answers to are:
>> 1. Is the default boosted to 64MB from Hadoop 1.0 to Hadoop 2.0?  I believe
>> it is, but want validation on that.
>
> The dfs.blocksize, which applies to HDFS, has not changed from its 64
> MB default.

I was referring to RawLocalFileSystem, not DistributedFileSystem.  I am 
fairly certain from my tests and from the code I've dug through that the 
default blocksize is still 32MB at the moment.  Please note that my 
questions here are fairly unconcerned with HDFS, as I'm not using it at 
all in >75% of my tests.

>> 2. Which one controls shuffle block-size?
>
> There is no "shuffle block-size", as shuffle goes to local filesystems
> and that has no block size concepts. Can you elaborate on this?

This was a plain ol' misconception/mistake on my part, still sticking 
around from when I started working in the Hadoop source just over a year 
back.  I mistook performance increases in TeraGen but performance 
decreases in TeraSort (noted by an elongated shuffle phase) when I 
increased file:///'s blocksize to suggest that the shuffling used the 
file:/// filesystem as well.  I now understand why this can happen, and 
appreciate you clarifying as my digging through the shuffle code has 
done that indeed, no chunking occurs on shuffle.  My apologies for the 
confusing question, based on errant inferences.

Thanks again to both of you!  However, if anyone has better intuition on 
what the file.blocksize parameter does, I'd be happy to hear it.

Best,

ellis

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