How to recover removed files from Silicon Graphic's Extent File System

[David Melman]

I have been request to help a desperate doctorial student who has lost
an entire year's worth of work on his Silicon Graphics (Iris) workstation
system version 2.0.  Out of ignorance, NO BACKUPS were done!  The machine
was taken down immediately afterwards.

Initially, I thought I was dealing with a conventional SysVR3 file
system.  I looked up in Bach's "The Design of the Unix Operating
System" and discovered that when the inode link count reaches zero, the
inode is left intact except for the file type (di_mode) field, which
gets zeroed out.  So, I thought all I need to do is run through the
i_list looking for his UID, and mark the file type.  I was going to
determine the file type (either regular file or directory) by looking
at the first data block.  Then fsck would find all the orphaned files
and pop them in lost+found.

Then I discovered that their file system is something called
the "Extent File System" which I have never heard of.  The structure
uses cylinder groups so I guess it has some Berkeley file system aspects (?).
Does it also just zero the file type field in the inode when freeing it?

Can anybody give me information about this file system to enable me
to restore these files?

David Melman
BITNET: melman at techunix.bitnet
Internet: melman%techunix.bitnet at cunyvm.cuny.edu