How to create an LVM snapshot

Creating an LVM snapshot gives an administrator a point-in-time view of a logical volume before a backup, risky change, or data check. The origin LV can keep running while the snapshot preserves older blocks, but the snapshot needs enough copy-on-write space for changes made after it is created.

The lvcreate –snapshot command creates a snapshot LV in the same volume group as the origin. For a standard thick LV, the snapshot size is the amount of VG free space reserved for changed blocks, not a full copy of the origin volume.

Snapshot space fills as the origin changes, and a full snapshot becomes invalid. Keep snapshots short-lived, monitor Data%, and use application-aware quiescing or backups for databases and other write-heavy services because an LVM snapshot is block-consistent rather than application-consistent.

1. List the origin logical volume.

   $ sudo lvs --options lv_name,vg_name,lv_size,lv_attr /dev/vgdata/projects
     LV       VG     LSize  Attr      
     projects vgdata 20.00g -wi-ao----

   Replace /dev/vgdata/projects with the VG and LV that should be captured. The o flag in Attr means the origin device is open, commonly because it is mounted or used by a process.

2. Check free space in the volume group.

   $ sudo vgs vgdata
     VG     #PV #LV #SN Attr   VSize    VFree 
     vgdata   2   1   0 wz--n- 200.00g 80.00g

   The snapshot size for a standard copy-on-write snapshot is allocated from VFree in the origin volume group.

3. Create the snapshot logical volume.

   $ sudo lvcreate --snapshot --size 5G --name projects_snap /dev/vgdata/projects
     Logical volume "projects_snap" created.

   Choose a size that can hold changed blocks for the snapshot's lifetime. If Data% reaches 100%, the snapshot becomes invalid.

4. List the snapshot and its origin.

   $ sudo lvs --options lv_name,origin,lv_size,data_percent,lv_attr /dev/vgdata/projects_snap
     LV            Origin   LSize Data%  Attr      
     projects_snap projects 5.00g 0.01   swi-a-s---

   The s target flag shows a classic snapshot. Origin should match the source LV, and Data% shows snapshot space already used.

5. Create a temporary mount point when the snapshot contains a filesystem.

   $ sudo mkdir -p /mnt/projects-snapshot

6. Mount an ext4 snapshot read-only for inspection.

   $ sudo mount -o ro,noload /dev/vgdata/projects_snap /mnt/projects-snapshot

   For XFS snapshots mounted beside the origin, use sudo mount -o ro,nouuid,norecovery /dev/vgdata/projects_snap /mnt/projects-snapshot. Skip mounting when the LV contains swap, a database device, a virtual machine disk, or another raw consumer.

7. Inspect a known file from the snapshot.

   $ sudo cat /mnt/projects-snapshot/app.conf
   release config

8. Unmount the snapshot after inspection.

   $ sudo umount /mnt/projects-snapshot

9. Monitor snapshot usage while it is kept.

   $ sudo lvs --options lv_name,origin,lv_size,data_percent,lv_attr /dev/vgdata/projects_snap
     LV            Origin   LSize Data%  Attr      
     projects_snap projects 5.00g 1.42   swi-a-s---

   Extend or remove the snapshot before Data% reaches 100%. A snapshot kept longer than needed consumes VG space and can slow write-heavy origins.
   Related: How to increase the size of an LVM logical volume
   Related: How to remove an LVM logical volume