Hadoop has configuration parameter hadoop.tmp.dir which, as per documentation, is "A base for other temporary directories." I presume, this path refers to local file system.
I set this value to /mnt/hadoop-tmp/hadoop-${user.name}. After formatting the namenode and starting all services, I see exactly same path created on HDFS. Doesn this mean, hadoop.tmp.dir refers to temporary location on HDFS?
Had a look around for information on this one. Only thing I could come up with was this post on the Amazon Elastic MapReduce Dev Guide:
In hadoop-site.xml, we set hadoop.tmp.dir to /mnt/var/lib/hadoop/tmp. /mnt is where we mount the “extra” EC2 volumes, which can contain a lot more data than the default volume. (The exact amount depends on instance type.) Hadoop's RunJar.java (the module that unpacks the input JARs) interprets hadoop.tmp.dir as a Hadoop file system path rather than a local path, so it writes to the path in HDFS instead of a local path. HDFS is mounted under /mnt (specifically /mnt/var/lib/hadoop/dfs/. So, you can write lots of data to it.