Master LVM Snapshots: Step‑by‑Step Guide to Create, Restore, and Merge

Preface

In the previous chapter we learned about LVM logical volume management and discovered that LVM can flexibly resize filesystems by adding or removing PEs. LVM also offers a crucial feature—snapshot technology—for backing up a filesystem.

Before diving into LVM, consider why a snapshot of any size completes in seconds.

LVM snapshots use copy‑on‑write (COW): at the instant a snapshot is created, the system records the current state of the logical volume, producing a blank snapshot that later stores only the blocks that change.

After creation, as the source LV changes, the altered data is copied to the snapshot. Restoring merges the snapshot with the original LV, returning the volume to its original state.

Note

Snapshot volume size must be large enough; it cannot exceed the free space of the volume group (VG).

The snapshot and the source LV must reside in the same VG, otherwise creation fails.

Creating LVM Snapshots

1. Experiment Preparation

First, create the logical volume:

1) Prepare four partitions

2) Create PV

3) Create volume group vg1

4) Create a 10G logical volume lv1

5) Format and mount to /data

6) Write test data

2. LVM Snapshot Experiment

Check current LV/VG usage:

lv1 is 10G in vg1, which has 40G total and 30G free, providing enough space for a snapshot.

1) Create snapshot

Command parameters:

-L: size of the snapshot volume
--snapshot: specify snapshot type
--name: snapshot name
/dev/vg1/lv1: target LV to snapshot

A snapshot is a special logical volume created with lvcreate. lvs – list logical volumes

2) Simulate data corruption

The original data "hello world" was maliciously changed to "hello worl". Using the LVM snapshot rollback we can recover the important data.

3) Mount snapshot

Encountered an error

Troubleshooting steps:

1) Run

dmesg | tail

2) The snapshot LV shares the same UUID as the original LV. XFS cannot mount two filesystems with identical UUIDs.

If the filesystem is XFS, mounting the snapshot will fail because the UUIDs clash. Resolve this by either changing the UUID or adding the nouuid option to ignore the duplicate.

3) Mount with nouuid option:

Problem solved, snapshot mounted at /data_bak

4) Restore data

When many small files have changed, copying from the snapshot becomes cumbersome. A simpler method is to merge the snapshot.

Restoring the Entire Logical Volume

1) Mount the snapshot

2) Export data from the snapshot to /opt

Export snapshot data to /opt (illustrated below)

3) Unmount, format, and remount the original LV

4) Restore the data to the original LV

Merge Snapshot (merge snapshot)

1) Simulate massive data loss that requires full LV restoration.

2) Unmount both the original LV and the snapshot LV

3) Merge the snapshot (ensure both volumes are unmounted)

Note: merging automatically deletes the snapshot volume.

4) Remount the original LV

The test file now contains "hello world", confirming that the entire volume has been restored to the state captured at snapshot creation.

Summary

Snapshots are a slightly advanced LVM topic, but the common use cases presented here are sufficient for most everyday scenarios and provide a solid foundation for further exploration.