TrueNAS Calculator

Estimate Storage Performance, Capacity, and ZFS Overhead

TrueNAS Storage Calculator

Calculation Results

Formula Overview:
1. Total Raw Capacity: Sum of all individual disk sizes.
2. Usable Capacity: Total Raw Capacity minus space used for ZFS redundancy (parity or mirroring). For Stripes, it’s the raw capacity. For Mirrors, it’s roughly half the raw capacity. For RAIDZ, it’s Total Raw Capacity – (Number of vdevs * (Number of disks per vdev) * Size of one disk * Number of parity disks per vdev).
3. Effective Usable Capacity: Usable Capacity reduced by the specified ZFS Usable Space Overhead percentage.
4. Parity/Spare Space: The total capacity dedicated to redundancy.
5. Performance Factor: For RAIDZ, this is related to the number of disks per vdev minus parity disks. For Mirrors, it’s the number of disks per mirror. Stripes have the highest theoretical performance but no redundancy.

ZFS Configuration Table

Configuration	Disks per vdev	vdevs	Total Disks	Raw Capacity (TB)	Redundancy Type	Parity Disks	Usable Capacity (TB)	Effective Usable (w/ Overhead) (TB)	Performance Index (Approx.)

Table detailing the estimated storage capacity and performance based on your TrueNAS configuration.

Performance vs. Capacity Chart

Visual representation of how usable capacity and performance scale with different disk configurations.

What is a TrueNAS Calculator?

A TrueNAS calculator is an essential tool for anyone planning or managing a TrueNAS storage system. It helps users estimate critical storage metrics such as total raw capacity, usable storage space after accounting for ZFS redundancy (like RAIDZ or mirroring), and the impact of ZFS overheads like snapshots, compression, and deduplication. Beyond capacity, it also provides insights into the potential performance characteristics of different pool configurations, allowing for informed decisions about hardware acquisition and system design.

Who should use it:

• IT professionals planning enterprise storage solutions.
• Home lab enthusiasts building custom NAS setups.
• System administrators optimizing existing TrueNAS deployments.
• Anyone seeking to understand the complex interplay of ZFS, hardware, and performance.

Common misconceptions:

• “More disks always mean more usable space”: While more disks increase raw capacity, the redundancy level (RAIDZ level, mirroring) significantly impacts usable space. A 10-disk RAIDZ2 pool has less usable space than a 10-disk stripe, despite offering high redundancy.
• “RAIDZ equals traditional RAID”: ZFS RAIDZ is fundamentally different and more robust, using variable stripe width and checksumming to prevent silent data corruption. It doesn’t suffer from the same performance degradation during rebuilds as traditional RAID.
• “ZFS overhead is negligible”: Features like snapshots, compression, and especially deduplication can consume significant amounts of space. Understanding and estimating this overhead is crucial for accurate capacity planning.

TrueNAS Calculator Formula and Mathematical Explanation

The TrueNAS calculator, specifically focusing on ZFS pools, employs several formulas to estimate capacity and performance. The core calculations revolve around disk size, redundancy levels, and ZFS features.

Capacity Calculations

1. Total Raw Capacity: This is the most straightforward calculation, representing the sum of the physical capacities of all disks in the pool.

Total Raw Capacity = Number of Disks × Individual Disk Size

Redundancy Calculations (The Heart of ZFS Pool Math)

a) Stripe (No Redundancy): No disks are lost to redundancy.

Usable Capacity (Stripe) = Total Raw Capacity

b) Mirror: Each pair of disks stores identical data. Effectively, half the total raw capacity is lost to redundancy.

Usable Capacity (Mirror) = Total Raw Capacity / 2 (Simplified; assumes even numbers of disks in pairs)

c) RAIDZ (RAIDZ1, RAIDZ2, RAIDZ3): Each vdev has a specific number of parity disks.

• RAIDZ1 uses 1 parity disk per vdev.
• RAIDZ2 uses 2 parity disks per vdev.
• RAIDZ3 uses 3 parity disks per vdev.

Parity Disks per vdev = Number of parity disks based on RAIDZ level (1, 2, or 3)

Usable Capacity (RAIDZ) = (Number of vdevs) × (Number of disks per vdev - Parity Disks per vdev) × Individual Disk Size

Total Redundant Space (RAIDZ) = (Number of vdevs) × Parity Disks per vdev × Individual Disk Size

ZFS Overhead Calculation

ZFS reserves space for features like snapshots, checksums, compression, and potential future expansion.

