



	_______________________________________________________________
	STACKER NOTE                                       STACKER NOTE
	Title: STACKER DRIVE SPACE REPORTING
	       (Applies to Stacker for Windows & DOS 2.x & 3.x)
	STAC FAX 31603  (6/93)
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	BACKGROUND

	  Stacker 2.0's SCHECK or Stacker 3.0 & 3.1's CHECK /D show 
	  exactly how the space has been used in the Stacker drive. 
	  Here is an example of an SCHECK & CHECK /D report:

	      Volume in drive C is STACVOL_DSK
	      No errors found
	      Stacker Drive Statistics:
	
			Stacker Drive              STACVOL File
			Drive C:                   D:\STACVOL.DSK
			------------------         -----------------
	Total Bytes:    147,095,552                73,550,336
	Bytes Used:      60,334,080  (41.0%)       33,004,032  (44.9%)
	Bytes Free:      86,761,472  (59.0%)       40,546,304  (55.1%)

	Bytes per Cluster     8,192                     2,048

	Stacker Drive Compression Ratio = 1.8:1
	Projected Bytes Free            = 74,096,640
	Fragmentation Level             = 0%

				    Note:

		    Not all the lines will be displayed if 
		    Stacker version 2.x SCHECK utility is used.

	What does it all mean?

	  The Left Column: This column contains information about your
	  "logical" drive's data.  The figures are "based on an        
	  anticipated compression ratio of 2.0:1."  In this example, 
	  your compressed drive (Total Bytes) has been established at
	  approximately 147 MB.  There are approximately 60 MB used
	  with enough clusters left on the compressed drive to 
	  accommodate approximately 86 MB of additional data.  Utility
	  programs which have the capability of reporting available
	  disk space could report either "Bytes Free" or "Projected
	  Bytes Free."

	  
	  
	  
	  
	  
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	Copyright 1993, Stac Electronics                    Page 1 of 3












	  The Right Column: This column shows the usage of actual 
	  "physical" hard disk space inside the STACVOL.DSK file.  The 
	  Stacvol file contains all of the data and free space for the 
	  Stacker drive C.  Its name and location are shown in the 
	  column header.  In our example, D:\STACVOL.DSK is about 73 MB 
	  in size and has used only 33,004,032 bytes of storage space.  
	  Therefore, there are 40,546,304 bytes of physical free space 
	  left in D:\STACVOL.DSK.

	Compression ratio

	  The Stacker drive compression ratio indicates the average 
	  compression of all the files in that drive.  The 
	  compressibility of the files is generally dependent on their 
	  contents.  In the above example, the 1.8:1 ratio is slightly 
	  less than a 2.0:1 ratio you typically get for an average
	  mixture of files.  This tells us that the files in this
	  Stacker drive are taking up slightly more space than if the
	  compression rtio was 2.0:1. 

	  A drive full of graphics files may achieve an average 
	  compression of 8.0:1 or more.  An 8.0:1 ratio would mean that 
	  the files are taking up 1/8 the space they would on a 
	  standard drive. 
    
	  Conversely, some files are already compressed, such as 
	  "zipped" files.  Stacker will attempt to compress these files 
	  even more however, since these files have already been 
	  previously compressed, little or no additional compression
	  occurs.  Previously compressed files will actually lower the
	  average compression ratio for the compressed drive in which
	  they reside.  If you used a third party utility to "zip" the
	  files, you could improve your average compression ratio by 
	  either storing these files on an uncompressed drive/diskette
	  or by "unzipping" the files and let Stacker compress them.

				     Note
  
		    The drive compression ratio does not display 
		    if the drive is empty.

	Projected bytes free

	  This number is Stacker's "estimate" of the available space 
	  left on the Stacker drive.  It is determined by one of the 
	  following methods:
	      
	      Method 1. By multiplying the physical bytes free (right 
			side) by the compression ratio if it is less
			than 2.0:1.  The example shows the compression
			ratio rounded off to tenths.  The actual
			compression ratio used in the calculation is
			not rounded off.
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	Copyright 1993, Stac Electronics                  Page 2 of 3

	      
	      
	      
	      
	      
	      
	      
	      
	      
	      
	      Method 2. The logical bytes free (left side) if your
			compression ratio is 2.0:1 or greater. 
			
	  The projected bytes free is limited to the smaller of these
	  values.

	  In our example, physical bytes free multiplied by the 
	  compression ratio would be 74,096,640.  However, there are 
	  enough unallocated clusters to accommodate the 86,761,472
	  bytes free from the left side column (based on an anticipated
	  compression ratio of 2.0:1).  The Projected Bytes Free become
	  the smaller of these two values.

				      Note
		    When the number for physical bytes free goes 
		    to zero, the drive is full.  You cannot "grow" 
		    the drive to create more physical space.  
		    Also, if the drive is less than 12% full, the 
		    projected bytes free will equal the "logical" 
		    bytes free number (left column).

	Expected Compression Ratio

	  If your average compression ratio significantly varies from
	  your original compression ratio (default is 2.0:1 unless 
	  modified) established when you installed Stacker, you can
	  either increase the number of allocation units or decrease
	  them.  You would do this by either adjusting the "Expected
	  Compression Ratio" from the Stacker Toolbox in version 3.x
	  or by using "Grow & Shrink" from the Stac Command Assistant
	  in version 2.x of Stacker.  You can only adjust the expected
	  compression ratio within a range that is either twice or half
	  the original value established at installation time (normally
	  2.0:1 unless you originally set it at another value).  By 
	  adjusting the Expected Compression Ratio (GROW operation in
	  version 2.x) you can adjust the figures being reported in the
	  left column to improve the accuracy of available space.  
	  Since your average compression ratio can fluctuate as you 
	  add, modify or delete files from your drive, modifying the 
	  expected compression ratio should only be done if there is a
	  "significant deviation" from the original compression ratio
	  (normally 2.0:1) established when you installed Stacker.
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	Copyright 1993, Stac Electronics                    Page 3 of 3
