How to Migrate /var/log to a Separate LVM Logical Volume on Linux

Managing log files is critical for server health. If the /var/log directory remains on the root partition, unchecked log growth can quickly fill up your main filesystem and bring the server to a halt.

This guide provides a step-by-step walkthrough on how to move /var/log to a dedicated, separate LVM (Logical Volume Management) Logical Volume using a new disk (in this example, /dev/xvdb).

Prerequisites: You must have a new, unpartitioned disk (/dev/xvdb in this guide) attached to your Linux server, and you must have sudo (root) privileges.

The Environment (Reference)

For this guide, we assume the following initial state:

Step 1: Initialize LVM Structure on the New Disk

We will convert the new disk (/dev/xvdb) into an LVM structure.

1.1 Create the Physical Volume (PV)

This initializes the raw disk as an LVM Physical Volume.

sudo pvcreate /dev/xvdb

1.2 Create the Volume Group (VG)

Create a Volume Group, which acts as a container for your LVs. We’ll name it vg_log.

sudo vgcreate vg_log /dev/xvdb

1.3 Create the Logical Volume (LV)

Create the Logical Volume for the logs, named lv_varlog, consuming 100% of the space in the new Volume Group.

# Create a 100% free Logical Volume named 'lv_varlog' in 'vg_log'
sudo lvcreate -l 100%FREE -n lv_varlog vg_log

The new device is now ready at /dev/vg_log/lv_varlog.

Step 2: Format and Prepare the Logical Volume

The LV needs a filesystem before it can store data.

2.1 Format the Logical Volume

Format the new LV with the ext4 filesystem.

sudo mkfs.ext4 /dev/vg_log/lv_varlog

2.2 Verify the LVM Creation

Use lsblk to confirm the LVM layer is visible.

lsblk

You should see the logical volume linked to $\mathbf{/dev/xvdb}$ (e.g., vg_log-lv_varlog).

Step 3: Migrate Existing Log Data

This is the most critical stage, where you temporarily copy the data.

3.1 Copy the Existing Logs

Use rsync to copy all contents from the current /var/log to the new mount point, ensuring all permissions and file attributes are preserved.

• The -a flag preserves permissions, ownership, and timestamps.
• The -x flag ensures rsync does not cross filesystem boundaries (e.g., into /var/log/journal).

sudo mkdir -p /var/log_old
sudo rsync -ax /var/log/ /var/log_old

3.2 Switch the Mount Points

1. mount it permanently

   sudo mount /dev/vg_varlog/lv_varlog /var/log

2. Copy you existing /var/log_old/ data into new logical volume lv_varlog in /var/log/

   sudo rsync -ax /var/log_old/ /var/log

Step 4: Make the Mount Permanent with /etc/fstab

To ensure the new LVM volume is mounted on every boot, you must add an entry to /etc/fstab.

4.1 Edit the fstab file

sudo nano /etc/fstab

4.2 Add the LV Entry

Add the following line to the end of the file. Using the LVM device path is reliable for Logical Volumes.

/dev/vg_log/lv_varlog /var/log ext4 defaults 0 2

4.3 Verify the fstab Entry

Test the new entry without rebooting:

sudo mount -a
df -h /var/log

The output should confirm the volume is mounted:

Filesystem                      Size Used Avail Use% Mounted on
/dev/mapper/vg_log-lv_varlog  9.8G 660M  8.6G  7% /var/log

Step 5: Final Verification and Cleanup

The migration is complete! Now you just need a final check and cleanup.

5.1 Final System Check

Confirm the mount point is active and logs are being written: Check real-time log activity

lsblk

sudo tail -f /var/log/syslog

5.2 Clean Up (Reclaim Space)

Once you are satisfied that the system is running correctly and logs are being written to the new volume, you can safely remove the old backup directory (/var/log_old) to free up space on your root partition.

# *** ONLY run this command when you are 100% sure the new setup is working ***
sudo rm -rf /var/log_old

Verify the Mount Point and Size

The most important step is confirming the logical volume is correctly attached to the directory.

You have successfully migrated your /var/log directory to a dedicated LVM Logical Volume, protecting your root partition from log file overflow!