Is a HDFS block sequential

Question

How can HDFS have a sequential block of 64MB when the underlying linux filesystem has only 4KB block sizes and a write of 64MB block cannot be sequential.

I am bit confused with this part. I am not able to get any explanation.

Can somebody help?

Answer 1

It seems like you are confused between the terms "contiguous" and "sequential". We have sequential reads/writes (from/to disk) and "contiguous" disk space allocation.

A single HDFS block of 64 MB will be written to disk sequentially. Therefore there is a fair chance that the data will be written into contiguous space on disk (consisting of multiple blocks next to each other). So the disk/block fragmentation will be much lower compared to a random disk write.

Furthermore, sequential reads/writes are much faster than random writes with multiple disk seeks. 

To learn more about this, check the following link: http://Difference between sequential write and random write.

Hope this answer your query to some extent.