file size vs size on disk

May 1, 2006 at 09:35:49
Specs: Windows 2000 Pro SP4, P-IV

When I right click a file to view it's properties there are two different values shown: one for file "size" and the other for "size on disk". The second value is always larger. Could someone please explain what the difference is because I'm in the process of allocating space for backed up user data on a network share and I need to multiply the average file size by the number of users to get an estimate of how much space to allocate to that share. Thanks in advance!

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May 1, 2006 at 10:14:02

If I had 12 ounces of fluid and only 10 ounce cups then it would require two cups to hold the 10 ounces of fluid.

20 ounces of storage space to hold 12 ounces.

40 ounces would require 4 cups or 40 ounces of available storage space.

Computers work in a similar way depending on the allocation unit sizes used on PC when it was formatted.

Look at the properties of a 1kb .txt file where the file size would show 1kb but it would then also show how much storage space it requires due to the size of your allocation units.

It only effects the percentage of the last unit required.

a 1001 ounce file would require 101 10 ounce allocation units leaving that last cup with 9 ounces of unusable space.


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May 1, 2006 at 10:20:44

The space allocated to files is determined by the cluster size for the formatted partition. Cluster size is higly variable in FAT32/FAT16 file systms, but Windows 2K/XP using NTFS has clusters of 4096 bytes.

To compute the real space allocated to files you need: (FileSize/4096) rounded to the next higher integer x 4096, e.g.

FileSize 37352 bytes
FileSize/4096 = 9.11 rounded to 10
Space Allocated 40960 bytes (4096 x 10)

If the file is then compressed the real space allocation may be lower then native FileSize.

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