The Free On-line Dictionary of Computing (30 December 2018):

installable file system

    (IFS or "File System Driver", "FSD") An
   API that allows you to extend OS/2 to access files stored
   on disk in formats other than FAT and HPFS, and access
   files that are stored on a network file server.

   For example an IFS could provide programs running under OS/2
   (including DOS and Windows programs) with access to files
   stored under Unix using the Berkeley fast file system.

   The other variety of IFS (a "remote file system" or
   "redirector") allows file sharing over a LAN, e.g. using
   Unix's Network File System protocol.  In this case, the
   IFS passes a program's file access requests to a remote file
   server, possibly also translating between different file
   attributes used by OS/2 and the remote system.

   Documentation on the IFS API has been available only by
   special request from IBM.

   An IFS is structured as an ordinary 16-bit DLL with entry
   points for opening, closing, reading, and writing files, the
   swapper, file locking, and Universal Naming Convention.  The
   main part of an IFS that runs in ring 0 is called by the
   OS/2 kernel in the context of the caller's process and
   thread.  The other part that runs in ring 3 is a utility
   library with entry points for FORMAT, RECOVER, SYS, and
   CHKDSK.

   EDM/2 article (http://edm2.com/0103/).

   (1999-04-07)