My PC-oriented brain says it’s easier to work with a million small files than one gigantic file. Hadoop says the opposite — big files are stored contiguously on disk, so they can be read/written efficiently. UNIX tar files work on the same principle, but Hadoop can’t read them directly because they don’t contain enough information for efficient splitting. So, I wrote a program to convert tar files into Hadoop sequence files.

Here’s some code (Apache license), including all the Apache jars needed to make it work:

tar-to-seq.tar.gz (6.1 MB)

Unpack it and run:

java -jar tar-to-seq.jar tar-file sequence-file

The output sequence file is BLOCK-compressed, about 1.4 times the size of a bzip2-compressed tar file. Each key is the name of a file (a Hadoop “Text”), the value is the binary contents of the file (a BytesWritable).

It took about an hour and a half to convert a 615MB tar.bz2 file to an 868MB sequence file. That’s slow, but it only has to be done once.