For the case in which mkextrafs is used to create local homedirs/projdirs:

Look for the desired mount point (/local) in /etc/fstab and use that if
it exists (i.e., that FS was already setup by the blockstore system or a
previous mkextrafs).

Otherwise, look for /var/emulab/boot/extrafs which should contain info
left behind by the local blockstore setup code indicating a FS or unused
device to use. For an unused device, rc.storage will identify the largest
available device that is at least 10MB.