lvreduce

The lvreduce command in Linux is used to reduce the size of a logical volume (LV) in an LVM (Logical Volume Manager) setup. This is a potentially risky operation as reducing the size of a logical volume can result in data loss if not done correctly. Therefore, it's important to ensure the filesystem is shrunk before reducing the logical volume's size.

Syntax

lvreduce [options] <logical-volume>

Where:

• <logical-volume> is the path to the logical volume you want to resize (e.g., /dev/vgname/lvname).

Common Options

1. -L or --size <size>: Specifies the new size of the logical volume. This is used when reducing the size.

   • Example: Resize the LV to 20GB:

     sudo lvreduce -L 20G /dev/vgname/lvname

2. -l or --extents <extents>: Specifies the size of the logical volume in terms of extents. An extent is a unit of space in the volume group.

   • Example: Resize the LV to use only 50% of the available extents:

     sudo lvreduce -l 50%FREE /dev/vgname/lvname

3. --resizefs: Automatically resizes the filesystem on the logical volume to match the new size of the logical volume.

   • Example: Resize the LV and filesystem together:

     sudo lvreduce -L 20G --resizefs /dev/vgname/lvname

4. --noresizefs: Prevents the automatic resizing of the filesystem. You would need to manually shrink the filesystem before running the lvreduce command.

   • Example:

     sudo lvreduce -L 20G --noresizefs /dev/vgname/lvname

5. -f or --force: Forces the reduction of the logical volume, even if there are potential issues (not recommended unless you're sure).

   • Example:

     sudo lvreduce -f -L 20G /dev/vgname/lvname

6. -d or --debug: Displays debugging information during the execution of the command.

   • Example:

     sudo lvreduce -d -L 20G /dev/vgname/lvname

How to Use lvreduce

Before you use lvreduce, ensure that:

1. Backup your data: Reducing the size of a logical volume, especially if you shrink it too much, can result in data loss. Always back up your data before performing such operations.

2. Shrink the filesystem: If you are reducing the size of a logical volume, it is important to shrink the filesystem first. Shrinking the filesystem ensures that no data will be lost during the reduction of the logical volume.

Steps to Reduce the Size of a Logical Volume

1. Shrink the Filesystem

Before you can safely reduce the size of the logical volume, you must shrink the filesystem. This step depends on the type of filesystem used.

• For ext4 filesystems, you can use resize2fs:

  sudo resize2fs /dev/vgname/lvname 20G

• For XFS filesystems, you cannot shrink them directly. If you are using XFS, you will need to back up the data, recreate the volume, and restore the data.

2. Reduce the Logical Volume Size

Once the filesystem is shrunk, you can safely reduce the logical volume size using lvreduce:

• To reduce the logical volume to 20GB:

  sudo lvreduce -L 20G /dev/vgname/lvname

• To reduce the logical volume by a certain number of extents (e.g., 50% of the free space):

  sudo lvreduce -l 50%FREE /dev/vgname/lvname

3. Resize the Filesystem

If you didn't use --resizefs during the lvreduce command, you'll need to resize the filesystem manually after reducing the LV. For example:

• For ext4 filesystems, use:

  sudo resize2fs /dev/vgname/lvname

4. Verify the Size

Once you’ve reduced the logical volume and filesystem, verify the changes:

sudo lvdisplay /dev/vgname/lvname

You can also check the filesystem size with:

df -h /mountpoint

Example of Reducing Logical Volume Size

1. Shrink the ext4 filesystem to 10GB:

   sudo resize2fs /dev/vgname/lvname 10G

2. Reduce the size of the logical volume to 10GB:

   sudo lvreduce -L 10G /dev/vgname/lvname

3. Resize the filesystem after reducing the logical volume size (optional if --resizefs was used):

   sudo resize2fs /dev/vgname/lvname

Important Notes:

• Data Backup: Always backup data before performing any operation that reduces the size of a logical volume.

• Shrinking the Filesystem: Always shrink the filesystem before shrinking the logical volume to avoid data loss.

• Filesystem Compatibility: Not all filesystems support shrinking. XFS, for instance, does not support shrinking directly, and you may need to recreate the volume.

• Use --resizefs Option: When reducing the LV size, it’s advisable to use the --resizefs option to resize the filesystem automatically, especially when increasing the LV size.

Conclusion

lvreduce is a powerful command for reducing the size of a logical volume in LVM. However, it requires careful handling, especially when shrinking volumes with existing data. Always make sure to shrink the filesystem first, and back up data before performing any size reductions.