RAIDZ2 Calculator

Calculate ZFS storage capacity and fault tolerance instantly.

Raw Capacity:
60.00 TB

Parity Reserved:
20.00 TB

Space Efficiency:
66.67 %

Fault Tolerance:
2 Drives

What is a RAIDZ2 Calculator?

A raidz2 calculator is an essential tool for system administrators and NAS enthusiasts building ZFS-based storage arrays. RAIDZ2 is the ZFS equivalent of RAID 6, utilizing dual parity to protect data against the simultaneous failure of up to two disks. While the basic math seems simple, a high-quality raidz2 calculator must account for binary vs. decimal conversions, ZFS “slop space,” and metadata overhead to provide an accurate real-world capacity estimate.

Who should use it? Anyone planning a TrueNAS, Proxmox, or FreeBSD storage server. Common misconceptions include thinking that a 10TB drive provides exactly 10TB of usable space. In reality, ZFS reserves a portion of the pool for administrative tasks, which our raidz2 calculator factors into the final result.

RAIDZ2 Calculator Formula and Mathematical Explanation

The underlying math for a raidz2 calculator involves subtracting the parity overhead from the raw capacity and then adjusting for filesystem-specific reservations. RAIDZ2 always uses the capacity equivalent of two drives for parity, regardless of the total drive count.

The Core Formula:

Usable Capacity = (N - 2) × S × (1 - O)

Variables used in the raidz2 calculator

Variable	Meaning	Unit	Typical Range
N	Number of Drives	Count	4 to 12 per VDEV
S	Individual Drive Size	TB / GB	1TB to 22TB+
O	ZFS Overhead / Slop	%	3% to 5%
P	Parity Drives	Count	Fixed at 2

Practical Examples (Real-World Use Cases)

Example 1: The Home Media Server

Suppose you are building a NAS with six 10TB hard drives. Using the raidz2 calculator, the raw capacity is 60TB. However, RAIDZ2 reserves two drives for parity.
Inputs: 6 drives, 10TB each.
Outputs: 40TB raw usable. After applying the 3.125% ZFS slop space, the raidz2 calculator shows approximately 38.75TB of actual usable storage. This configuration allows any two drives to fail without data loss.

Example 2: Enterprise Backup Target

An enterprise uses twelve 18TB drives in a single RAIDZ2 VDEV.
Inputs: 12 drives, 18TB each.
Outputs: Raw capacity is 216TB. Parity takes 36TB. The raidz2 calculator determines that the usable capacity is roughly 174.3TB after ZFS overhead. This provides a space efficiency of 83.3% while maintaining high redundancy.

How to Use This RAIDZ2 Calculator

1. Enter Drive Count: Input the total number of physical disks you plan to include in your RAIDZ2 VDEV. Note that RAIDZ2 requires a minimum of 3 drives, but 4 or more is standard.
2. Set Drive Capacity: Enter the size of the smallest drive in your array. Remember that in ZFS, if you mix drive sizes, the raidz2 calculator logic dictates that all drives will be treated as the size of the smallest disk.
3. Adjust Overhead: The default is 3.125% (1/32nd), which is what ZFS typically reserves to prevent the pool from becoming completely full and locked.
4. Review Results: The primary result shows your usable capacity. The chart visualizes how much is lost to parity and overhead.
5. Copy and Plan: Use the “Copy Results” button to save your specs for hardware procurement.

Key Factors That Affect RAIDZ2 Calculator Results

• VDEV Width: The number of drives (N) affects efficiency. A wider VDEV increases efficiency but increases “resilver” (rebuild) times during a failure.
• Binary vs. Decimal: Drive manufacturers sell 1TB as 1,000,000,000,000 bytes. Operating systems see it as ~931GiB. Our raidz2 calculator uses the input units provided.
• Ashift Settings: Using the wrong alignment (e.g., ashift=9 on 4Kn drives) can cause massive hidden storage waste not captured by simple calculators.
• Padding Overhead: RAIDZ with small block sizes (volblocksize) can lead to significant padding overhead, sometimes losing up to 50% more space than expected.
• Reserved Slop Space: ZFS keeps a small portion of the pool (usually 3.2%) to ensure the system can still perform deletions even when “full.”
• Compression: Unlike traditional RAID, ZFS compression (LZ4 or ZSTD) can actually make your “usable” space larger than the physical limit for compressible data.

Frequently Asked Questions (FAQ)

Q: Can I expand a RAIDZ2 VDEV later?
A: Traditionally, no. You had to add another VDEV. However, newer ZFS versions support RAIDZ expansion, though it is a long process. Always use a raidz2 calculator to plan correctly the first time.

Q: Is RAIDZ2 better than RAID 6?
A: Yes, ZFS RAIDZ2 prevents “silent data corruption” via checksumming, which traditional RAID 6 cannot detect.

Q: What is the minimum number of drives?
A: Technically 3, but that acts like a RAIDZ1 with an extra parity disk. 4 is the logical minimum for a raidz2 calculator to make sense.

Q: How many drives can I lose?
A: Exactly two. If a third drive fails before the first two are replaced and resilvered, the entire pool is lost.

Q: Does drive speed matter for capacity?
A: No, the raidz2 calculator only cares about capacity, but speed affects how long the array stays in a “degraded” state.

Q: Why is my usable space lower than the calculation?
A: You likely have a small volblocksize on a ZVOL, causing parity padding overhead.

Q: Should I use RAIDZ3 instead?
A: If you are using drives larger than 10TB in VDEVs wider than 12 drives, RAIDZ3 (triple parity) is safer.

Q: Does RAIDZ2 affect performance?
A: Yes, write performance is roughly equal to the speed of a single drive, while read performance scales with the number of data drives.

Related Tools and Internal Resources

• RAIDZ1 Calculator – Estimate capacity for single-parity ZFS pools.
• ZFS Compression Guide – Learn how LZ4 and ZSTD impact your real-world storage limits.
• NAS Drive Comparison – Find the best drives for your RAIDZ2 configuration.
• Storage IOPS Calculator – Calculate the performance of your disk array.
• RAID Rebuild Timer – Estimate how long it takes to replace a failed disk.
• ZFS Ashift Checker – Ensure your pool is optimized for modern 4K sector drives.