Disk Debate

By Derek Walker

Originally published in EUG #07

I would like to add my tuppence worth to the DFS/ADFS debate between Ron Marshall and Christopher Chadwick by trying to clarify the structure of both filing systems.

The DFS is known as a "flat-file" structure with the 'root' directory having as many as 31 sub-directories/files. These sub-directories cannot contain further sub-directories. Having 31 files and no further sub-directories does limit the free space usage on an 80 track disk. Remember, this filing system was introduced with 40 track drives were the norm - 100k max and 32 files tended to fill the disk.

The other problem touched on by Ron/Chris is the "Can't Extend" problem with DFS. When a file is enlarged and no longer fits into its original space on the disk, the filing system saves the new file at the end of the current list of files (Hopefully there is plenty of room there) and deletes the old file. The space left by the old file is not reallocated by the filing system and is left unused until the disk is compacted (*COMPACT). The DFS disk can be quickly filled and fragmented with the need for frequent compaction.

ADFS has similar (but larger) constraints, the hierarchy of ADFS limits the number of files/sub-directories in a directory of 47 by 127 deep. A great many files, but very cumbersome - try remembering (or typing) a path-name that is deeper than 6. File-retrieval time is also increased the deeper into the hierarchy you do. There are of course excellent products on the market to help you find your way around the ADFS file hierarchy, e.g. Slogger's Click - Extree.

ADFS also gives the "Compaction Required" (Can't Extend) message but less frequently because ADFS reallocates the space left by deleted files. i.e. it will re-use the space if the file you wish to save is the same size or smaller than that deleted.

DFS File Structure
              |                |      |                   |
          dir name.filename    B.     C.     etc.   to   31

ADFS File Structure
                 |   |          |                    |
                 A.  files   and/or dirs            47
             ____|____                          _____|_____
            |         |                        |           |
            B.       47                        C.         47
       |         |
       D.       47                     and so on....

As a user of both filing systems on both the Electron and BBC I have found that they are equally essential, not because of how many files they can hold or their capacity, but because of the available software.

Derek Walker, EUG #7