See last week's photocopy for:

• Data compression
• Access methods
• Levels in a file management system
• Access control

WINDOWS NT

Usually OS is associated with a particular file system (e.g. FAT for DOS and i-nodes for UNIX). NT designed to be independent of file system. Supports multiple systems:

• FAT
• HPFS (OS/2 high performance file system)
• NTFS

NTFS has the following features:

• File system recovery restores disk-based data after system failure.
• Can handle large storage media, up to 17 billion gigabytes.
• Extra security features including execute-only files.
• Unicode filenames
• Compliant with POSIX
• Features for future extensibility.
• Supports CD-ROM file system.

File objects provide a memory-based representation of shareable physical resources. When file is opened, I/O manager returns a handle for an NT file object. This is a virtual file. Virtual file is treated like any other object, unless reading or writing takes place, in which case IO manager is called.

Here are some of the properties (attributes) and methods (services) related to file objects.

ATTRIBUTES: file name, device type, byte offset, share mode, open mode, file disposition

SERVICES: create, open, read, write, query file information, flush buffers.

To distinguish between file object and the file itself - the same physical file can be opened by two users in share mode - it is the same physical file but there will be two file objects as far as NT is concerned. At IO time, obviously, the resources cannot be shared, so file object has to ensure exclusive access when writing to disk.

WIN95/98

File management subsystem made up of the following components:

• IFS (installable file system) manager communicates with permanent storage devices eg cd disk burner and can install and manage their file systems.
• VFAT manages hard drives and floppies.

IBM/VMS

VSAM (Virtual Storage Access Methods) file is a master catalog and contains an entry for each file.

Each file used by a program must be described in a special DD (data definition) statement. This is part of the JCL preceding any job. The DD statement tells the system which IO device to use, the volume serial number of any specific volume needed, etc.

DATAIN is the name of the DD statement. DD indicates it's a data definition statement. DSN (data set name) is the MVS equivalent of filename.

DISP indicates that the disposition of this file is that it is an "old" file (already existing) and that it should be kept after being used.

UNIT indicates the type of hardware (Model 2314 disk pack).

Last portion indicates the serial number of the volume.

If file has been previously activated by a previous executing then DD statement can be shorted because location and hardware are already stored in the catalog. When a file is a mentioned in a DD statement, the catalog is searched.

OPEN and CLOSE are the two routines users call from programs to gain IO support. Can do sequential access, indexed sequential access, direct access, and virtual storage access (VSAM).

VSAM is important because it was the first device independent access method.

A VTOC (volume table of contents) is maintained in each DASD (direct access system device). Occupies two cylinders. Space is allocated in tracks. User must specify how much space they need.