When setting up FreeNAS (now TrueNAS Core), one of the most persistent recommendations is the "1GB RAM per 1TB storage" rule. But the storage community remains divided on whether this applies to:

• Raw physical disk capacity
• Usable storage after redundancy (e.g., ZFS RAIDZ configurations)

The TrueNAS documentation states:

"For general storage use, we recommend 1GB of RAM per 1TB of stored data"

Key observations from the source code (FreeNAS 11.3+):

# ZFS ARC memory allocation heuristic
def calculate_arc_size(total_ram, storage_capacity):
    # Default to 50% of RAM or 1GB per TB, whichever is smaller
    arc_max = min(total_ram * 0.5, storage_capacity * 1.07)
    return arc_max

Case 1: Physical Disk Basis

4x 4TB disks in RAIDZ1 (12TB usable):

• Physical: 16TB → 16GB RAM recommended
• Usable: 12TB → 12GB RAM could suffice

Case 2: Compression Impact

# zfs get compression tank
NAME  PROPERTY     VALUE     SOURCE
tank  compression  lz4       local

With LZ4 compression achieving 1.5:1 ratio, effective storage becomes 18TB from 12TB physical. RAM requirement becomes ambiguous.

For enterprise deployments with heavy simultaneous access:

# Recommended vm.zone_reclaim_mode for high-performance systems
sysctl vm.zone_reclaim_mode=1

Home/NAS use cases can often get by with 0.5GB per TB if:

• Single-stream access patterns
• No deduplication
• Limited snapshot retention

Critical parameters in /etc/sysctl.conf:

# ZFS memory management
vfs.zfs.arc_max=8589934592  # 8GB limit example
vfs.zfs.arc_min=4294967296  # 4GB floor
vfs.zfs.prefetch_disable=0

Use this arcstat command to monitor real usage:

arcstat -f "time,hits,miss,read,hit%,mru,mfu,size,c" 1

Healthy systems show:

• Hit rate > 90%
• MRU/MFU ratio between 0.8-1.2

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When configuring FreeNAS (now TrueNAS Core), administrators often encounter conflicting recommendations about memory requirements. The official documentation states:

# FreeNAS/TrueNAS memory baseline
minimum_ram = 8GB  # Absolute minimum for basic functionality
recommended_ram = 1GB * (physical_disk_capacity_TB / redundancy_factor)

The correct interpretation is that FreeNAS requires 1GB of RAM per 1TB of physical disk capacity, not usable storage. This distinction becomes critical in these scenarios:

• RAID-Z2 configurations (losing 2 disks to parity)
• Mirrored vdevs (50% storage efficiency)
• Deduplication setups (requires additional overhead)

Consider these real-world configurations:

# Scenario 1: 4x4TB drives in RAID-Z1 (12TB usable)
physical_capacity = 16TB
required_ram = 16GB  # Not 12GB

# Scenario 2: 6x8TB drives in RAID-Z2 (32TB usable) 
physical_capacity = 48TB
required_ram = 48GB  # Not 32GB

The 1GB/TB rule primarily supports ZFS's Adaptive Replacement Cache (ARC). The ARC uses RAM to:

• Cache frequently accessed data
• Store filesystem metadata
• Maintain deduplication tables (if enabled)

For optimal performance in write-heavy environments, consider this adjustment:

# Advanced tuning for write-intensive workloads
sysctl -w vfs.zfs.arc_min=8589934592  # 8GB minimum ARC
sysctl -w vfs.zfs.arc_max=34359738368 # 32GB maximum ARC

These use cases demand additional RAM beyond the 1GB/TB guideline:

• Running jails/plugins: Add 2-4GB per jail
• iSCSI targets: +25% RAM for protocol overhead
• Deduplication: 5GB RAM per TB of deduplicated data

Check actual ARC usage with:

# View ZFS memory statistics
arcstat.py  # Shows ARC size, hit ratio, and pressure
zpool iostat -v  # Detailed pool performance metrics

For long-term monitoring, create a cron job:

#!/bin/bash
# Log ARC stats hourly
echo "$(date) - ARC Stats" >> /var/log/arc_stats.log
arc_summary.py >> /var/log/arc_stats.log