Each node has a tasktracker with a number of map slots. A map slot hosts as mapper. A mapper executes map tasks. If there are more map tasks than slots obviously there will be multiple rounds of mapping.
The map function is called once for each input record. A block is typically 64MB and can contain a multitude of record, therefore a map task = run the map() function on all records in the block. Number of blocks = no. of map tasks (not mappers) Furthermore you have to make a distinction between the two layers. You have a layer for computations which consists of a jobtracker and a set of tasktrackers. The other layer is responsible for storage. The HDFS has a namenode and a set of datanodes. In mapreduce the code is executed where the data is. So if a block is in datanode 1, 2 and 3, then the map task associated with this block will likely be executed on one of those physical nodes, by tasktracker 1, 2 or 3. But this is not necessary, thing can be rearranged. Hopefully this gives you a little more insigth. Regards, Dieter 2014-02-25 7:05 GMT+01:00 Sugandha Naolekar <sugandha....@gmail.com>: > One more thing to ask: No. of blocks = no. of mappers. Thus, those many > no. of times the map() function will be called right? > > -- > Thanks & Regards, > Sugandha Naolekar > > > > > > On Tue, Feb 25, 2014 at 11:27 AM, Sugandha Naolekar < > sugandha....@gmail.com> wrote: > >> Hello, >> >> As per the various articles I went through till date, the File(s) are >> split in chunks/blocks. On the same note, would like to ask few things: >> >> >> 1. No. of mappers are decided as: Total_File_Size/Max. Block Size. >> Thus, if the file is smaller than the block size, only one mapper will be >> invoked. Right? >> 2. If yes, it means, the map() will be called only once. Right? In >> this case, if there are two datanodes with a replication factor as 1: only >> one datanode(mapper machine) will perform the task. Right? >> 3. The map() function is called by all the datanodes/slaves right? If >> the no. of mappers are more than the no. of slaves, what happens? >> >> -- >> Thanks & Regards, >> Sugandha Naolekar >> >> >> >> >
