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RE: a question on NameNode
Awesome, thanks.

So, what if DN2 is down, i.e. it is not sending any blocks' report.  Then NN (I guess) will figure out that it has 2 blocks (3,4) that has no home and that (without replication) it has no way of reconstructing the file A.txt. It must spit the error then.


From: Kai Voigt [mailto:[EMAIL PROTECTED]]
Sent: Monday, November 19, 2012 9:31 AM
Subject: Re: a question on NameNode


Am 19.11.2012 um 15:27 schrieb "Kartashov, Andy" <[EMAIL PROTECTED]<mailto:[EMAIL PROTECTED]>>:
I am learning that NN doesn't persistently store block locations. Only file names and heir permissions as well as file blocks. It is said that locations come from DataNodes when NN starts.

So, how does it work?

Say we only have one file A.txt in our HDFS that is split into 4 blocks 1,2,3,4 (no replication), with block 1-2 residing on DN1 and blocks 3,4 on DN2.

When we start NN it reads it metastore and tries to locate and map the locations of 4 blocks of file A.txt??

when a NameNode starts, it does that in safe mode. Like you said, it doesn't know where the blocks are. The DataNodes send a list of all of their local block IDs (so called block reports). Once the NameNode knows about the locations of most blocks (99,9%, configurable number), it will leave safe mode and HDFS is back.


Kai Voigt
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