Disk organization in MS DOS

Answer 1

Unlike the host's DFS or ADFS filing Systems, DOS does not require that each file occupies a contiguous area of disc. This gives major advantages in convenience, since it provides a self-maintaining system which never requires such operations as compacting, rendering errors like can't extend impossible. The only error in this category which is never seen in DOS is Disc full. A subsequent DIR will show that either this is literally true, or that the file to be written to the disc will not fit into the remaining unallocated spaces

The converse of this convenience is that, over time, as files are extended or contracted and new files are added or old ones deleted, individual files become very fragmented. This has two possible implications for the user.

The first, which is unavoidable, is that pieces of file may become literally scattered around the disc at virtually random locations. This eventually can make itself felt when file access times become extended. It is of little consequence for small files, but can have a very noticeable performance implication for regularly used large files.

The solution is to format a new disc and copy the data to it file by file. The copying must be carried out at the file level because copying the whole disc track by track, (i.e. using the DISK program) will also take the existing fragmented structure with it, defeating the objective.

Copying the files individually reorganises them, by joining the fragments together on the new disc. The result is that the newly copied files are again stored in contiguous areas and disc performance is restored. Assuming two floppy drives this can be simply achieved by the command. copy A*.*.* B:

Using this technique will require that copying is carried out on a directory by directory basis, and that the appropriate directories have already been created on the target drive. Although the effects of file fragmenting may be less noticeable for longer periods with a hard disc, this problem and its solution (unfortunately very much more tedious in this case) apply equally.

The second implication of fragmented files is that, in the event of a physical disc failure or the accidental deletion of a file, retrieving and assembling the fragments of a file in the correct order by recovering hardware sectors from disc can be an extremely lengthy and laborious process.

As usual, but even more especially important for DOS disc formats than for BBC native mode discs, the very best advice is to attempt to totally avoid this possibility by ensuring adequate backups are taken on a separate disc at frequent intervals during lengthy file updating sessions.

By: Rajnish Gupta(Moradabad)