RAIDZ Calculator
Plan your ZFS storage pool with precision. Calculate usable capacity, fault tolerance, and efficiency for RAID-Z configurations.
Formula: (Drives – Parity) × Capacity × ZFS Overhead Factor
Visual representation of storage distribution
What is a RAIDZ Calculator?
A raidz calculator is an essential tool for system administrators and NAS enthusiasts using the ZFS file system. Unlike traditional RAID levels, ZFS uses RAID-Z to provide data redundancy while overcoming the "write hole" phenomenon. This calculator helps you determine how much actual storage space you will have after accounting for parity drives, ZFS metadata overhead, and the conversion from decimal Terabytes (TB) to binary Tebibytes (TiB).
Who should use it? Anyone building a TrueNAS server, a Proxmox storage node, or a custom Linux ZFS pool. Understanding your raidz calculator results ensures you don't run out of space unexpectedly due to parity requirements or the 80% capacity rule recommended for ZFS performance.
Common misconceptions include the idea that RAID-Z1 is identical to RAID 5. While they share similar parity ratios, ZFS handles data blocks differently, meaning the raidz calculator must account for specific ZFS overheads that traditional RAID calculators ignore.
RAIDZ Calculator Formula and Mathematical Explanation
The math behind a raidz calculator involves several layers of subtraction and conversion. The primary goal is to find the "Slop Space" and "Padding Overhead" which are unique to ZFS.
The basic derivation follows this logic:
- Raw Capacity: Total Drives × Drive Size
- Parity Deduction: (Total Drives – Parity Level) × Drive Size
- Binary Conversion: Decimal TB × (10^12 / 2^40)
- ZFS Overhead: Usable Capacity × 0.966 (approximate 3.4% for metadata and slop)
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| N | Number of Disks | Count | 3 – 255 |
| P | Parity Level (Z1, Z2, Z3) | Disks | 1, 2, or 3 |
| C | Advertised Capacity | TB | 1TB – 24TB |
| S | Slop Space / Metadata | Percentage | 1/64th to 3% |
Practical Examples (Real-World Use Cases)
Example 1: The Home Media Server
A user has 6 drives, each 12TB, and wants to use RAID-Z2 for high reliability. Using the raidz calculator:
- Inputs: 6 Drives, 12TB each, RAID-Z2.
- Raw: 72 TB.
- Parity: 24 TB (2 drives).
- Usable (TiB): Approximately 40.5 TiB after ZFS overhead and binary conversion.
Example 2: The Performance Flash Pool
An editor uses 4 SSDs of 4TB each in RAID-Z1. The raidz calculator shows:
- Inputs: 4 Drives, 4TB each, RAID-Z1.
- Raw: 16 TB.
- Parity: 4 TB (1 drive).
- Usable (TiB): Approximately 10.2 TiB.
How to Use This RAIDZ Calculator
Follow these steps to get the most accurate results for your ZFS pool planning:
- Enter Drive Count: Input the total number of disks you plan to put in a single vdev.
- Select Capacity: Use the manufacturer's TB rating (e.g., 18 for an 18TB drive).
- Choose RAID-Z Level: Select Z1 for 1-disk redundancy, Z2 for 2-disk, or Z3 for 3-disk.
- Set Ashift: Use Ashift 12 for almost all modern mechanical and solid-state drives.
- Interpret Results: Focus on the "Usable TiB" value, as this is what your operating system will actually report.
Decision-making guidance: If your raidz calculator efficiency is below 50%, consider using mirrored vdevs for better performance and similar space utilization.
Key Factors That Affect RAIDZ Calculator Results
- Binary vs Decimal: Hard drive makers sell in decimal (1000^4), but ZFS calculates in binary (1024^4). This causes a ~9% "loss" immediately.
- Ashift (Sector Alignment): Incorrect ashift settings can lead to massive "padding overhead," where small blocks take up more space than necessary.
- ZFS Slop Space: ZFS reserves about 1/64th of the pool to ensure it can always perform administrative tasks even when "full."
- Parity Ratio: In a 3-drive RAID-Z1, 33% is lost to parity. In a 10-drive RAID-Z1, only 10% is lost.
- Recordsize: Larger recordsizes (e.g., 1MB) generally improve efficiency on RAID-Z pools compared to the default 128K.
- Vdev Expansion: Historically, you couldn't add disks to a RAID-Z vdev. While this is changing, it's best to plan your final capacity using the raidz calculator now.
Frequently Asked Questions (FAQ)
Is RAID-Z1 safe for large drives?
Most experts recommend RAID-Z2 for drives larger than 8TB due to the long rebuild times and the risk of a second drive failure during resilvering.
Why does my 10TB drive only show 9.09 TiB?
This is the difference between decimal TB and binary TiB. The raidz calculator accounts for this conversion automatically.
Can I mix drive sizes in RAID-Z?
Yes, but ZFS will treat all drives as if they are the size of the smallest drive in the vdev.
What is the "Write Hole"?
It's a data corruption risk in traditional RAID 5/6 during power loss. RAID-Z eliminates this through its copy-on-write architecture.
How many drives can I have in one RAID-Z vdev?
While the limit is high, it is generally recommended to keep vdevs between 3 and 12 drives for optimal performance.
Does RAID-Z improve read speeds?
Yes, read speeds generally scale with the number of data (non-parity) drives in the vdev.
What is the best RAID-Z level for 4 drives?
RAID-Z2 is safest, providing 2-disk fault tolerance, but RAID-Z1 is common for non-critical data to maximize space.
Does this calculator include L2ARC or SLOG?
No, L2ARC and SLOG are cache devices and do not increase the usable storage capacity of your pool.
Related Tools and Internal Resources
- ZFS Performance Guide – Optimize your pool for speed.
- RAID vs RAID-Z – Understanding the architectural differences.
- NAS Storage Calculator – Compare different NAS brands and file systems.
- ZFS Pool Optimization – Advanced tuning for RAID-Z vdevs.
- Hard Drive Reliability Stats – Choose the best disks for your ZFS pool.
- Data Backup Strategies – Why RAID is not a backup.