If you have a file named “
foo.txt” with a size of 500 bytes and you create a new file that is 10 bytes in size and save it with the same name, the original file will be overwritten. Does the operating system, whether it’s Windows or Linux, automatically free the unused space (which is 410 bytes) in this situation?
In another scenario, if you use the dd utility to create both the old and new versions of “
foo.txt,” will the operating system automatically free the unused space? It’s possible that all file writes use the same system calls provided by the operating system, which could result in consistent handling of file overwrites across different programs.
When we create or modify a file, the operating system allocates a certain amount of space to it. However, when we overwrite a file with a new one that is smaller in size, there may be some unused space left in the file. In this blog post, we will explore whether the operating system automatically frees up this unused space or not. We will also discuss the use of the “dd” utility and its impact on the allocation of file space.
What Happens When a File is Overwritten?
When a file is overwritten, the operating system deletes the old file and creates a new one with the same name. The new file may be smaller or larger than the old file. If the new file is smaller, there may be some unused space left in the file. However, whether the operating system automatically frees up this unused space depends on the file system used.
File Systems and Unused Space
Different file systems handle unused space differently. In general, file systems that support sparse files will automatically free up unused space when a file is overwritten with a smaller one. A sparse file is a type of file that allows the operating system to allocate disk space only for the portions of the file that contain actual data, rather than for the entire file. This can save a significant amount of disk space.
File systems that do not support sparse files, such as FAT and NTFS, may not automatically free up unused space when a file is overwritten with a smaller one. Instead, the unused space may be marked as “free” but still allocated to the file. This can result in wasted disk space.
Using the dd Utility
The “dd” utility is a command-line tool that is commonly used in Unix-based systems to create and modify disk images. It can also be used to create and overwrite files. When we use the “dd” utility to create and overwrite files, the file system used will determine whether unused space is automatically freed up or not.
If we use the “dd” utility to create both the old and new versions of a file, the file system will handle the unused space in the same way as if we had created and overwritten the file using another program. However, if we use the “dd” utility to create a sparse file, the file system will automatically allocate disk space only for the portions of the file that contain actual data. This can result in significant disk space savings.
In conclusion, whether the operating system automatically frees up unused space when a file is overwritten with a smaller one depends on the file system used. File systems that support sparse files will generally free up unused space, while those that do not may not. The “dd” utility can be used to create and overwrite files, and can also be used to create sparse files that can save disk space. When working with files, it is important to be aware of the file system used and its handling of unused space in order to avoid wasted disk space.
The answer to these two questions depends on the operating system used.
For Windows, the answer is no. Windows does not automatically free up the space no longer used when a file is overwritten. When a file is overwritten, the old file data still remains in the file system, taking up space. The only way to reclaim the space is to delete the file and rely on the OS to run its garbage collection to reclaim the space.
In contrast, Linux does automatically free up the space no longer used when a file is overwritten. When a file is overwritten, the old file data is removed from the file system and the space is reclaimed.
To illustrate this difference, let’s consider the following example with a file called foo.txt that is 500 bytes in size. If we were to create a new file that is 10 bytes in size and save it as foo.txt, overwriting the file, Windows would still show the file as taking up 500 bytes of space, while in Linux the file would only show as taking up 10 bytes of space.
In the second scenario, where we use the dd utility to create both the old and new foo.txt as described above, the behavior is the same. Windows will still show the file as taking up 500 bytes of space, while in Linux the file will only show as taking up 10 bytes of space.
This difference in behavior can be seen in the following example. On Windows we can run the following code to create a 500 byte foo.txt file:
echo "hello world!" > foo.txt
and then run the following code to overwrite the file with a 10 byte file:
echo "!" > foo.txt
In Windows, the file size will still be 500 bytes, while in Linux the file size will be 10 bytes.
In summary, Windows does not automatically free up the space no longer used when a file is overwritten, while Linux does. When overwriting a file on Windows, the space no longer used will remain in the file system unless the file is deleted and the OS runs its garbage collection to reclaim the space. On Linux, the space no longer used is reclaimed automatically when the file is overwritten.
If you have any further questions regarding how to overwrite a file size of 500 bytes, please feel free to reach out and I will be happy to help.
In both scenarios you described, the operating system (OS) will generally free the space that is no longer being used when you overwrite a file. This is because when you overwrite a file, you are essentially writing new data to the same location on the storage device where the old file was stored. The OS will recognize that the old file is no longer needed and will free the space that it was occupying.
There are some nuances to how this process works, depending on the specific OS and file system being used. In some cases, the OS may not immediately free the space that is no longer being used when a file is overwritten. Instead, it may mark the space as being available for reuse, and the space will be freed when the OS determines that it is needed for a new file. This process is known as “deallocation” or “trimming,” and it can help improve the performance of the storage device by minimizing the number of write operations that are required.
In general, however, you can expect that the OS will ensure that the space no longer being used is freed when you overwrite a file, regardless of the program or utility you use to perform the overwrite.
It seems that your question is based on the assumption that unused space is always available for other use and that overwriting a file with a new file of the same name means that the new file is written to the same location. However, these assumptions are incorrect. In addition, referring to “space no longer used” and “freed space” means the same thing because space is either allocated to a file or not.
As explained by davidgo, drives allocate space in blocks or sectors, so I will use the term “blocks” for simplicity. Even small files that are less than a block in size are allocated the entire block. This means that small files have unused space in the block that cannot be used for anything else. When a large file that uses multiple blocks is replaced by a smaller file that uses fewer blocks, there are whole blocks that are no longer needed.
When a file is overwritten with a new file, the new file is not actually written to the same location. Instead, it is saved in a new location, using as many blocks as it needs. The reference in the filesystem’s file table to the old file’s blocks is then modified, and those blocks become unassigned to any file and available for reuse. The content of the old file remains in the blocks, but it is ignored until the space is needed, which is why deleted files can be recovered.
Regarding dd, it can be used in various ways, but if we focus on simply writing a new file with the same name, the process would be the same as described above.