The Extent File System was the default filesystem used by IRIX from GL2-W2.4 (pre-IRIX 4.x) through IRIX 5.3. Additionally, it was the filesystem used for all CDs produced for IRIX by SGI.
EFS was introduced in GL2-W2.4, a pre-IRIX System V based product to replace the original System V UFS. It was deprecated starting in 1994, with the release of an IRIX 5.3 XFS distribution, and replaced by XFS with IRIX 6.0 on all new installs.
The EFS filesystem is relatively primitive by modern standards, being a synchronous (non-journaling, no soft updates) filesystem with a fixed block size of 512 bytes. The maximum filesystem size is 8G, and the maximum file size is 2G.
For disc images that use EFS, a program called EFS2Tar has been developed in go.
It has no error handling but is very reliable at recovering them into tarballs that can be used under any OS that supports tar.
efs2tar -in ~/foo.img -out ~/bar.tar is an example of usage once installed.