Raidz2 Calculator

Estimate ZFS Storage Capacity, Fault Tolerance, and Efficiency

What is a RAIDZ2 Calculator?

A raidz2 calculator is a specialized tool used by system administrators and home lab enthusiasts to estimate the storage capacity of a ZFS pool utilizing the RAID-Z2 level. RAID-Z2 is the ZFS equivalent of traditional RAID 6, providing double-parity protection. This means your storage array can survive the simultaneous failure of any two drives without data loss.

Using a raidz2 calculator is essential because ZFS capacity isn’t as simple as adding up drive sizes. Factors like parity overhead, “slop space” reservations, and the difference between decimal and binary units significantly impact the final usable space. Whether you are building a server for TrueNAS or managing an enterprise ZFS cluster, calculating these values beforehand ensures you meet your storage requirements.

RAIDZ2 Calculator Formula and Mathematical Explanation

The core logic behind a raidz2 calculator relies on the N-2 principle. In a RAID-Z2 configuration, two disks worth of capacity are always reserved for parity information, regardless of the total number of disks in the VDEV.

The mathematical derivation for raw usable capacity is:

C_usable = (N – 2) × S

Where:

• N: Total number of drives in the VDEV.
• 2: Parity disks required for RAIDZ2.
• S: The capacity of the smallest drive in the group.

Variables Table

Variable	Meaning	Unit	Typical Range
N	Number of Disks	Count	4 to 12 per VDEV
S	Drive Size	TB / TiB	1 TB to 24 TB+
P	Parity Overhead	Drives	Fixed at 2
ZFS Slop	Reserved Space	%	~3.125% (1/32)

Practical Examples (Real-World Use Cases)

Example 1: The 8-Bay Home Server

Imagine a user building a media server with 8 drives, each being 14TB. Using the raidz2 calculator, the raw capacity is 112TB (8 x 14). Since RAIDZ2 uses 2 drives for parity, the usable capacity is (8 – 2) x 14 = 84TB. After accounting for ZFS overhead and binary conversion (TiB), the user would see approximately 74 TiB in their OS.

Example 2: Enterprise Backup Node

A company deploys 12 drives of 20TB each in a single RAIDZ2 VDEV. The raidz2 calculator shows a usable capacity of 200TB (10 x 20). This configuration offers a storage efficiency of 83.3%, making it a high-density solution for backups while still allowing for two drive failures during the lengthy resilvering process of 20TB disks.

How to Use This RAIDZ2 Calculator

1. Select Number of Drives: Enter the total count of hard drives or SSDs you plan to include in your VDEV. For RAIDZ2, 4 is the minimum, but 6-10 is the “sweet spot” for performance and efficiency.
2. Enter Drive Size: Input the capacity listed on the drive’s label (e.g., 18TB).
3. Choose Unit: Toggle between TB (decimal) or TiB (binary) to match how your operating system displays storage.
4. Review Results: The raidz2 calculator instantly updates the usable capacity, parity loss, and storage efficiency.
5. Consult the Chart: View the visual breakdown to see how much of your hardware investment is going toward data vs. protection.

Key Factors That Affect RAIDZ2 Calculator Results

• ZFS Slop Space: ZFS reserves about 1/32nd of the pool capacity to ensure the system can still function (like deleting files) even when the pool is technically “full.”
• Binary vs. Decimal: Drive manufacturers sell in decimal (1 TB = 1,000,000,000,000 bytes), but ZFS and Linux/Windows use binary (1 TiB = 1,099,511,627,776 bytes). This results in a ~9% “loss” in perceived capacity.
• Pool Filling Limit: It is widely recommended never to fill a ZFS pool beyond 80% to avoid severe performance degradation. The raidz2 calculator shows raw usable, but your “functional” usable is lower.
• Ashift and Recordsize: Misaligned ashift (sector size) or suboptimal recordsize can cause “padding overhead,” where small blocks consume more space than expected.
• Swap Partitions: Some OS like TrueNAS Core create a small swap partition (usually 2GB) on every drive by default, slightly reducing the available size per disk.
• VDEV Expansion: RAID-Z2 VDEVs cannot be expanded by adding single disks easily (though ZFS expansion is becoming available in newer versions). Usually, you expand by adding another VDEV or replacing all disks one by one.

Frequently Asked Questions (FAQ)

Is RAIDZ2 better than RAID 6?

Functionally, they both provide 2-drive redundancy. However, ZFS RAIDZ2 avoids the “write hole” error found in hardware RAID cards and provides data checksumming to prevent silent data corruption.

How many drives can I lose in RAIDZ2?

You can lose exactly two drives. If a third drive fails before the first two are replaced and resilvered, the entire pool is lost.

What is the minimum number of drives for RAIDZ2?

Technically, you can create a RAIDZ2 with 3 drives, but it would have the same usable capacity as a 3-drive RAIDZ1 but with more overhead. 4 drives is the practical minimum.

Does the raidz2 calculator account for formatting?

Yes, this raidz2 calculator accounts for standard ZFS slop space and unit conversions which are the primary “formatting” losses in ZFS.

Can I mix drive sizes in a RAIDZ2 VDEV?

You can, but the raidz2 calculator will base the capacity on the smallest drive in the group. The extra space on larger drives will be wasted.

Why is my TiB result lower than the TB result?

Because 1 TiB is 1024^4 bytes while 1 TB is 1000^4 bytes. The TiB unit is a larger “bucket,” so fewer of them fit into the same total number of bytes.

Is RAIDZ2 slow during resilvering?

Resilvering (rebuilding) can be slow on large HDDs. RAIDZ2 is safer than RAIDZ1 because it allows another failure during this high-stress period.

What is the maximum number of drives for one VDEV?

While ZFS supports many, most experts suggest 10-12 drives per VDEV. For more drives, use multiple RAIDZ2 VDEVs striped together.

Related Tools and Internal Resources

• RAID Calculator – Compare standard RAID 0, 1, 5, 6, and 10 configurations.
• RAIDZ1 Calculator – Estimate capacity for single-parity ZFS pools.
• ZFS Optimization Tips – Learn how to tune your recordsize and ashift for maximum performance.
• RAIDZ Performance Guide – Understanding IOPS and throughput in ZFS arrays.
• TrueNAS Hardware Requirements – Best practices for choosing ECC RAM and HBA controllers.
• Backup Strategy Calculator – Plan your 3-2-1 backup strategy based on your usable ZFS capacity.