fs.s3.buffer.dir defines the tmp folder where files will be written to before getting sent to S3. Right now this is limited to a single folder which causes to major issues.
1. You need a drive with enough space to store all the tmp files at once
2. You are limited to the IO speeds of a single drive
This solution will resolve both and has been tested to increase the S3 write speed by 2.5x with 10 mappers on hs1.