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From "Mich Talebzadeh" <m...@peridale.co.uk>
Subject Total memory available to NameNode
Date Thu, 26 Mar 2015 10:16:34 GMT
Is there any parameter that sets the total memory that NameNode can use?




Mich Talebzadeh




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From: Mirko Kämpf [mailto:mirko.kaempf@gmail.com] 
Sent: 25 March 2015 16:08
To: user@hadoop.apache.org; mich@peridale.co.uk
Subject: Re: can block size for namenode be different from wdatanode block size?


Correct, let's say you run the NameNode with just 1GB of RAM.
This would be a very strong limitation for the cluster. For each file we need about 200 bytes
and for each block as well. Now we can estimate the max. capacity depending on HDFS-Blocksize
and average File size.





2015-03-25 15:34 GMT+00:00 Mich Talebzadeh <mich@peridale.co.uk>:

Hi Mirko,

Thanks for feedback.

Since i have worked with in memory databases, this metadata caching sounds more like an IMDB
that caches data at start up from disk resident storage.

IMDBs tend to get issues when the cache cannot hold all data. Is this the case the case with
metada as well?



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From: Mirko Kämpf <mirko.kaempf@gmail.com> 

Date: Wed, 25 Mar 2015 15:20:03 +0000

To: user@hadoop.apache.org<user@hadoop.apache.org>

ReplyTo: user@hadoop.apache.org 

Subject: Re: can block size for namenode be different from datanode block size?


Hi Mich,


please see the comments in your text.



2015-03-25 15:11 GMT+00:00 Dr Mich Talebzadeh <mich@peridale.co.uk>:


The block size for HDFS is currently set to 128MB by defauilt. This is

Correct, an HDFS client can overwrite the cfg-property and define a different block size for
HDFS blocks. 

My point is that I assume this  parameter in hadoop-core.xml sets the
block size for both namenode and datanode. 

Correct, the block-size is a "HDFS wide setting" but in general the HDFS-client makes the

However, the storage and
random access for metadata in nsamenode is different and suits smaller
block sizes.

HDFS blocksize has no impact here. NameNode metadata is held in memory. For reliability it
is dumped to local discs of the server.

For example in Linux the OS block size is 4k which means one HTFS blopck
size  of 128MB can hold 32K OS blocks. For metadata this may not be
useful and smaller block size will be suitable and hence my question.

Remember, metadata is in memory. The fsimage-file, which contains the metadata 
is loaded on startup of the NameNode.


Please be not confused by the two types of block-sizes.


Hope this helps a bit.








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