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From "Mich Talebzadeh" <m...@peridale.co.uk>
Subject Re: can block size for namenode be different from wdatanode block size?
Date Wed, 25 Mar 2015 15:34:52 GMT
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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-----Original Message-----
From: Mirko Kämpf <mirko.kaempf@gmail.com>
Date: Wed, 25 Mar 2015 15:20:03 
To: user@hadoop.apache.org<user@hadoop.apache.org>
Reply-To: 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>:

> Hi,
> The block size for HDFS is currently set to 128MB by defauilt. This is
> configurable.
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 blocks.

> 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
is loaded on startup of the NameNode.

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

Hope this helps a bit.

> Thanks,
> Mich

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