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When working with Btrfs filesystems, quota management is handled through qgroups (quota groups). Unlike traditional filesystems where quotas are set per user/group, Btrfs implements quotas at the subvolume level using this hierarchical grouping system.

Here are the essential commands you'll need:

# Enable quotas (required before first use)
sudo btrfs quota enable /path

# Set a quota limit (e.g., 21GB for subvolume)
sudo btrfs qgroup limit 21G /path/to/subvolume

# Show current usage
sudo btrfs qgroup show -re /path

While btrfs qgroup show displays usage statistics, it doesn't explicitly show the configured limit. To view both usage and limits:

# The most comprehensive view:
sudo btrfs qgroup show -reF /path

# Sample output format:
# Qgroupid    Referenced    Max referenced    Exclusive    Max exclusive
# --------    ----------    --------------    ---------    -------------
# 0/5         15.00GiB      21.00GiB          14.50GiB     20.00GiB

For scripting purposes, you might prefer these approaches:

# JSON output for parsing (Btrfs v5.6+)
sudo btrfs qgroup show -pcre --raw /path | jq

# Filter specific qgroup info
sudo btrfs qgroup show -re /path | awk '/0\/[0-9]+/ {print "Limit:", $4}'

Remember these technical details:

• Quotas must be enabled for the filesystem before use
• All values are shown in binary units (1GiB = 1024^3 bytes)
• The Max referenced column shows your configured limit
• Quotas apply to the entire subvolume, including snapshots

Here's how I typically manage quotas in production:

# 1. Enable quotas if not already active
if ! btrfs quota show /mnt/data &> /dev/null; then
    sudo btrfs quota enable /mnt/data
fi

# 2. Set a 50GB limit for a project subvolume
sudo btrfs qgroup limit 50G /mnt/data/projects/webapp

# 3. Verify the configuration
sudo btrfs qgroup show -reF /mnt/data/projects/webapp | head -n 3

# 4. Set up monitoring (cron job example)
*/30 * * * * /usr/bin/btrfs qgroup show -re /mnt/data | mail -s "Btrfs Quota Report" admin@example.com

When working with Btrfs filesystems, quota management is handled through qgroups (quota groups). While setting quotas is straightforward, retrieving the configured limits requires specific commands that aren't always well-documented.

To view both usage and limits for a subvolume, use:

btrfs qgroup show -re /path/to/subvolume

The -r flag shows recursive information, while -e displays only the exclusive amounts (space used solely by this subvolume).

For a comprehensive view of all qgroups and their limits:

btrfs qgroup show -pcre --raw /mount/point

This command breaks down the output with:

• -p: Path information
• -c: Shows child-parent relationships
• -r: Recursive display
• -e: Exclusive usage
• --raw: Displays numbers in bytes

Let's examine a real-world scenario:

# Set quota limit (21GB)
sudo btrfs qgroup limit 21G /mnt/data/subvol1

# Check current status
sudo btrfs qgroup show -re /mnt/data/subvol1

Sample output would display:

qgroupid         rfer         excl     max_rfer     max_excl
--------         ----         ----     --------     --------
0/256      2147483648            0   22548578304           0

The key columns to note:

• max_rfer: Referenced limit (total including shared blocks)
• max_excl: Exclusive limit (only unique blocks)
• Values are in bytes by default (use --human-readable for friendly units)

For script processing, use JSON format:

btrfs qgroup show -re --raw /path | jq .

This requires jq installed and provides machine-readable output for automation.

For regular monitoring, consider setting up a cron job:

*/30 * * * * root btrfs qgroup show -re /mnt/data > /var/log/btrfs_quota.log

Combine with tools like inotifywait for real-time notifications when approaching limits.