A read only union filesystem for sets of read only directories. directories are merged, files appear on a first in list order.
This file system was primarily designed for a package manager that works by mounting many packages in a read only union, then creating a chroot of the combined packages. The packages themselves are never edited so we can safely keep the whole union index in memory.
Mount three directories in a union.
$ pkgfs -f -oauto_unmount,kernel_cache ./dir1 ./dir2 ./dir3 ./mnt