I'm planning to set up a small Hadoop cluster where the slave nodes boot and run from a central PXE server, to simplify deployment and updates, and to enable all of the disks on the slaves to be (almost) monopolized by HDFS.
However, I suppose I'll still have to reserve some space on the slave nodes for /tmp and /var/log, I don't want to just put these in a ramdisk because I'd like them there for debugging after crashes (and because RAM is scarcer than disk).
So the machines might boot off the remote PXE server, mount their / read-only from there, then mount /dev/sda1 through sdd1 for the HDFS data partitions, /dev/sda2 for /tmp, and /dev/sdb2 for /var/log.
My question is, are there any other directories that will need to be writable? Assuming we get Hadoop etc. to log into /var/log.
(And is this a sensible architecture in general?)
EDIT: don't worry about swap, I'm planning to make these swapless, the OOM killer is preferable to thrashing.