Effective Usable Capacity = Usable Capacity × (1 - (ZFS Usable Space Overhead (%) / 100))

Performance Estimation (Simplified Index)

Performance in ZFS is complex and depends heavily on workload, record size, disk type (HDD/SSD), and cache configuration. This calculator provides a simplified index based on the number of “data” disks per vdev.

a) Stripe: Highest theoretical performance, limited by disk speed and count.

Performance Factor (Stripe) = Number of vdevs × Number of disks per vdev

b) Mirror: Performance is roughly equivalent to a single disk in the mirror, as reads can come from either disk, but writes must complete on both.

Performance Factor (Mirror) = Number of vdevs × (Number of disks per vdev / 2) (Number of active data paths)

c) RAIDZ: Performance is influenced by the number of data disks per vdev. More data disks generally lead to better potential throughput.

Performance Factor (RAIDZ) = Number of vdevs × (Number of disks per vdev - Parity Disks per vdev)

Variables Table

Variable	Meaning	Unit	Typical Range / Notes
Pool Type	ZFS redundancy configuration (RAIDZ1, RAIDZ2, RAIDZ3, Mirror, Stripe)	N/A	RAIDZ1, RAIDZ2, RAIDZ3, Mirror, Stripe
Disks per vdev	Number of physical disks within a single ZFS vdev.	Count	3-20 (RAIDZ), 2-20 (Mirror), 1-20 (Stripe)
vdev Count	Number of vdevs aggregated into the pool.	Count	1-20
Individual Disk Size	Capacity of one physical storage device.	Terabytes (TB)	0.5 TB – 20+ TB
ZFS Usable Space Overhead (%)	Percentage of usable space reserved for ZFS features.	Percent (%)	0% – 50% (Highly variable based on features used)
Record Size (KiB)	The block size for data stored in ZFS.	Kibibytes (KiB)	4 KiB – 1024 KiB (e.g., 16, 32, 64, 128, 1M)
AShifT Value	Log base 2 of the physical sector size of the drive.	Integer (Log2)	9 (512B), 10 (1KiB), 11 (2KiB), 12 (4KiB)
Parity Disks per vdev	Number of disks in a RAIDZ vdev dedicated to data redundancy.	Count	1 (RAIDZ1), 2 (RAIDZ2), 3 (RAIDZ3)

Practical Examples (Real-World Use Cases)

Let’s explore how the TrueNAS calculator helps in practical scenarios:

Example 1: Home Media Server Build

Goal: Build a reliable home server for storing photos, videos, and music, prioritizing data safety over maximum raw capacity.

Inputs:

• Pool Type: RAIDZ2
• Disks per vdev: 5
• vdev Count: 2
• Individual Disk Size: 8 TB
• ZFS Usable Space Overhead: 15%
• Record Size: 128 KiB
• AShifT: 10

Calculator Output:

• Total Raw Capacity: 80 TB (10 disks * 8 TB)
• Usable Capacity (After Redundancy): 48 TB (2 vdevs * (5 disks – 2 parity disks) * 8 TB)
• Effective Usable Capacity: 40.8 TB (48 TB * (1 – 0.15))
• Parity/Spare Space: 32 TB (2 vdevs * 2 parity disks * 8 TB)
• Performance Factor (Approx.): 6 (2 vdevs * 3 data disks)

Interpretation: This setup provides excellent data redundancy against two drive failures per vdev. With 40.8 TB of effective usable space, it’s ample for a media library. The performance factor of 6 suggests good throughput for typical media serving needs.

Example 2: Virtualization Host Storage

Goal: Create a high-performance storage pool for running multiple virtual machines (VMs), prioritizing speed and responsiveness.

