Understanding Used Space in Capacity Reporter


Introduction

Total space used displayed in Capacity Reporter can conflict for several reasons.

Total Space Used is calculated using Operating System values, while the total of Space Used by users is calculated by adding together the size of files.  These differences cause discrepancies in the totals reported. As file systems get bigger and bigger, the discrepancy can become very large.

 

AO = Overhead consumed by Disk Allocation, Volume Managers, and File Systems.
M = MinFree (Super User Reserved Space) ranging from 1% to 10% of the disk.
DO = Overhead consumed by directory descriptors.
FO = Extra space consumed by files due to the cluster group size.
SO = Overhead consumed by symbolic links and indexes.
FSU = File Space Used by the files calculated by adding the file sizes.
FS = Free Space available on the file system.
TC = Total Capacity of a Disk.

Solaris Calculations of Total Space Used for File Systems vs Total Space Used by Users of the Same File System

Comparison

Total Space Used for File Systems Total Space Used by Users of File Systems
Total Space Used = Total capacity* - Free Space Agent adds up the actual size of files on the file system
*TC = M + DO + FO + SO + FSU + FS (AO not included) FSU
 

In Solaris, the output of 'df -k' excludes the MinFree value from Total Space Used and Total Free Space.
For example: If the total capacity of an empty disk is 10 MB, and the minfree setting is 10% (1 MB in this case), the space available values will differ by 1 MB:

Windows NT Calculations of Total Space Used for File Systems vs Total Space Used by Users of the Same File System

Comparison

Total Space Used for File Systems Total Space Used by Users of File Systems
NTFS Formatted File Systems
Total Space Used = Total capacity* - Free Space Agent adds up the actual size of files on the file system
*TC = DO + FO + SO + FSU + FS FSU

FAT Formatted File Systems

Total Space Used = Total capacity* - Free Space Agent adds up the actual size of files on the file system
TC = FO + FSU + FS FSU

NTFS Example:

This can be verified by running chkdsk from the command prompt against the file system in question.  

Sample Results from chkdsk: 

FAT Example:

For any operating system that supports it, the FAT file system uses the following cluster sizes:
   Drive size            
   (logical volume)      FAT type   Sectors     Cluster size
   -----------------------------------------------------------------------
       15 MB or less     12-bit       8           4 KB
       16 MB - 127 MB    16-bit       4           2 KB
      128 MB - 255 MB    16-bit       8           4 KB
      256 MB - 511 MB    16-bit      16           8 KB
      512 MB - 1,023 MB  16-bit      32          16 KB
    1,024 MB - 2,048 MB  16-bit      64          32 KB
    2,048 MB - 4,096 MB  16-bit     128          64 KB
   *4,096 MB - 8,192 MB  16-bit     256         128 KB Windows NT 4.0 only
   *8,192 MB - 16384 MB  16-bit     512         256 KB Windows NT 4.0 only 

Capacity Reporter Setting Can Affect Space Used Results for Devices and Users

Capacity Reporter can report different results for space used on a disk, volume, or partition than it reports for space used by users. The used space value differs, depending upon:

In addition, the space used calculations differ:

So, when file compression is used, and when the option to report logical size is selected, space used by users is often reported as a greater value than space used on the device.

For example, if a device contains compressed and uncompressed files, the calculations vary as follows, depending upon the option setting:

Capacity Reporter Option Setting Space used by users Space used by devices Notes
Report logical size 1 MB +1 MB = 2 MB 330 KB + 1 MB = 1.33 MB + Overhead The combination of file compression and reporting logical size of files often results in a Space used by users value > Space used by devices value
Report physical size 330 KB + 1 MB = 1.33 MB 330 KB + 1 MB = 1.33 MB + Overhead Selecting the option to report physical size of files is much more likely to result in the Space used by users value agreeing with the space used by devices value