HDFS data transfer is faster than SCP based transfer?

Question

I have a use case that requires transfer of input files from remote storage using SCP protocol (using jSCH jar). To optimize this use case, I have pre-loaded all my input files into HDFS and modified my use case so that it copies required files from HDFS. So, when tasktrackers works, it copies required number of input files to its local directory from HDFS.

All my tasktrackers are also datanodes. I could see my use case has run faster. The only modification in my application is that file copy from HDFS instead of transfer using SCP. Also, my use case involves parallel operations (run in tasktrackers) and they do lot of file transfer. Now all these transfers are replaced with HDFS copy.

Can anyone tell me HDFS transfer is faster as I witnessed? Is it because, it uses TCP/IP? Can anyone give me reasonable reasons to support the decrease of time?

Comments

Is it a single file? Lots of files? How big are the files? Is the copy on a single node or are you running some kind of a MapReduce program?

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It is not a single file. Lot of small files. Files are stored in HDFS and map operations copies required files from hdfs. One map process running in one node only. Each file will be about 16MB

Answer 1

When u put the data or write into HDFS, 64kb of data is written on client side and then it is pushed through pipeline and this process continue till 64mb of data is written which is the block size defined by the client.

While on the other hand scp will try to buffer the entire data. Passing chunks of data would be faster than passing larger data.

Please check how writing happen in HDFS. That will give you clear picture

Answer 2

There's a lot of difference here, although both do use TCP underneath, but do note that SCP securely encrypts data but stock HDFS configuration does not.

You can also ask SCP to compress data transfer via the "-C" argument btw - unsure if you already applied that pre-test - it may help show up some difference. Also, the encryption algorithm can be changed to a weaker one if security is not a concern during the transfer, via "-c arcfour".