How does partitioning work in Spark

Question

I'm trying to understand how partitioning is done in Apache Spark. Can anyone help please?

Here is the scenario:

a master and two nodes with 1 core each
a file sample.txt of 10 MB in size
How many partitions does the following create?

rdd = sc.textFile(sample.txt)
Does the size of the file have any impact on the number of partitions?

Answer 1

By default a partition is created for each HDFS partition, which by default is 64MB (from the Spark Programming Guide).

It's possible to pass another parameter defaultMinPartitions which overrides the minimum number of partitions that spark will create. If you don't override this value then spark will create at least as many partitions as spark.default.parallelism.

Since spark.default.parallelism is supposed to be the number of cores across all of the machines in your cluster I believe that there would be at least 3 partitions created in your case.

You can also repartition or coalesce an RDD to change the number of partitions that in turn influences the total amount of available parallelism.
Hope it will answer your query to some extent.