Volume Management

Volume management (that is, detecting when the wrong volume is mounted and 
notifying the operator to take corrective action) is handled directly 
through OS/2 and the device driver. Each FSD is responsible for generating 
a volume label and 32 -bit volume serial number. These are stored in a 
reserved location in logical sector zero at format time. Because an FSD is 
the only system component to touch this information, an FSD is not 
required to store it in a particular format. OS/ 2 calls the FSD to 
perform operations that might involve it. The FSD is required to update 
the volume parameter block (VPB) whenever the volume label or serial 
number is changed. 

When the FSD passes an I/O request to an FS helper routine, the FSD passes 
the 32-bit volume serial number and the user's volume label (through the 
VPB) . When the I/O is performed, OS/2 compares the requested volume 
serial number with the current volume serial number it maintains for the 
device. This is an in -storage test (no I/O required) performed by 
checking the drive parameter block 's (DPB) VPB of the volume mounted on 
the drive. If unequal, OS/2 signals the critical error handler to prompt 
the user to insert the volume having the serial number and label 
specified. 

When OS/2 detects a media change in a drive, or the first time a volume is 
accessed, OS/2 determines which FSD is responsible for managing I/O to 
that volume. OS/2 allocates a VPB and polls the installed FSDs (by calling 
the FS_MOUNT entry point) until an FSD indicates that it does recognize 
the media. 

Note:  The FAT FSD is the last in the list of installed FSDs and acts as 
the default FSD when no other FSD recognition takes place.