📸 LVM Snapshots

Creating a snapshot is instantaneous regardless of LV size:

# Create a snapshot of lv_home with 5GB CoW space
sudo lvcreate -s -L 5G -n lv_home_snap /dev/vg_data/lv_home

# Snapshot naming convention — append date for clarity
sudo lvcreate -s -L 5G \
    -n lv_home_snap_$(date +%Y%m%d) \
    /dev/vg_data/lv_home

# Verify snapshot created
sudo lvs
sudo lvdisplay /dev/vg_data/lv_home_snap

Mount a snapshot read-only to browse files at the point-in-time state, or read-write for testing changes without touching the original:

# Mount snapshot read-only (safest — preserves snapshot integrity)
sudo mkdir -p /mnt/snapshot
sudo mount -o ro /dev/vg_data/lv_home_snap /mnt/snapshot

# Browse files as they were at snapshot time
ls /mnt/snapshot
diff /mnt/snapshot/etc/passwd /etc/passwd

# Mount snapshot read-write (for testing)
sudo mount /dev/vg_data/lv_home_snap /mnt/snapshot

# XFS snapshots require nouuid option (duplicate UUID issue)
sudo mount -o ro,nouuid /dev/vg_data/lv_home_snap /mnt/snapshot

# Unmount when done
sudo umount /mnt/snapshot

If a snapshot's CoW store fills up it becomes invalid and is automatically deactivated. Monitor and extend before that happens:

# Check snapshot fill level
sudo lvs | grep snap
sudo lvdisplay /dev/vg_data/lv_home_snap | grep "Allocated"

# Watch snapshot usage live
sudo watch -n10 'sudo lvs | grep snap'

# Extend snapshot CoW space before it fills
sudo lvextend -L +2G /dev/vg_data/lv_home_snap

# Configure LVM to auto-extend snapshots (lvm.conf)
sudo vi /etc/lvm/lvm.conf
# Find and set:
# snapshot_autoextend_threshold = 70  (extend when 70% full)
# snapshot_autoextend_percent = 20    (extend by 20%)

Merging a snapshot reverts the original LV to the snapshot's state — useful for undoing a bad system update or failed migration:

# Unmount the origin LV first if possible
sudo umount /home

# Merge snapshot back into origin (schedules the merge)
sudo lvconvert --merge /dev/vg_data/lv_home_snap

# If the LV is in use (e.g., root filesystem), merge happens on next boot
# LVM will automatically complete the merge during next activation
sudo reboot

# After reboot — verify merge completed (snapshot LV will be gone)
sudo lvs
# lv_home_snap should no longer appear

When a snapshot is no longer needed, remove it to free the CoW space:

# Unmount first if mounted
sudo umount /mnt/snapshot

# Remove the snapshot LV
sudo lvremove /dev/vg_data/lv_home_snap

# Remove without interactive confirmation (for scripts)
sudo lvremove -f /dev/vg_data/lv_home_snap

# Verify it is gone and space is returned
sudo lvs
sudo vgs   # VFree should increase by the snapshot CoW size

The classic use case — take a snapshot, back it up, remove it. Gives you a consistent backup of a live filesystem:

#!/bin/bash
# lvm-snapshot-backup.sh
# Consistent online backup using LVM snapshot

LV_ORIGIN="/dev/vg_data/lv_home"
LV_SNAP="/dev/vg_data/lv_home_snap"
SNAP_SIZE="5G"
SNAP_MOUNT="/mnt/snap_backup"
BACKUP_DEST="/backup/home_$(date +%Y%m%d_%H%M%S).tar.gz"

echo "=== LVM Snapshot Backup ==="
echo "Origin:  $LV_ORIGIN"
echo "Dest:    $BACKUP_DEST"
echo ""

# Step 1: Create snapshot
echo "Creating snapshot..."
sudo lvcreate -s -L "$SNAP_SIZE" -n lv_home_snap "$LV_ORIGIN"

# Step 2: Mount snapshot read-only
sudo mkdir -p "$SNAP_MOUNT"
sudo mount -o ro "$LV_SNAP" "$SNAP_MOUNT"

# Step 3: Backup the snapshot (consistent point-in-time)
echo "Backing up..."
sudo tar -czf "$BACKUP_DEST" -C "$SNAP_MOUNT" .
BACKUP_EXIT=$?

# Step 4: Unmount and remove snapshot (always — even if backup failed)
sudo umount "$SNAP_MOUNT"
sudo lvremove -f "$LV_SNAP"

# Step 5: Report
if [ $BACKUP_EXIT -eq 0 ]; then
    echo "Backup complete: $BACKUP_DEST"
    ls -lh "$BACKUP_DEST"
else
    echo "ERROR: Backup failed (exit code $BACKUP_EXIT)" >&2
    exit 1
fi

For databases, flush and lock before snapshotting to guarantee a clean, consistent state:

#!/bin/bash
# Database-consistent LVM snapshot for MySQL/MariaDB

DB_USER="root"
DB_PASS="$MYSQL_ROOT_PASSWORD"
LV_ORIGIN="/dev/vg_data/lv_mysql"
SNAP_NAME="lv_mysql_snap_$(date +%Y%m%d)"

echo "Flushing and locking MySQL tables..."

# Step 1: Flush tables and acquire read lock
mysql -u"$DB_USER" -p"$DB_PASS" -e "FLUSH TABLES WITH READ LOCK;"

# Step 2: Create snapshot immediately (while lock is held)
sudo lvcreate -s -L 10G -n "$SNAP_NAME" "$LV_ORIGIN"

# Step 3: Release the lock — DB is fully operational again
mysql -u"$DB_USER" -p"$DB_PASS" -e "UNLOCK TABLES;"

echo "Lock released. Snapshot: $SNAP_NAME"
echo "DB fully operational — backup from snapshot at leisure."

# Step 4: Mount and backup the snapshot
sudo mkdir -p /mnt/dbsnap
sudo mount -o ro /dev/vg_data/"$SNAP_NAME" /mnt/dbsnap
sudo rsync -av /mnt/dbsnap/ /backup/mysql_$(date +%Y%m%d)/

# Step 5: Cleanup
sudo umount /mnt/dbsnap
sudo lvremove -f /dev/vg_data/"$SNAP_NAME"
echo "Done."

Thin-provisioned LVs and their snapshots are more space-efficient for environments with many snapshots of the same origin:

# Create a thin pool LV (the backing store)
sudo lvcreate -L 500G --thinpool tp_pool vg_data

# Create a thin LV from the pool (no fixed size commitment)
sudo lvcreate -V 200G --thin -n lv_thin_home vg_data/tp_pool

# Format and mount as normal
sudo mkfs.xfs /dev/vg_data/lv_thin_home
sudo mount /dev/vg_data/lv_thin_home /home

# Create a thin snapshot (much more efficient than classic snapshots)
sudo lvcreate -s -n lv_thin_home_snap vg_data/lv_thin_home

# Thin snapshots share unchanged blocks — no CoW size needed
sudo lvs
sudo lvs -o name,attr,size,pool_lv,origin,data_percent