     
     HD-EFF (Hard Drive Efficiency) is a program to analyze the files 
     stored on your hard drive and calculate storage efficiency for 
     different cluster sizes.  HD-EFF is a variation of PC Magazine's
     CHKDRIVE utility.
     
     PC Magazine (June 27, 1995) has published a detailed discussion of 
     hard drive storage efficiency in its Tutor column.  However, the 
     magazine's utility incorrectly calculates storage efficiency, and 
     the difference is quite significant.

     DOS divides your drive's data storage space into clusters.  Files 
     are stored in units of whole clusters.  If a file requires only 
     part of a cluster, the unused space in that cluster cannot be 
     reallocated to store another file and is thus wasted.  The larger 
     your cluster size, the greater the potential for wasted space.  
     Note that you cannot directly choose the cluster size your drive 
     uses.  Cluster size is a byproduct of the size of the partitions 
     or logical volumes you set up on your hard disk.

     The PC mag utility is supposed to calculate how much wasted space 
     you would have for different cluster sizes, based on the actual 
     files on your drive.  It should compute storage efficiency by 
     comparing the ratio of filesize to cluster space used.  The formula 
     is further tweaked by considering the space each file also requires 
     in the drive's FAT (File Allocation Table).  Thus, the correct 
     formula for storage efficiency would be:

        Efficiency (%) = 100 * (Size) / (Size + Slack + Fatspace)

     where Size is the total size of all files on the drive, Slack is 
     the total wasted space in each file's last cluster, and Fatspace 
     is the total space required in the FAT for all files.  The latter 
     two values are derived for various assumed cluster sizes so you 
     can review your storage efficiency options.

     The magazine text's definition of overhang is contrary to the 
     calculation of overhang performed by the program.  The program 
     computes overhang as the amount of space each file requires in its 
     last cluster, not the amount of unused space.  (FWIW, I prefer to 
     define overhang as the program calculates it: the amount of space 
     a given file "overhangs" into the last cluster.  I call the unused 
     space slack.  The magazine's program uses what I call overhang in 
     the calculation of efficiency, but in the text defining "overhang" 
     uses what I call slack.)

     While the magazine named their utility CHKDRIVE, I find there are 
     already too many utilities floating around with variations on the 
     spelling of "CheckDrive", so I have named the modification HD-EFF.  
     If you obtained a copy of PC Magazine's CHKDRIVE, try comparing its 
     results with those of the corrected program HD-EFF.  As with all PC 
     Magazine utilities, CHKDRIVE is the copyrighted work of Ziff-Davis 
     Publishing Company.


     Dan Goodell                                     1261 Hookston Rd
     CIS: 71520,3116                                 Concord CA 94518


     Addendum:  The original 1995 HD-EFF utility has been revised, with 
     version 1.1 now displaying the cluster size currently in use on the 
     drive.
