Raj's almost right. In times of high load or space fillup on a local DN, the NameNode may decide to instead pick a non-local DN for replica-writing. In this way, the Node A may get a "copy 0" of a replica from a task. This is per the default block placement policy.
P.s. Note that HDFS hardly makes any differences between replicas, hence there is no hard-concept of a "copy 0" or "copy 1" block, at the NN level, it treats all DNs in pipeline equally and same for replicas. On Sat, Aug 25, 2012 at 4:14 AM, Raj Vishwanathan <rajv...@yahoo.com> wrote: > But since node A has no TT running, it will not run map or reduce tasks. When > the reducer node writes the output file, the fist block will be written on > the local node and never on node A. > > So, to answer the question, Node A will contain copies of blocks of all > output files. It wont contain the copy 0 of any output file. > > > I am reasonably sure about this , but there could be corner cases in case of > node failure and such like! I need to look into the code. > > > Raj >>________________________________ >> From: Marc Sturlese <marc.sturl...@gmail.com> >>To: hadoop-u...@lucene.apache.org >>Sent: Friday, August 24, 2012 1:09 PM >>Subject: doubt about reduce tasks and block writes >> >>Hey there, >>I have a doubt about reduce tasks and block writes. Do a reduce task always >>first write to hdfs in the node where they it is placed? (and then these >>blocks would be replicated to other nodes) >>In case yes, if I have a cluster of 5 nodes, 4 of them run DN and TT and one >>(node A) just run DN, when running MR jobs, map tasks would never read from >>node A? This would be because maps have data locality and if the reduce >>tasks write first to the node where they live, one replica of the block >>would always be in a node that has a TT. Node A would just contain blocks >>created from replication by the framework as no reduce task would write >>there directly. Is this correct? >>Thanks in advance >> >> >> >>-- >>View this message in context: >>http://lucene.472066.n3.nabble.com/doubt-about-reduce-tasks-and-block-writes-tp4003185.html >>Sent from the Hadoop lucene-users mailing list archive at Nabble.com. >> >> >> -- Harsh J
