Connectivity

There are two classes of file system drivers: 

oFSDs that use a block device driver to do I/O to a local or remote device 
. These are called local file systems. 
oFSDs that access a remote system without a block device driver. These are 
called remote file systems. 
  

The connection between a drive letter and a remote file system is achieved 
through a command interface provided with the FSD (FS_ATTACH). 

When a local volume is first accessed, OS/2 sequentially asks each 
installed FSD to accept the media, by calling each FSD's FS_MOUNT entry 
point. If no FSD accepts the media, it is then assigned to the default FAT 
file system. Any further attempt that is made to access an unrecognized 
media, other than by FORMAT, results in an 'Invalid media format' message. 

When a volume has been recognized, the relationship between drive, FSD, 
volume serial number, and volume label is remembered. The volume serial 
number and label are stored in the volume parameter block (VPB). The VPB 
is maintained by OS /2 for open files (I/O based on file-handles), 
searches, and buffer references . The VPB represents the media. 

Subsequent requests for a volume that has been removed require polling the 
installed FSDs for volume recognition by calling FS_MOUNT. The volume 
serial number and volume label of the VPB returned by the recognizing FSD 
and the existing VPB are compared. If the test fails, OS/2 signals the 
critical error handler to prompt the user for the correct volume. 

The connection between media and VPB is remembered until all open files on 
the volume are closed and search and cache buffer references are removed. 
Only volume changes cause a redetermination of the media at the time of 
next access.