Inputs:

• Pool Type: Mirror
• Disks per vdev: 2 (This defines the mirror pair)
• vdev Count: 3
• Individual Disk Size: 2 TB SSDs
• ZFS Usable Space Overhead: 25% (Snapshots for VM rollback are crucial)
• Record Size: 16 KiB (Often optimal for VM disk I/O)
• AShifT: 12

Calculator Output:

• Total Raw Capacity: 12 TB (6 disks * 2 TB)
• Usable Capacity (After Redundancy): 6 TB (3 mirrors * 2 TB per mirror)
• Effective Usable Capacity: 4.5 TB (6 TB * (1 – 0.25))
• Parity/Spare Space: 6 TB (Equivalent to usable capacity in mirror)
• Performance Factor (Approx.): 3 (3 vdevs * 1 active path per mirror)

Interpretation: While the effective usable space of 4.5 TB might seem limited for the raw capacity, the mirror configuration provides fast read/write performance and resilience. The calculator highlights that for VM workloads, speed often outweighs raw capacity efficiency. The high ZFS overhead (25%) accounts for frequent snapshotting and potential ZIL (ZFS Intent Log) usage.

How to Use This TrueNAS Calculator

Using the TrueNAS calculator is designed to be intuitive:

1. Select Pool Type: Choose the desired redundancy level (RAIDZ1, RAIDZ2, RAIDZ3, Mirror, or Stripe) from the dropdown.
2. Input Disk Configuration:
   • For RAIDZ, enter the number of disks planned for *each* vdev (e.g., 5 for a 5-disk RAIDZ2 vdev).
   • Enter the total number of vdevs you intend to combine into the pool (e.g., 2 vdevs for a dual-vdev pool).
   • For Mirrors, ‘Disks per vdev’ refers to the number of disks in each mirror pair (minimum 2).
   • For Stripes, ‘Disks per vdev’ is simply the number of disks in that stripe (usually 1).
3. Specify Disk Size: Enter the capacity of a single physical drive in Terabytes (TB).
4. Estimate ZFS Overhead: Input a percentage (0-50%) representing how much space you anticipate ZFS features (snapshots, compression, etc.) will consume relative to the calculated usable space. Start with 10-20% and adjust based on your planned usage.
5. Set Record Size & AShift: Choose the appropriate Record Size (KiB) based on your primary workload and the correct AShift value corresponding to your disks’ physical sector size.
6. Calculate: Click the “Calculate” button.

How to read results:

• Total Raw Capacity: The sum of all disk capacities before any redundancy or ZFS overhead.
• Usable Capacity (After Redundancy): The theoretical capacity available after accounting for the space lost to parity or mirroring.
• Effective Usable Capacity: The most realistic estimate of available storage space after both redundancy and ZFS overheads are applied. This is the figure you should primarily rely on for storage planning.
• Parity/Spare Space: The amount of storage dedicated solely to redundancy. Understanding this helps gauge the trade-off for data protection.
• Performance Factor (Approx.): A relative indicator of potential performance. Higher numbers suggest better potential throughput, but actual performance depends on many factors.

Decision-making guidance: Use the results to compare different configurations. If you need more space, consider higher-capacity disks or a less redundant pool type (if acceptable). If performance is key, evaluate configurations with more data disks per vdev or consider SSDs.

Key Factors That Affect TrueNAS Results

Several factors significantly influence the output of a TrueNAS calculator and the real-world performance/capacity of your ZFS pool:

1. Redundancy Level (RAIDZ vs. Mirror vs. Stripe): This is the most direct factor impacting usable capacity. RAIDZ3 offers the highest redundancy (tolerating 3 drive failures per vdev) but sacrifices the most capacity compared to RAIDZ1 or a stripe. Mirrors offer good read performance and fast resilvering but are capacity-inefficient (50% usable).
2. Number of Disks per vdev: For RAIDZ, increasing disks per vdev improves capacity efficiency (e.g., 10 disks in RAIDZ2 lose 2 disks to parity, yielding 80% efficiency for that vdev) but can slow down resilvering (rebuild time) after a drive failure.
3. Number of vdevs: Adding more vdevs increases both raw and usable capacity and can significantly boost performance due to increased parallelism. However, ensure your pool doesn’t become overly complex or difficult to manage.
4. Individual Disk Size: Larger disks increase raw capacity linearly. However, the usable capacity percentage remains the same for a given RAIDZ level. Larger disks also mean longer resilvering times, increasing the window of vulnerability during a rebuild.
5. ZFS Features (Overhead): This is a critical, often underestimated factor.
   • Snapshots: Essential for backups and versioning, but each snapshot consumes space for changed data blocks. Frequent snapshotting of large datasets can dramatically increase storage needs.
   • Compression: Algorithms like LZ4 (recommended) offer good compression ratios with minimal CPU overhead, saving space and potentially increasing read throughput (as less data needs to be read from disk).
   • Deduplication: Can save space if you have many identical blocks of data, but it requires vast amounts of RAM and can severely impact write performance due to the need to constantly check a massive hash table. It’s often impractical for large pools.
   • Record Size: Mismatched record sizes (e.g., large records for small files or databases) lead to wasted space (slop space) within blocks.
6. AShifT Value & Drive Sector Size: Incorrect AShift settings (e.g., using AShift 9 for drives with 4K sectors) can lead to severe performance degradation (4x slowdown) and potential data corruption. Always match AShift to the physical sector size (typically 10 for 1KiB/4Kn drives, 12 for 4KiB drives).
7. Workload Type: Transactional workloads (databases, VMs) benefit from smaller record sizes and faster media (SSDs), while sequential workloads (media streaming, backups) can handle larger record sizes and HDDs effectively.
8. RAM and CPU: While not directly in the calculator’s capacity formulas, sufficient RAM is crucial for ZFS performance (ARC cache) and features like deduplication. CPU power affects compression/deduplication performance.
9. Network Throughput: Even with a fast storage pool, network limitations can bottleneck overall data transfer speeds.

