hdfs - Checksum verification in Hadoop

Question

Do we need to verify checksum after we move files to Hadoop (HDFS) from a Linux server through a Webhdfs ?

I would like to make sure the files on the HDFS have no corruption after they are copied. But is checking checksum necessary?

I read client does checksum before data is written to HDFS

Can somebody help me to understand how can I make sure that source file on Linux system is same as ingested file on Hdfs using webhdfs.

Answer 1

If your goal is to compare two files residing on HDFS, I would not use "hdfs dfs -checksum URI" as in my case it generates different checksums for files with identical content.

In the below example I am comparing two files with the same content in different locations:

Old-school md5sum method returns the same checksum:

$ hdfs dfs -cat /project1/file.txt | md5sum
b9fdea463b1ce46fabc2958fc5f7644a  -

$ hdfs dfs -cat /project2/file.txt | md5sum
b9fdea463b1ce46fabc2958fc5f7644a  -

However, checksum generated on the HDFS is different for files with the same content:

$ hdfs dfs -checksum /project1/file.txt
0000020000000000000000003e50be59553b2ddaf401c575f8df6914

$ hdfs dfs -checksum /project2/file.txt
0000020000000000000000001952d653ccba138f0c4cd4209fbf8e2e

A bit puzzling as I would expect identical checksum to be generated against the identical content.