Info Panel


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Clicking on a blue lozenge displays a menu showing the units in which a particular field can be displayed. The size of a “block” on an HFS+ file system is generally 4KB (where KB means 1,024 bytes). Other units supported by iDefrag include:

Supported Units
Unit Name Size (bytes)

Block File system block 4,096 (typical)
Byte Byte/Octet 1
KB kilobyte (kibibyte) 1,024
MB Megabyte (Mebibyte) 1,048,576 (1,024 KB)
GB Gigabyte (Gibibyte) 1,073,741,824 (1,024 MB)
TB Terabyte (Tebibyte) 1,099,511,627,776 (1,024 GB)
KB10 (kilobyte) 1,000
MB10 (Megabyte) 1,000,000
GB10 (Gigabyte) 1,000,000,000
TB10 (Terabyte) 1,000,000,000,000

You may notice that the number of blocks displayed is not a round number. This is not a bug. The program is displaying the exact size of the file, not the number of blocks it occupies on the disk (which is usually the next largest integer). The number of bytes will always be a round number, however.

Historically, computer scientists and engineers involved with digital hardware of one sort or another have used units based on powers of 2 rather than the powers of 10 used by the SI unit system. Since 210 = 1,024, which is approximately 1,000, it became customary to use the SI abbreviations “k”, “M”, “G”, “T”, et cetera to represent multiples of 1,024 rather than 1,000.

In 1998, the IEC decided to lend credence to the base 10 units by decreeing that the base 2 units should be called “kibibyte”, “Mebibyte”, ..., with abbreviations “KiB”, “MiB”, “GiB” and so on. In many respects, this is unfortunate because current computer hardware is so tied to base 2 that in most circumstances a base 10 unit actually doesn’t make sense; for example, disks are almost without exception divided into sectors of 512 bytes, making two sectors exactly 1 KiB, but a rather unfortunate 1.024 kB using the IEC’s preferred form.

If you wish to change to IEC-style units, you can edit the names of the units by changing the .strings file within the iDefrag application bundle. (Similarly if you have a preference for the word “octet” instead of “byte”, although most of the machines where the two were different are now long gone.)

Unfortunately, the manufacturers of disk mechanisms spotted that they could quote larger figures if they used powers of 10 instead of 1,024, so when looking at disks, it is often useful to be able to display figures in units based upon powers of 10 rather than 2.

One particularly poor example of use of units is the case of the “1.44MB” floppy disk, where MB is used to mean 1,024,000 bytes. The only other place known to the author where such a travesty takes place is in the telephony world, where 1Mbps sometimes means 1,024,000 bits per second.