How to Use the Command 'fallocate' (with Examples)

The fallocate command is a versatile utility found in Unix-based operating systems, which is designed to efficiently manage disk space allocation for files. Unlike some other utilities, fallocate reserves or deallocates space for files without zeroing, making it faster for particular operations. This command is particularly useful when you want to manage files for large data sets or various applications that require rapid space allocation or deallocation. More comprehensive information about the command can be found here .

Use case 1: Reserve a file taking up 700 MiB of disk space

Code:

fallocate --length 700M path/to/file

Motivation:

This use case is relevant when you anticipate needing a specific amount of disk space for future data but haven’t yet generated the data itself. By preallocating the disk space, you ensure that the space is available when needed, preventing disruptions due to running out of storage. This can be particularly useful in environments where storage capacity is frequently near its limit or when working with large multimedia files or databases that will expand.

Explanation:

• fallocate: This is the command being used to allocate disk space.
• --length 700M: This argument specifies the size of the space to be reserved. Here, 700M indicates that 700 MiB of disk space will be preallocated.
• path/to/file: This is the path to the target file. If the file does not exist, fallocate will create it and reserve the specified amount of space.

Example Output:

Executing this command would result in the creation of a file with 700 MiB of allocated disk space at the specified path. There won’t be any output displayed on successful execution, but checking the file size will confirm the reservation of space.

Use case 2: Shrink an already allocated file by 200 MiB

Code:

fallocate --collapse-range --length 200M path/to/file

Motivation:

Occasionally, files or databases may not require as much space as initially allocated. This situation might occur due to better-than-expected data compression or a reduction in data volume. In such cases, freeing up unused space is beneficial for optimizing storage use, especially in systems with limited disk space resources.

Explanation:

• fallocate: The command used to manage disk space for a file.
• --collapse-range: This option instructs fallocate to deallocate a specified range of bytes.
• --length 200M: Specifies the amount of space to deallocate. Here, it indicates that the file will be reduced by 200 MiB.
• path/to/file: Denotes the target file whose allocated space will be reduced.

Example Output:

Executing this command will decrease the file size by 200 MiB. There is typically no output displayed, but inspecting the file’s properties would show the newly reduced size.

Use case 3: Shrink 20 MB of space after 100 MiB in a file

Code:

fallocate --collapse-range --offset 100M --length 20M path/to/file

Motivation:

This particular use case is useful when you only need to free up a specific region of a file. For example, you may be removing obsolete content or unused data structures that were placed after an initial segment of data. By selectively deallocating space, you can effectively manage file sizes and remain compliant with storage constraints.

Explanation:

• fallocate: The command managing file space.
• --collapse-range: Indicates that a specific segment of allocated space within the file will be removed.
• --offset 100M: Specifies the starting point for space deallocation, indicating that 100 MiB into the file is where the operation will begin.
• --length 20M: Indicates the amount of space to deallocate beginning from the offset, in this case, 20 MiB.
• path/to/file: Represents the target file where the space collapse operation will be performed.

Example Output:

Upon running this command, 20 MiB of space will be removed starting 100 MiB into the file, effectively altering the file’s structure and size. As with previous cases, there is no standard output, but viewing the file’s properties will confirm the change.

Conclusion:

Employing fallocate offers a convenient and efficient means of managing disk space allocation for files, particularly on systems where disk space management is a critical concern. The examples discussed demonstrate how to preallocate necessary space and reclaim unused space, allowing for better resource management and potentially improved system performance.