Using lvreduce to PROPERLY shrink Logical Volume [Cheat Sheet]

Introduction to lvreduce command

lvreduce command reduces the size of the logical volume. You have to be careful when reducing the logical volume because data in the reduced part will be lost.

 

Are you new to LVM and still learning how it works?

We have written detailed articles covering different areas of managing logical volumes, which you can follow using the given links:

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Manage Logical Volume in Linux - One STOP Solution
Understand LVM Architecture
Create LVM during installation RHEL/CentOS 7/8
How to use LVM Snapshot for Backup and Restore
Create Mirrored Logical Volume
Create Striped Logical Volume

 

Pre-requisites and Important Points Before You Start

The data is stored on disk in chunks which are distributed across your entire allotted disk so shrinking a storage may lead to data loss or partition corruption so it is very important to backup your data.

Shrinking a non-root i.e. data logical volume will not require reboot but it must be un-mounted first. So if any application or process is using this path then shrinking is not possible. You can use lsof | grep <partition> to check the list of processes using the <partition>

Shrinking a root logical volume (mounted on /) is not possible without reboot as it cannot be unmouned. So this must be performed in rescue mode and will have some extra steps. I have already covered detailed steps to extend or shrink root logical volume with examples so I will not repeat the same here.

I am using ext4 partition for all my examples, some of the commands may vary if you have a different file system such as XFS in which case resize2fs has to be replaced with xfs_growfs command. You can check xfs_growfs man page for more information.

 

Different examples to use lvreduce command

The syntax for lvreduce command is as follows:

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# lvreduce option LV_path 

 

1. Reduce  the size of the logical volume in units of megabytes

The -L or --size option of lvreduce command reduces the logical volume in units of megabytes.

With the - sign, the size will be subtracted from the actual size of the logical volume.

The following command reduces the size of the logical volume by 40 MiB.

# lvreduce -L -40M /dev/vol_grp/lvol1

OR

# lvreduce -L -40M /dev/vol_grp/lvol1

Sample Output:

lvreduce command to reduce the size of logical volume

Without the - sign, the given size will be the new size of the logical volume. The size should be smaller than the existing size.

# lvreduce -L 40M /dev/vol_grp/lvol2

OR

# lvreduce -L 40M /dev/vol_grp/lvol2

Sample Output:

lvreduce command to set the reduce size of logical volume

 

2. Reduce the size of the logical volume by logical extents

You can reduce the logical volume size in units of logical extents by using -l or --extents option. With the - sign, the value is subtracted from the actual size and without the - sign, the value will be the new reduced size of the logical volume.

The following example reduces the logical volume lvol3 in the volume group vol_grp by 10 extents.

# lvreduce -l -10 /dev/vol_grp/lvol3

OR

# lvreduce -l -10 /dev/vol_grp/lvol3

Sample Output:

lvreduce command to reduce the size of logical volume by logical extents

 

3. Resize filesystem together with the logical volume

If the logical volume contains a file system, you must ensure that the file system is not using the space in the logical volume that is being reduced. In that case, you need to use -r or --resizefs option to prevent data loss.

# lvreduce -r -L 40M /dev/vol_grp/lvol0

OR

# lvreduce --resizefs -L 40M /dev/vol_grp/lvol0

Sample Output:

lvreduce command to reduce logical volume size with filesystem

NOTE:
When you use this option, the lvreduce tries to shrink the filesystem before reducing the logical volume. If shrinking the filesystem fails, the lvreduce command will fail and not attempt to reduce the logical volume.

 

How to properly shrink non-root logical volume [Step-by-Step]

Reducing the size of the logical volume can lead to data loss on a filesystem located on that volume. You must first decrease the size of the filesystem to avoid possible data loss.

If you follow the steps below to reduce the logical volume then you don't have to worry about any data loss. But it is still strictly recommended to backup your filesystem before performing any such operation.

 

Step-1: Unmount the filesystem

If the logical volume you are reducing is mounted to a filesystem other than the root, you have to unmount the mount point with the umount command.

root@ubuntu-PC:~# df -h /test
Filesystem                 Size  Used Avail Use% Mounted on
/dev/mapper/vol_grp-lvol3   74M   60K   68M   1% /test
root@ubuntu-PC:~# 
root@ubuntu-PC:~# umount /test

 

Step-2: Check the filesystem for any errors

It is important to check the filesystem for errors before reducing its size by using e2fsck command. The -f option is used to forcefully check the file system, even if the filesystem is clean.

root@ubuntu-PC:~# e2fsck -f /dev/mapper/vol_grp-lvol3
e2fsck 1.45.5 (07-Jan-2020)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/mapper/vol_grp-lvol3: 11/20480 files (9.1% non-contiguous), 1683/20480 blocks

 

Step-3: Reduce the filesystem

The next step is to reduce the filesystem by using the resize2fs command.

The following command reduces the filesystem /test to 40M.

root@ubuntu-PC:~# resize2fs /dev/mapper/vol_grp-lvol3 40M
resize2fs 1.45.5 (07-Jan-2020)
Resizing the filesystem on /dev/mapper/vol_grp-lvol3 to 10240 (4k) blocks.
The filesystem on /dev/mapper/vol_grp-lvol3 is now 10240 (4k) blocks long.

 

Step-4: Reduce the logical volume

Now, we will reduce the logical volume to 40M using the lvreduce command.

root@ubuntu-PC:~# lvreduce -L 40M /dev/vol_grp/lvol3
  WARNING: Reducing active logical volume to 40.00 MiB.
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce vol_grp/lvol3? [y/n]: y
  Size of logical volume vol_grp/lvol3 changed from 80.00 MiB (20 extents) to 40.00 MiB (10 extents).
  Logical volume vol_grp/lvol3 successfully resized.

 

Step-5: Mount the filesystem

After reducing the size of the logical volume, you can mount the filesystem back using the mount command.

root@ubuntu-PC:~# mount /dev/vol_grp/lvol3 /test
root@ubuntu-PC:~# 
root@ubuntu-PC:~# df -h /test
Filesystem                 Size  Used Avail Use% Mounted on
/dev/mapper/vol_grp-lvol3   34M   60K   31M   1% /test

 

Conclusion

We hope this article helps you to understand how to reduce the logical volume size using the lvreduce command. This article also covers the steps by steps instructions to properly reduce the logical volume without any data loss.

If you have any question or feedback, please let us know in the comment section below.

 

What’s Next

lvremove command examples in Linux [Cheat Sheet]
10+ lvchange command examples in Linux [Cheat Sheet]

 

Further Reading

man page for lvreduce command

 

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