Snapshots

A snapshot is an instant point-in-time copy of a logical volume. Creating one takes milliseconds, regardless of how large the volume is.

How snapshots work

Snapshots use a technique called copy-on-write (CoW). When you create a snapshot:

  1. LVM creates a new, small LV linked to the original
  2. Nothing is copied yet — the snapshot initially contains zero data
  3. As long as the original data doesn’t change, the snapshot just reads from the original
  4. The moment a block on the original is about to be modified, LVM first copies the original version of that block into the snapshot, then lets the write proceed

This means:

  • Creating a snapshot is near-instant, no matter the source size
  • The snapshot stays small if the source data doesn’t change much
  • You always have the original state preserved
T=0: Create snapshot
─────────────────────────────────────────────
  lv_home (origin)    │  snap_home (snapshot)
  ┌───────────────┐   │  ┌─────────────────┐
  │ A  B  C  D  E │   │  │  [empty]        │
  └───────────────┘   │  │  reads: origin  │
                      │  └─────────────────┘

T=1: Block B is modified on origin
─────────────────────────────────────────────
  lv_home (origin)    │  snap_home (snapshot)
  ┌───────────────┐   │  ┌─────────────────┐
  │ A  B' C  D  E │   │  │  [old B saved]  │
  └───────────────┘   │  │  reads: A from origin
  (B is now B')       │  │          B from snapshot
                      │  │          C,D,E from origin
                      │  └─────────────────┘

The snapshot presents the original view of the data (with old B), even though the origin has moved on.

Creating a snapshot 

lvcreate -L 20G -s -n snap_home /dev/vg_tank/lv_home
Flag Meaning
-L 20G Reserve 20 GB for changed blocks
-s This is a snapshot
-n snap_home Name of the snapshot LV
/dev/vg_tank/lv_home The source (origin) LV

The snapshot size (20G here) doesn’t need to match the source. It only needs to hold the changes that will accumulate before you delete the snapshot. For a brief backup snapshot, 10–20% of the source is usually enough. If the snapshot fills up, it becomes invalid.

Verify:

lvs

  LV        VG      Attr       LSize  Origin   Snap%  ...
  lv_home   vg_tank owi-aos--- 200.00g                ...
  snap_home vg_tank swi-a-s--- 20.00g lv_home  0.00   ...

Snap% shows how full the snapshot’s reserved space is. Watch this — if it hits 100%, the snapshot is lost.

Using a snapshot for backup

Mount the snapshot (read-only is safest):

mkdir /mnt/snap
mount -o ro /dev/vg_tank/snap_home /mnt/snap

Now back it up:

tar -czf /backup/home_backup.tar.gz -C /mnt/snap .
rsync -a /mnt/snap/ /backup/home/

The origin (/home) is still live and in use the entire time. Your backup is from a frozen point in time.

When done:

umount /mnt/snap
lvremove /dev/vg_tank/snap_home

Restoring from a snapshot

If something went wrong and you need to roll back to the snapshot state:

# Unmount the origin
umount /home

# Merge the snapshot back into the origin
lvconvert --merge /dev/vg_tank/snap_home

LVM will merge on the next activation (usually on reboot if the LV was mounted). After the merge, the origin is back to the snapshot’s point in time, and the snapshot LV disappears.

If the LV is not mounted, the merge happens immediately. If it was mounted, LVM marks it for merge on next mount.

Thin provisioning (advanced)

Regular snapshots allocate their space from the parent VG. Thin provisioning is a more powerful model:

  • Create a thin pool inside the VG
  • Create thin LVs inside the pool
  • LVs only use actual disk space when data is written
  • Snapshots of thin LVs are extremely efficient
# Create a 500 GB thin pool
lvcreate -L 500G --thinpool tp_pool vg_tank

# Create thin LVs (these can sum to more than 500 GB — overprovisioning)
lvcreate -V 200G --thin -n lv_thin1 vg_tank/tp_pool
lvcreate -V 200G --thin -n lv_thin2 vg_tank/tp_pool

# Snapshot a thin LV (instant, very cheap)
lvcreate -s -n snap_thin1 vg_tank/lv_thin1

Thin provisioning allows you to create more virtual space than physical space (overprovisioning). This is useful but risky — if real usage exceeds the pool size, the pool fills up and all thin LVs in it go offline. Monitor thin pool usage carefully with lvs.

Snapshot monitoring 

# Watch snapshot fill percentage
watch -n 5 "lvs | grep snap"

# Get detailed snapshot info
lvdisplay /dev/vg_tank/snap_home

Set up automatic snapshot extension (LVM 2.02.99+):

# In /etc/lvm/lvm.conf, set:
snapshot_autoextend_threshold = 70   # extend when 70% full
snapshot_autoextend_percent = 20     # extend by 20%

Summary

Operation Command
Create snapshot (20G reserve) lvcreate -L 20G -s -n snap_name /dev/vg/lv_name
Mount snapshot read-only mount -o ro /dev/vg/snap_name /mnt/snap
Check snapshot usage lvs (watch Snap% column)
Delete snapshot lvremove /dev/vg/snap_name
Restore (merge back) lvconvert --merge /dev/vg/snap_name