When working with BTRFS subvolumes where compression is enabled (-o compressed), developers often need to distinguish between:

• Logical size (uncompressed data size)
• Physical size (actual disk space used)

For a quick overview of compressed storage efficiency:

sudo btrfs filesystem df /path/to/subvolume

Sample output:

Data, single: total=25.00GiB, used=16.23GiB
System, single: total=32.00MiB, used=16.00KiB
Metadata, single: total=2.00GiB, used=1.45GiB

For comprehensive subvolume statistics including compression ratios:

sudo btrfs subvolume show /path/to/subvolume

Key metrics to examine:

    Name:                   mysubvol
    UUID:                   xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
    Parent UUID:            -
    Received UUID:          -
    Creation time:          2023-01-01 12:00:00
    Subvolume ID:           256
    Generation:             12345
    Gen at creation:        6789
    Parent ID:              5
    Top level ID:           5
    Flags:                  -
    Snapshot(s):
    Disk total:             15.23GiB  <-- Physical size (compressed)
    Exclusive:              8.45GiB

Here's a Python script to extract both compressed and uncompressed sizes:

import os
import subprocess

def get_btrfs_sizes(path):
    # Get uncompressed size (logical)
    du = subprocess.run(['du', '-sb', path], capture_output=True, text=True)
    uncompressed = int(du.stdout.split()[0])
    
    # Get compressed size (physical)
    btrfs = subprocess.run(['btrfs', 'filesystem', 'usage', '-b', path], 
                         capture_output=True, text=True)
    compressed = int([line for line in btrfs.stdout.split('\n') 
                     if 'Device size' in line][0].split()[-1])
    
    return uncompressed, compressed

uncomp, comp = get_btrfs_sizes('/mnt/btrfs/subvol')
print(f"Compression ratio: {uncomp/comp:.2f}x")

For more detailed breakdown including compression effectiveness:

sudo btrfs filesystem usage /path/to/subvolume

This shows both allocated and used space with compression factored in.

Remember these benchmarks when choosing methods:

When working with Btrfs subvolumes using compression (mounted with -o compress), standard tools like du only show uncompressed sizes. For administrators needing accurate disk usage metrics, this presents a significant limitation.

The btrfs filesystem du command provides the most accurate view of compressed storage usage:

# Show compressed disk usage for a subvolume
sudo btrfs filesystem du -s /path/to/subvolume

# Detailed per-file breakdown
sudo btrfs filesystem du /path/to/subvolume

For systems without recent btrfs-progs, consider these methods:

Method 1: Using btrfs fi usage

sudo btrfs fi usage /mount/point -b

Method 2: Compressed size estimation

# Requires Linux 4.14+ and btrfs-progs v4.16+
sudo compsize -x /path/to/subvolume

The native btrfs commands are significantly faster than recursive du scans because they:

• Access filesystem metadata directly
• Don't traverse directory trees
• Leverage Btrfs's internal accounting

Here's a typical output comparison:

# Traditional du (uncompressed size)
$ du -sh /mnt/btrfs/subvol
15G    /mnt/btrfs/subvol

# Btrfs actual compressed size
$ sudo btrfs filesystem du -s /mnt/btrfs/subvol
     Total   Exclusive  Set shared  Filename
   6.12GiB     6.12GiB       0.00B  /mnt/btrfs/subvol

Remember that:

• Shared extents between snapshots aren't included in these calculations
• The numbers represent actual disk usage, not logical file sizes
• Compression ratios vary based on file types and compression algorithm