Difference Between Size and Size on Disk (with Table)

The difference between size and size on disk is in terms of the meaning of each of these terms. While size refers to the real size of a file, size on disk is the measure of the number of bytes the file occupies on the hard disk. People are frequently confused by these two values as they are often different. This difference arises from the seminal dissimilarity in the meaning of each term.

When a file is stored on the hard drive, it tends to occupy a certain number of bytes. This is connoted by the size on disk. On the other hand, size of the file simply means the actual size of a given file that includes solely the data stored within it.

Comparison Table Between Size and Size on Disk

Parameters of ComparisonSizeSize on Disk
DefinitionSize of a file refers to the actual size of the file.Size on disk refers to the size of the file when stored on the hard disk.
MeasurementSize of the file is usually smaller than size on disk.Size on disk is usually larger than the size of the file.
Wasted SpaceThe actual size of the file does not waste space.Size on disk wastes cluster space.
Additional Storage LocalesThe additional sectors where the same file is stored is not included in the size of a file.The additional sectors where the same file is stored is included in the size on disk.
CompressionCannot reduce the size below the size on disk.Can reduce the size on disk for a given file.

What is Size?

Size refers to the actual size of a given file. File size simply measures the amount of data stored on a particular file. When we click on the file properties icon, it displays two outcomes. One is the actual size of the file and the other is the size of the file on disk.

The actual size of the file is presented in the form of bytes. It connotes the actual byte count of any given file. This value is usually much lesser than the size of the same file on the disk.

The amplification of its bytes count happens due to the storage of the file in clusters on the hard drive. The size of the file is usually rounded up and includes the unused disk space as well. This leads to the bloating of byte counts on the hard drive for the same file.

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What is Size on Disk?

Size on disk refers to the size of a given file when it is stored on a hard drive. The hard drive uses clusters to store files. This measure of space that the cluster allocation of a file occupies on the hard drive is known as size on disk. In other words, the byte count of the file on disk is higher than the normal byte count of the same file.

One of the most prominent features of size on disk is that it is usually larger than the actual size of the file. The size on disk may be compressed, but it still remains a little over the actual size.

The discrepancy between file size on disk and its actual size happens because if the file size is not divisible by the available clusters or is smaller than the smallest available cluster, there will be empty residual space on that cluster.

Although the file is not occupying the entire cluster, the size on disk shows the file using the whole cluster and hence displays an enhanced bytes count.

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Main Differences Between Size and Size on Disk

  1. The main difference between size and size on disk is in terms of the definition of each of these terms. The former connotes the actual size of a given file, while the latter stands for the size of the file on the hard disk- measured in terms of the number of bytes it occupies.
  2. The second difference between the two terms can be noted in terms of their discrepancies in measurement. While the actual size of a file may be smaller, the size of the same file on the disk is displayed as significantly larger.
  3. Additionally, the size on disk takes into account all the different sectors where the given file may be saved. This is a reason for its amplified size. Whereas, the actual size of the file does not take into account the additional sectors of the file.
  4. When a file is stored on the disk, it is stored using clusters. This means that if the file size is not wholly divisible by the available clusters, there will be wasted space left in open clusters. Hence size on disk results in a certain amount of wasted space, which is not the case while dealing with the actual size of the file.
  5. Compression may reduce the file size on the disk- lowering it more than the actual file size. Compression does not lower the actual size over the size on disk for the same file.

Conclusion

Size and size on disk are terms that often sound confusing to users. There exist important differences between the two terms in terms of definition, their measurements, usage of space, etc.

The main difference between the two can be noted in terms of the definition of each, where size means the actual size of any given file, while size on disk refers to the size of the file when it is stored on the hard drive. The latter is usually larger in byte counts in comparison to the former.

Size on disk can be reduced by using compression. However, compression cannot be used to reduce the actual size of the file to a measurement lower than the size on disk.

Moreover, the former does not waste space as the actual size of the file only comprises vital data. However, the size on disk of a file refers to its storage in clusters and sectors on the hard drive and thus also includes unused, wasted space in its bytes measurement.

These differences must be understood properly to reduce wastage and to ensure the optimal utilization of the hard drive space available.

References

  1. http://www.cse.buffalo.edu/faculty/tkosar/cse710_spring14/papers/gpfs.pdf
  2. https://static.usenix.org/event/fast08/tech/full_papers/zhu/zhu.pdf