Filesystem considerations for build of spack packages

Not all filesystems are equal… some can't be used for building of certain software (e.g., due to limited atime resolution, or racy deletion semantics, or limited ownership/permission POSIX properties).

On beefy machines, we recommend:

• /dev/shm to be used (but memory considerations are important, cf. #69 → "memory per core")
• to avoid any distributed filesystems (definitely no GPFS, but NFS is also no fun at all)
• for FUSE-mounted ones, it depends… overlayfs things are okay, some others are not (to be investigated further)