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Computer Appreciation

In brief it can be described as….

The allocation of file space within a DOS partition is recorded and maintained within DOS's File Allocation Tables (FATs). The FATs make up a map of the utilization of space on any floppy or hard disk with one entry in the FAT for each allocatable cluster of sectors. Each entry in the FAT can indicate one of four possible conditions for the clusters of sectors it represents: It can be unused and available for allocation, unused and marked as bad to prevent its use, in use and pointing to the next cluster of the file, or in use as the last cluster of a file.

If each entry in the FAT points to the next, who points to the first entry? This is the role of the file's directory entry. It contains the name of the file, the file's exact length, the time and date of the file's last modification, file attribute flags, and the identity of file's first cluster. In a sense, a file's directory entry forms the head of the file's allocation chain with each link thereafter pointing to the next link in the chain.

This system, while quite workable and efficient, does have its dangers. These dangers center around the fact that the FAT contains the ONLY record of disk space utilization and a stubborn failure to correctly read a single sector of the FAT could render hundreds of files unrecoverable. This danger explains the popularity of several utility programs, which create a back-up copy of the File Allocation Table and Root Directory with each system boot-up. They provide some hope of recovery from the cataclysmic loss of the FAT's data.

The original designers of DOS were aware of the importance of the FAT and do provide a duplicate copy immediately following the first, but its physical proximity to the original renders it little better than none, and DOS has long been notorious for failing to intelligently utilize this extra copy of FAT information even in the event of a primary FAT failure. (DOS 3.3 seems to be much smarter in this regard.)

Important as FAT reliability is, it's not generally the prime source of DOS file corruption, since even with perfect data retrieval, it's still possible to scramble DOS's files like crazy. The primary cause of DOS file system troubles are user error, program bugs, and "glitches." The advent of TSR "rule breaking" resident multitasking-style software has further complicated the scene.

 

When a new file is created or "opened," information about it is maintained inside DOS. The file's name, status, and first cluster are all held in internal tables. Then, as the file grows, free clusters are "checked out" of the File Allocation Table and allocated to the file's chain of clusters.

Now here's the crucial fact which causes so much trouble: No matter how big the newly created file becomes, a directory entry for the file is ONLY created when the file is finally and properly CLOSED. Until then the file exists only as a chain of allocated clusters filled with the file's data. If anything occurs to prevent the error-free closing of this file we have a real problem because the file's data is occupying a chain of "checked out" disk clusters, but there is no anchoring directory entry to point to the first cluster in the chain!

A chain of clusters without an anchoring directory entry is called a "lost chain." It exists, it contains data, but there's no record of the file's name, exact size, or purpose.

Lost cluster chains are frequently created when programs abort abnormally, when TSR's crash the system suddenly, when the computer user forgets to write a TSR's files out to disk before shutting the system down, or when a task in a multi-tasking system is not terminated. (It's easy to forget that a file was left open in a suspended background task.) Additionally, any damage to DOS's root directory or subdirectories can "liberate" chains of lost clusters.

DOS provides the CHKDSK (pronounced Check Disk) command to help its users keep an eye on just these sorts of problems. CHKDSK provides a comprehensive verification of DOS's filing system integrity and provides a means for straightening things out. When the CHKDSK command is given, the parentage of all cluster chains is checked, allocation chains are "followed" to be sure they don't cross over other chains (creating cross-linked files), and several other system integrity checks are performed.

In the case of lost chains, CHKDSK will offer to convert these into files by anchoring them to the root directory. Then any suitable text editor can be used to open these new files for the sake of identifying them and moving them back to where they belong.

Unfortunately the structure of DOS filing systems lacks the fundamental redundancy required to provide simple and error-free recovery from many forms of damage. Even the tools and techniques available from third party suppliers can't surmount these problems. The best bet is to understand DOS's weak spots, make certain that all opened files are closed successfully, perform a weekly CHKDSK command to collect accumulating file fragment "debris" and back up your hard disks regularly.

