How to use the command 'btrfs scrub' (with examples)

The btrfs scrub command is a tool used with the Btrfs (B-tree file system) to verify data integrity and detect potential errors. It’s particularly useful because Btrfs is a copy-on-write file system aimed at fault tolerance, repair, and easy administration. Regularly running a scrub can help ensure that all checksums of data and metadata are correct, which can preemptively catch and address bit rot or silent corruption.

Regular scrubs are recommended, typically once a month, to maintain system reliability. Below are use cases with examples to demonstrate the practical application of btrfs scrub.

Use case 1: Starting a scrub

Code:

sudo btrfs scrub start path/to/btrfs_mount

Motivation: Starting a scrub on a Btrfs file system is an essential maintenance task that scans the entire file system to verify the integrity of data and metadata. This process helps detect and potentially repair corrupted sectors, ensuring the file system remains healthy. Regular usage can prevent data loss by identifying issues before they become critical.

Explanation:

• sudo: Grants the necessary root permissions to run the scrub, as file system maintenance requires elevated privileges.
• btrfs: References the Btrfs file system command group.
• scrub: Invokes the scrub functionality.
• start: Initiates the scrubbing process.
• path/to/btrfs_mount: Specifies the directory path where the Btrfs volume is mounted.

Example Output:

Scrub started on /mnt/btrfs, fsid d5b21634-9671-47b5-8e9f-9c6388aa5b14 (pid=12345)

Use case 2: Showing the status of an ongoing or last completed scrub

Code:

sudo btrfs scrub status path/to/btrfs_mount

Motivation: Checking the status of a scrub operation allows users to monitor the progress or results of the ongoing process, ensuring that it is running smoothly and there are no immediate errors that need attention. It helps assess whether the last scrub detected any issues with the file system.

Explanation:

• sudo: Uses superuser permissions to access detailed system operation data.
• btrfs: Command group for Btrfs file system operations.
• scrub: Refers to the scrubbing operation.
• status: Requests information about the current or most recently completed scrub.
• path/to/btrfs_mount: Specifies the location of the Btrfs file system being monitored.

Example Output:

scrub status for d5b21634-9671-47b5-8e9f-9c6388aa5b14
	scrub started at Fri Oct 15 09:15:00 2023 and finished after 00:20:45
	total bytes scrubbed: 500.00GiB with 0 errors

Use case 3: Canceling an ongoing scrub

Code:

sudo btrfs scrub cancel path/to/btrfs_mount

Motivation: There might be situations where you need to cancel an ongoing scrub, such as when system resources are constrained, or the scrub is interfering with other critical operations. Canceling allows you to effectively manage system workload and prioritize important tasks.

Explanation:

• sudo: Provides administrative permissions to terminate file system operations.
• btrfs: The command group specific to Btrfs systems.
• scrub: Indicates a scrubbing operation.
• cancel: Stops the currently running scrub process.
• path/to/btrfs_mount: Designates the mounted Btrfs file system that you wish to halt scrubbing.

Example Output:

scrub canceled for /mnt/btrfs

Use case 4: Resuming a previously canceled scrub

Code:

sudo btrfs scrub resume path/to/btrfs_mount

Motivation: Resuming a scrub allows continuity where maintenance efforts can be picked up from where they were interrupted, making it efficient as you do not have to start the process from scratch. This is particularly beneficial to save time and computational resources in extensive file systems.

Explanation:

• sudo: Confers requisite superuser powers to modify file system status.
• btrfs: Commands associated with managing Btrfs file systems.
• scrub: Relates to a scrubbing operation.
• resume: Restarts a scrub operation which was previously halted.
• path/to/btrfs_mount: Identifies the Btrfs mount point to continue scrubbing.

Example Output:

scrub resumed for /mnt/btrfs (pid=12346)

Use case 5: Starting a scrub, but waiting for it to finish before exiting

Code:

sudo btrfs scrub start -B path/to/btrfs_mount

Motivation: Ensuring completion of a scrub before proceeding with subsequent operations can be crucial in environments where data consistency checks are essential before performing tasks like backup or other maintenance routines. This ensures that all operations happen in known-good states.

Explanation:

• sudo: Calls for administrative privileges.
• btrfs: Commands tailored for Btrfs file systems.
• scrub: Commands scrubbing functionality.
• start: Starts the scrub operation.
• -B: A flag to block the command prompt from exiting until the scrub process is complete.
• path/to/btrfs_mount: Specifies the path to the Btrfs file system.

Example Output:

Scrub started at Wed Oct 15 10:00:00 2023 and finished after 00:15:30
total bytes scrubbed: 300.00GiB with 0 errors

Use case 6: Starting a scrub in quiet mode

Code:

sudo btrfs scrub start -q path/to/btrfs_mount

Motivation: Using quiet mode can be beneficial in scripting or automated environments where you prefer minimal output to avoid clutter in the logs, only opting for critical information or errors to be displayed.

Explanation:

• sudo: Ensures authorized access for the operation.
• btrfs: Commands related to Btrfs operations.
• scrub: Points to the scrub functionality.
• start: Begins the scrubbing process.
• -q: Quiet mode option to suppress verbose output.
• path/to/btrfs_mount: Directs the operation to the mounted Btrfs volume.

Example Output:

<No output unless errors occur>

Conclusion:

The btrfs scrub command provides various options to manage and maintain the integrity of Btrfs file systems. Each use case is designed to offer flexibility and control over how scrubs are initiated, monitored, canceled, or resumed, with different modes to suit specific operational requirements. Regular use of these commands can ensure that the data remains robust against corruption and other integrity issues.