Truenas Raid Calculator

Plan your ZFS storage pools with precision and speed

What is the TrueNAS RAID Calculator?

The truenas raid calculator is an essential utility for storage administrators and home lab enthusiasts using the OpenZFS file system. Unlike traditional hardware RAID, ZFS (the heart of TrueNAS) handles data integrity, parity, and snapshots differently. This truenas raid calculator helps you estimate exactly how much storage you will have after accounting for the “binary tax,” RAIDZ parity, and the critical 20% performance reservation.

Using a truenas raid calculator ensures that you don’t over-provision your hardware. Many new users are surprised to find that 100TB of raw disks results in significantly less usable space. This tool bridges the gap between marketing numbers and real-world availability.

TrueNAS RAID Calculator Formula and Mathematical Explanation

The math behind ZFS storage involves several layers of reduction. The truenas raid calculator follows these logic steps:

1. Raw Capacity: Total Disks × Disk Size.
2. Parity Deduction: Depending on the RAIDZ level, parity drives are subtracted (RAIDZ1 = 1, RAIDZ2 = 2, RAIDZ3 = 3).
3. ZFS Metadata Overhead: Roughly 1/128th of the pool is reserved for internal ZFS structures.
4. TiB Conversion: Manufacturers use 10^12 (Decimal), while TrueNAS uses 2^40 (Binary). The conversion factor is approximately 0.9095.
5. Slop Space Reservation: TrueNAS reserves small amounts for “slop” space to prevent pool locking.

Variables used in truenas raid calculator logic

Variable	Meaning	Unit	Range
N	Number of Physical Disks	Count	1 – 255
S	Advertised Disk Size	Terabytes (TB)	0.1 – 30+
P	Parity Count (RAIDZ level)	Count	0 – 3
Q	Performance Padding	Percentage (%)	10% – 20%

Practical Examples (Real-World Use Cases)

Example 1: The Modern Media Server

A user buys 8 drives of 14TB each. They choose RAIDZ2 for high data safety. The truenas raid calculator shows:

– Raw: 112 TB

– Parity (2 disks): 28 TB

– Decimal Usable: 84 TB

– Binary Usable (TiB): ~76.4 TiB

– Safe Limit (80%): 61.1 TiB.
This interpretation allows the user to know they can store roughly 60 TiB of movies safely.

Example 2: The High-Performance SSD Pool

A business uses 4 SSDs of 4TB each in a Mirror (RAID 10 equivalent) setup. The truenas raid calculator calculates:

– Raw: 16 TB

– Redundancy: 8 TB (Mirroring halves capacity)

– Binary Usable: 7.27 TiB.
Crucially, the truenas raid calculator reminds them that for SSD performance, keeping 20% free is even more critical than for HDDs.

How to Use This TrueNAS RAID Calculator

1. Input Disk Count: Enter the total number of drives in your specific VDEV. Note that TrueNAS pools are made of one or more VDEVs.

2. Set Disk Size: Use the TB value printed on the drive label (e.g., 8, 12, 18).

3. Select RAID Level: RAIDZ2 is the gold standard for pools with 6-12 drives. RAIDZ3 is for 12+ drives. Mirroring is for performance.

4. Adjust Padding: Leave at 20% unless you are strictly using the pool for archival storage where performance doesn’t matter.

5. Read the Chart: The visual bar shows how much of your expensive storage is “lost” to math and safety.

Key Factors That Affect TrueNAS RAID Calculator Results

• The Binary Tax (TiB vs TB): Windows and TrueNAS report in TiB (1024^4), but you buy disks in TB (1000^4). This accounts for a ~9% loss.
• RAIDZ Parity: RAIDZ1 can lose 1 drive, RAIDZ2 can lose 2, and RAIDZ3 can lose 3 without data loss.
• Ashift (Sector Size): Modern 4Kn drives require ashift=12. Incorrect setting can waste space on small files.
• Pool Fragmentation: As a Copy-on-Write (COW) system, TrueNAS performance degrades as the pool fills.
• Metadata & Snapshots: Every snapshot takes a small amount of space, which the truenas raid calculator assumes is part of your 20% reserve.
• Swap Space: TrueNAS usually creates a 2GB swap partition on every drive, slightly reducing the raw capacity available for data.

Frequently Asked Questions (FAQ)

How accurate is this truenas raid calculator?

This truenas raid calculator provides a highly accurate estimate based on standard OpenZFS math. However, specific file sizes and compression ratios (LZ4/ZSTD) can increase your effective capacity.

Why is my usable space lower than 80%?

Between the decimal-to-binary conversion and RAIDZ2 parity, it is common to see only 60-70% of raw capacity as “usable” in a truenas raid calculator.

Can I expand a RAIDZ pool later?

Recent TrueNAS versions allow RAIDZ expansion, but historically you had to add entire VDEVs. Check your version before relying on the truenas raid calculator for future planning.

Does disk size affect parity?

Yes, parity is based on the size of the smallest disk in the VDEV. If you mix 8TB and 12TB drives, the truenas raid calculator will treat them all as 8TB.

Is RAIDZ1 safe for large drives?

For drives over 8TB, RAIDZ2 is recommended. The truenas raid calculator highlights the risks of parity loss during the long rebuild times of large drives.

What is the ‘Slop Space’?

ZFS reserves about 1/32nd of the pool to ensure it never becomes 100% full, which would prevent you from deleting files to free up space.

Should I use Mirroring or RAIDZ?

Mirroring (RAID 10) offers better IOPS. RAIDZ offers better capacity. Use the truenas raid calculator to compare the trade-offs.

Can I calculate for multiple VDEVs?

Simply calculate one VDEV in the truenas raid calculator and multiply the result by your total number of VDEVs.

Related Tools and Internal Resources

• ZFS Pool Performance Guide – Learn how layout affects your speed.
• Hard Drive Reliability Database – Choose the right disks for your pool.
• RAIDZ vs RAIDZ2 Comparison – Deep dive into fault tolerance.
• ZFS Compression Calculator – See how LZ4 saves you space.
• TrueNAS Hardware Selection – Best CPUs and RAM for ZFS.
• Backup Strategy Planner – RAID is not a backup!

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