"Disk Optimizers" which promise to increase the throughput and performance of old and well used hard disk drives number among the most popular of the general use hard disk utilities.

We've seen how DOS's file allocation system operates. Files are composed of clusters, which in turn are composed of sectors. And while the group of sectors which comprise a cluster are by definition contiguous, the cluster linking scheme which DOS employs allows a file's clusters to be scattered across the disk's surface. Since the file's directory entry specifies the file's first cluster, and each succeeding cluster entry in the file allocation table specifies the next one, the file's contents could be literally anywhere on the disk. The term "file fragmentation" refers to the condition where a file's clusters are not consecutively numbered. Let's first examine how a disk's files might become fragmented.

When a file is deleted from a disk, its directory entry is flagged as unused and each cluster, which the file occupies, is flagged in the system's FAT as being free for use. If the surrounding clusters are still in use by other files, this creates a "hole" of free space in the disk.

Now suppose that a new file is copied from a floppy disk onto the hard disk. As DOS reads the new file's data from the floppy, it must allocate space for this file on the hard disk. So each time another cluster of sectors is needed, DOS searches through the file allocation table to find the next available cluster. In our example, DOS would discover the clusters, which had been freed by the first file we deleted and allocate them for use by
the new file. Then, when all of the clusters in the free space hole had been used, DOS would be forced to continue its search deeper into the drive. When space was found further in, the file's contents would be partially stored near the beginning of the disk and partially nearer to the end. The file would then consist of at least two fragments.

 

During the normal course of daily computer usage, many files are being constantly created, copied, extended, deleted, and replaced. When a word processor creates an automatic backup file, the original file is typically renamed to identify it as a backup file and a new file is created. Every new file creation is an opportunity for fragmentation. The files, which are being modified most often, are most subject to extensive fragmentation since any search by DOS for a free file cluster is almost guaranteed to produce a new discontinuity. With continued use, it's typical for much of the disk's file data to become haphazardly scattered across the surface of the disk drive.

But since DOS's cluster allocation scheme was specifically designed to manage such scattering, what's the problem? Any time the drive's head moves, two things occur: Time is consumed, and the drive experiences some mechanical wear and tear. If a file's data is scattered across the surface of the disk, the drive's head is forced to move a large distance many times to read a single file. If the file is a database whose records are being accessed at random, this excessive head motion can degrade the overall system performance tremendously and induce many other wear-related disk drive problems.

The extra time wasted in cluster fragment chasing is directly proportional to the drive's average head access time. The prior generation of 65 to 80 millisecond stepping motor drives lose far more performance to fragmentation than the latest sub-28 millisecond drives.

Disk optimisers like SoftLogic Solutions' DISK OPTIMIZER, Norton's SPEEDDISK, Central Point's COMPRESS, and Golden Bow's VOPT operate by physically rearranging the allocation of files on the disk. They relocate file cluster fragments while simultaneously updating the system's File Allocation Tables to reflect the new cluster locations. When finished, every file on the disk consists of a single contiguous run of consecutively
numbered clusters. Once the disk drive's head has been positioned to the beginning of the file, the entire file can be read or randomly accessed with an absolute minimum of head motion. Besides improving the system's overall performance, file defragmentation minimizes the mechanical wear and tear placed upon the drive's hardware. If some disaster should befall your system's Root Directory or File Allocation Table, contiguous
files are also much easier to find and recover than files with severe fragmentation.

Since file fragmentation is a continually occurring fact of living with DOS, periodic defragmentation, like hard disk backup, should become part of every serious DOS user's regimen.

c) File allocation table


Figure 5: File Allocation Table

  • Allocation of disk blocks to files recorded in a file allocation table (e.g. MS-DOS, OS/2)
  • Sequential Access
  • Hold all or part of file allocation table in store to reduce number of disk accesses
  • Maximum of two disk accesses per file block access if blocks for the file are referenced on one file map block (i.e. ideally file blocks should be clustered on the disk)

Using DOS (Chap2)

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