Frequently Asked Questions (FAQ)

Q1: What is the difference between a Stripe and a Mirror in TrueNAS?

A Stripe simply combines disks without redundancy, offering maximum raw capacity but no protection against drive failure. A Mirror duplicates data across two or more disks, providing redundancy but halving the effective usable capacity.

Q2: Is RAIDZ2 or RAIDZ3 better for my TrueNAS server?

RAIDZ2 is a good balance for most uses, protecting against two drive failures. RAIDZ3 offers higher protection (three failures) but uses more disks for parity, reducing usable capacity efficiency. Choose RAIDZ3 if you have many disks per vdev, use large HDDs prone to longer resilvers, or require maximum data safety.

Q3: How much ZFS overhead should I expect?

It varies greatly. For a basic setup with compression enabled, 10-20% might suffice. If you plan heavy use of snapshots, especially for VMs, or consider light deduplication, budget 30-50% or even more. Avoid enabling deduplication unless you have ample RAM (32GB+, ideally 64GB+).

Q4: Does the calculator account for pool expansion?

This calculator focuses on a single pool configuration. ZFS pools can often be expanded by adding more vdevs (if the pool type allows, like mirrors or RAIDZ) or by replacing disks with larger ones (requires downtime and careful sequential replacement). The calculator helps plan the *initial* setup.

Q5: What does “Performance Factor” mean in the results?

It’s a simplified metric indicating the relative number of active data paths or disks contributing to performance within the pool. Higher numbers generally imply better potential throughput. It’s not a direct IOPS or bandwidth measurement but a comparative index.

Q6: Can I mix different disk sizes in a RAIDZ vdev?

No, ZFS requires all disks within a single RAIDZ vdev to be of the same size. The usable capacity of the vdev is determined by the smallest disk. You *can* mix different sized vdevs within the same pool, but it’s generally not recommended for simplicity and optimal performance.

Q7: What is the best AShift value?

The best AShift value directly corresponds to the physical sector size of your drives (in bytes) on a log2 scale. For modern 4K sector drives (often advertised as 4KB), AShift 12 (log2 of 4096) is usually correct. For older drives or specific “Advanced Format” drives, check documentation. Using the wrong AShift significantly hurts performance.

Q8: How does record size affect performance and capacity?

A smaller record size (e.g., 16K) is efficient for small files or databases, reducing slack space. A larger record size (e.g., 128K) is better for large files (like VM images or media) as it reduces metadata overhead and can improve sequential throughput. Choosing the wrong size wastes space and can impact performance.

Related Tools and Internal Resources

• NAS Build Cost Calculator – Estimate the total cost of building your own NAS.
• ZFS RAID Calculator – Detailed breakdown of ZFS RAID levels, their pros and cons.
• Storage Capacity Planning Guide – Comprehensive guide to calculating your storage needs.
• RAID Level Comparison Chart – Compare different RAID types beyond ZFS.
• SSD vs. HDD Performance Analysis – Understand the performance differences for your storage.
• TrueNAS Performance Tuning Tips – Optimize your TrueNAS system for speed.

Explore our resources to make informed decisions about your storage infrastructure.