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a question on NameNode

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??

Andy Kartashov
IT Architecture, Co-op
1340 Pickering Parkway, Pickering, L1V 0C4
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