How to manage block devices using 'blockdev' (with examples)

How to manage block devices using 'blockdev' (with examples)

The blockdev command is a powerful utility in Linux used for managing, querying, and manipulating block devices directly from the command line. Block devices are physical or virtual devices that provide buffered I/O access. These devices usually include hard drives, USB drives, and other storage media. With blockdev, system administrators can perform a variety of tasks, such as retrieving device information, setting device parameters, and optimizing device operations.

Use case 1: Print a report for all devices

Code:

sudo blockdev --report

Motivation:

Generating a comprehensive report for all block devices is essential for system administrators to get an overview of the connected storage devices. This report provides critical information such as device size, sector size, and read-ahead settings, which can be useful for monitoring, troubleshooting, and configuring devices.

Explanation:

  • sudo: This command requires elevated privileges to access device information, thus it is prefixed with sudo to run it as a superuser.
  • blockdev: The command used for block device management.
  • --report: This option instructs blockdev to generate a detailed report of all block devices attached to the system.

Example output:

RO    RA   SSZ   BSZ   StartSec            Size   Device
rw  128  512  512         0        2000398934016   /dev/sda
rw  128  512  4096         0         500107862016   /dev/sdb

Use case 2: Print a report for a specific device

Code:

sudo blockdev --report /dev/sdXY

Motivation:

When you need detailed information about a specific device, such as /dev/sda1, running a report for that particular device allows you to focus on its individual characteristics and configuration. This is especially useful when examining the properties of a newly added or problematic device.

Explanation:

  • sudo: Needed to execute blockdev with the necessary permissions.
  • blockdev: The command for performing operations on block devices.
  • --report: Generates a report for the specified device.
  • /dev/sdXY: The specific device for which the report is to be generated. Replace sdXY with the actual device identifier.

Example output:

RO    RA   SSZ   BSZ   StartSec            Size   Device
rw  128  512  512         0        500107862016   /dev/sdXY

Use case 3: Get the size of a device in 512-byte sectors

Code:

sudo blockdev --getsz /dev/sdXY

Motivation:

Fetching the size of a device in 512-byte sectors provides insights into its storage capacity, which can be crucial for disk management tasks, planning partitions, or verifying if a device meets certain storage requirements.

Explanation:

  • sudo: Ensures the command runs with the required permissions.
  • blockdev: The utility for managing block devices.
  • --getsz: Requests the size of the block device in units of 512-byte sectors.
  • /dev/sdXY: The targeted block device for which the size is being queried.

Example output:

976773168

Use case 4: Set read-only

Code:

sudo blockdev --setro /dev/sdXY

Motivation:

Setting a device to read-only mode is important in scenarios where data integrity needs to be protected from accidental modification or deletion. It is a preventive measure often used in maintenance and forensic investigations.

Explanation:

  • sudo: Provides the necessary permissions for executing the operation.
  • blockdev: The command operating on block devices.
  • --setro: Sets the device to read-only mode.
  • /dev/sdXY: The block device that is to be switched to read-only mode.

Example output:

No output is shown upon successful execution, indicating the operation has completed without errors.

Use case 5: Set read-write

Code:

sudo blockdev --setrw /dev/sdXY

Motivation:

Reverting a device back to read-write mode is essential when modifications need to be made to the device’s data. This is common after maintenance or when assurance of data integrity is no longer critical.

Explanation:

  • sudo: Executes the command with elevated privileges.
  • blockdev: Utility for block device operations.
  • --setrw: Changes device permissions to allow both reading and writing.
  • /dev/sdXY: The block device being adjusted to read-write mode.

Example output:

As with the read-only operation, no output is expected on successful completion.

Use case 6: Flush buffers

Code:

sudo blockdev --flushbufs /dev/sdXY

Motivation:

Flushing buffers forces all pending, cached data to be written to storage, ensuring the device’s data is fully synchronized with the filesystem. This is a critical operation when data integrity needs to be assured before device removal or system shutdown.

Explanation:

  • sudo: Necessary for granting proper execution access.
  • blockdev: The command in operation.
  • --flushbufs: Commits all buffered data to the physical device.
  • /dev/sdXY: The specific device whose buffers are being flushed.

Example output:

No direct output is provided, indicating the flush operation was successful.

Use case 7: Get the physical block size

Code:

sudo blockdev --getpbsz /dev/sdXY

Motivation:

Retrieving the physical block size of a device can help optimize how data is read and written, especially when aligning partitions for performance enhancements. Knowing the block size is also vital for efficient filesystem and RAID setups.

Explanation:

  • sudo: Allows command execution with sufficient privileges.
  • blockdev: Tool for block device interrogation and management.
  • --getpbsz: Fetches the physical block size of the device in bytes.
  • /dev/sdXY: Target block device for the query.

Example output:

4096

Use case 8: Set the read-ahead value to 128 sectors

Code:

sudo blockdev --setra 128 /dev/sdXY

Motivation:

Adjusting the read-ahead value can improve the performance of sequential read operations by prefetching additional blocks into memory. This is useful in workloads with predictable read patterns, enhancing efficiency and speed.

Explanation:

  • sudo: Required to carry out the command successfully due to permissions.
  • blockdev: The command interfacing with block devices.
  • --setra: Sets the number of sectors to read ahead.
  • 128: Specifies the intended read-ahead value in sectors.
  • /dev/sdXY: The block device on which the read-ahead is being modified.

Example output:

Adjusting read-ahead settings does not produce a visible output, signaling successful execution without errors.

Conclusion:

Understanding and managing block devices are vital for maintaining optimal system performance and data integrity. The blockdev command provides a broad array of functionalities to control, query, and manipulate block devices directly from your Linux terminal. Whether it’s setting device permissions, optimizing read/write operations, or querying device specifics, blockdev proves to be an essential utility for system administrators and users alike.

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