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
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