How to Use the Command 'lvreduce' (with examples)

The lvreduce command is a powerful utility in the Linux operating system used to decrease the size of a logical volume within a volume group. Logical volumes are an essential part of Logical Volume Management (LVM), which allows for flexible disk storage management in Linux environments. Through the lvreduce command, system administrators can efficiently resize logical volumes to manage available disk space better, adapt to the needs of applications or users, and improve system performance without necessarily requiring a reboot.

Use case 1: Reducing a Volume’s Size to 120 GB

Code:

lvreduce --size 120G logical_volume

Motivation:

At times, a logical volume might have been allocated more space than necessary. This could occur due to initial over-estimation of needs or changes in storage requirements over time. Reducing a logical volume’s size to a specific capacity allows for the reclamation of unused space, which can then be reallocated for other purposes, such as creating new volumes or expanding existing ones that require additional capacity. By explicitly setting the desired size, system administrators gain precision in managing storage resources.

Explanation:

• lvreduce: This is the command used to initiate the reduction process of the logical volume size.
• --size 120G: This argument specifies that the logical volume should be reduced to exactly 120 gigabytes. The --size flag is used to define the target size for the logical volume after reduction.
• logical_volume: This placeholder represents the path or name of the logical volume to be reduced. The user needs to replace this with the actual name or path of the volume they intend to resize.

Example Output:

WARNING: Reducing active and open logical volume to 120.00 GiB.
THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce logical volume logical_volume? [y/n]: y
  Reducing logical volume logical_volume to 120.00 GiB
  Logical volume logical_volume successfully resized

In this example output, a warning is issued to notify the user about potential data loss or corruption if the file system is not resized adequately beforehand. The confirmation prompt is an additional measure to ensure that critical operations are intentional.

Use case 2: Reducing a Volume’s Size by 40 GB and Adjusting the Filesystem

Code:

lvreduce --size -40G -r logical_volume

Motivation:

Sometimes, it becomes necessary to reduce a logical volume’s size while simultaneously updating the underlying filesystem. Reducing the logical volume without prior filesystem adjustment may lead to data corruption or loss because the filesystem still believes it has a larger storage area than the physical space available. The -r option automatically resizes the filesystem to match the new logical volume size, ensuring data integrity and avoiding manual filesystem adjustments. This use case is convenient when quick changes are necessary, reducing both logical volume and filesystem size efficiently.

Explanation:

• lvreduce: Similar to the previous example, this command launches the logical volume reduction process.
• --size -40G: This argument indicates that the logical volume size should be reduced by 40 gigabytes. The negative sign - before 40G signifies a decrement by the specified value.
• -r: This flag is crucial as it tells the system to resize the filesystem in conjunction with the logical volume shrinkage. It ensures that the filesystem is appropriately reduced to align with the logical volume’s new size.
• logical_volume: The user must substitute this placeholder with the correct path or name of the logical volume needing adjustment.

Example Output:

fsck from util-linux 2.34
/dev/mapper/volume-group-logical_volume: clean, 1029414/7213119 files, 8471928/28892140 blocks
resize2fs 1.45.5 (07-Jan-2020)
Resizing the filesystem on /dev/mapper/volume-group-logical_volume to 6809160 (4k) blocks.
The filesystem on /dev/mapper/volume-group-logical_volume is now 6809160 (4k) blocks long.

  WARNING: Reducing active and open logical volume to 190.00 GiB.
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
  Do you really want to reduce logical volume logical_volume? [y/n]: y
  Reducing logical volume logical_volume to 190.00 GiB
  Logical volume logical_volume successfully resized

In the output above, some extra steps appear, such as running fsck for filesystem checking and resize2fs for resizing the filesystem directly. These extra steps automatically ensure data integrity, reducing potential risks considerably.

Conclusion:

Efficient storage management is critical in maintaining scalable and high-performing systems. The lvreduce command offers administrators a flexible tool for resizing logical volumes, whether targeting a specific size or decreasing incrementally while maintaining filesystem integrity. Understanding how to employ lvreduce with options like --size and -r can empower Linux administrators to optimize storage usage and mitigate risks associated with storage adjustments.