When working with Btrfs snapshots, traditional approaches like mounting and running diff -r can be inefficient for several reasons:

• Metadata overhead from mounting multiple snapshots
• I/O bottlenecks when scanning entire directory structures
• Lack of filesystem-aware comparison logic

Btrfs actually provides built-in mechanisms for efficient snapshot comparison:

# Compare two snapshots by subvolume ID
btrfs subvolume find-new /path/to/snapshot1 12345 | grep -v "transid" > changes.txt
btrfs subvolume find-new /path/to/snapshot2 12345 | grep -v "transid" >> changes.txt
sort changes.txt | uniq -u

For binary-efficient comparison, use the Btrfs send/receive pipeline:

# Generate binary diff between snapshots
btrfs send -p /snapshots/old /snapshots/new | btrfs receive --dump

This outputs a detailed change list including:

• File additions/deletions
• Inode modifications
• Extended attribute changes
• Subvolume operations

Testing on a 50GB subvolume with 200,000 files:

Combine with other Btrfs tools for targeted comparisons:

# Find files modified in last snapshot only
btrfs subvolume find-new /current-root $(btrfs subvolume list / | grep previous | awk '{print $2}')

• For regular monitoring, consider using inotifywait with Btrfs events
• Combine with btrfs filesystem df to understand storage impact
• Use --chunk-root option for large filesystem comparisons

BTRFS (B-tree File System) provides built-in snapshot capabilities, but comparing snapshots efficiently isn't always straightforward. While you could theoretically mount both snapshots and run traditional diff tools, this approach has significant performance drawbacks:

• High I/O overhead from reading entire files
• Memory pressure when comparing large datasets
• Slow performance for deep directory structures

BTRFS actually includes specialized tools for snapshot comparison that operate at the filesystem level:

# Compare two snapshots by subvolume ID
btrfs subvolume find-new /path/to/parent-snapshot 12345 | grep -v "transid" > changes.txt

# Alternative method using send/receive dry-run
btrfs send -p /snapshots/snap1 /snapshots/snap2 --no-data | btrfs receive --dump

The send/receive method is particularly efficient because:

• It operates at the BTRFS metadata level
• Doesn't require reading file contents unless necessary
• Can be piped directly to analysis tools

Here's a bash script that compares snapshots and outputs changed files:

#!/bin/bash

SNAP1=$1
SNAP2=$2
OUTPUT_FILE=$3

btrfs subvolume find-new "$SNAP1" $(btrfs subvolume show "$SNAP2" | \
    grep "Received UUID" | awk '{print $3}') | \
    grep -v "transid" | \
    awk '{print $4}' | \
    sort | uniq > "$OUTPUT_FILE"

echo "Comparison complete. Changed files listed in $OUTPUT_FILE"

For more complex comparisons, consider these approaches:

# Compare checksums of files between snapshots
btrfs filesystem du -s --checksum /snapshots/*

# Use btrfs inspect-internal commands for low-level analysis
btrfs inspect-internal tree-stats /path/to/snapshot

While BTRFS-native methods are generally fastest, sometimes external tools like rsync or rdiff might be appropriate for:

• Cross-filesystem comparisons
• Cases where you need traditional diff output
• Comparing snapshots on different machines