Extent File System,
SGI's early block-device filesystem, in wide use on their
pre-Irix-6.0 workstations. Not to be confused with extfs (latterly, ext2fs), Linux's "Extended- (and Second Extended-) Filesystem".
Irix 6 adds support for a replacement filesystem ("XFS"), which is
journalling-capable; since post-6.5 kernels are
incapable of mounting EFS volumes read-write, SG's intention appears to
be to reserve EFS format for software distribution CDs, presumably
because it's useful to have a filesystem format available for this where the
index nodes and related file metadata take up a small(er)
percentage of the disk.