Size vs Size on Disk: Difference and Comparison

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

Key Takeaways

1. Size refers to the total amount of space occupied by a file or folder; size on disk refers to the amount of space used by the file or folder on the storage device.
2. Size on disk considers the file’s allocation unit size, the smallest amount of space that can be allocated for a file; size does not consider allocation unit size.
3. The size of the disk is larger than the size due to the amount of unused space allocated for a file, known as slack space.

Size vs. Size on Disk

Size is the total amount of data stored in a file or folder, regardless of how it is stored. Size on disk is the amount of space a file or folder takes up on a storage device, including any additional space used for storage allocation. This can be larger than the actual size of the file or folder.

Comparison Table

Parameters of Comparison	Size	Size on Disk
Definition	The size 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.
Measurement	The file size is smaller than the size of the disk.	The size of the disk is larger than the size of the file.
Wasted Space	The actual size of the file does not waste space.	Size on disk wastes cluster space.
Additional Storage Locales	The additional sectors where the same file is stored are not included in the file size.	The additional sectors where the same file is stored are included in the size of the disk.
Compression	You cannot reduce the size below the size of the 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 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 file’s actual size, 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 much lesser than the size of the same file on the disk.

The amplification of its bytes count happens due to storing the file in clusters on the hard drive. The size of the file is 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.

What is the Size of the 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 the size of the disk.

In other words, the file’s byte count 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 larger than the file’s actual size. The size of the disk may be compressed, but it 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 the disk shows the file using the whole cluster and displays an enhanced bytes count.

Main Differences Between Size and Size on Disk

1. The main difference between size and size on disk is in terms of each definition. 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 disk size considers all the different sectors where the given file may be saved. This is a reason for its amplified size. At the same time, the file size does not consider 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 the size of the 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 the disk for the same file.

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