RAIDZ2 Storage Calculator
Calculate Usable Capacity and Redundancy for ZFS RAIDZ2 Pools
RAIDZ2 Configuration
Enter the usable capacity of a single physical disk in Gibibytes (e.g., 4000 for a 4TB drive).
RAIDZ2 requires a minimum of 4 disks (2 data + 2 parity). Enter the total number of disks in the vdev. Minimum is 5 for a functional pool with at least 3 data disks.
RAIDZ2 Configuration Table
| Metric | Value | Unit | Notes |
|---|---|---|---|
| Total Disks | N/A | Count | Total physical disks in the vdev. |
| Disk Size | N/A | GiB | Usable capacity per disk. |
| Parity Disks | 2 | Count | Number of disks dedicated to parity (RAIDZ2 always has 2). |
| Data Disks | N/A | Count | Disks available for storing user data. |
| Total Raw Capacity | N/A | GiB | Sum of all disk sizes. |
| Parity Overhead | N/A | GiB | Storage consumed by parity information. |
| Usable Capacity | N/A | GiB | The actual storage space available for data. |
| Failure Tolerance | 2 Disks | Count | Number of simultaneous disk failures the pool can withstand. |
Capacity Utilization Chart
A visual representation of how disk space is allocated between data and parity.
What is a RAIDZ2 Calculator?
A RAIDZ2 calculator is a specialized tool designed to help users and system administrators determine the usable storage capacity and data redundancy levels for a ZFS storage pool configured with RAIDZ2 (also known as RAID 6 triple-parity). In essence, it quantifies how much actual data you can store and how many simultaneous drive failures your pool can survive, based on the number of disks and their individual sizes.
Who should use it? Anyone planning or managing a ZFS storage solution where data integrity and resilience are paramount. This includes home users with large media libraries, small to medium businesses needing reliable file servers, creative professionals handling large datasets, and enterprise environments requiring robust data protection. If you’re setting up a NAS, a SAN, or a dedicated storage server using ZFS, a RAIDZ2 calculator is an invaluable aid.
Common misconceptions:
- Misconception 1: More disks always mean proportionally more usable space. While adding disks increases total capacity, the parity overhead in RAIDZ2 means usable space doesn’t scale linearly with the number of disks, especially with small pool sizes.
- Misconception 2: RAIDZ2 is the same as traditional RAID 6. While functionally similar, ZFS RAIDZ implementations are more sophisticated, offering better performance and different failure modes compared to hardware RAID.
- Misconception 3: Any drive size can be mixed freely. ZFS RAIDZ levels typically use the size of the *smallest* disk in the vdev as the effective size for all disks, impacting overall capacity significantly. This calculator assumes uniform disk sizes for simplicity.
RAIDZ2 Formula and Mathematical Explanation
The core calculation for RAIDZ2 is straightforward once you understand the principles of parity. RAIDZ2 uses two independent parity blocks distributed across the disks. This allows the system to reconstruct data even if two disks fail simultaneously.
Step-by-step derivation:
- Total Raw Capacity: This is the sum of the capacities of all physical disks in the storage pool (vdev).
- Parity Overhead: In RAIDZ2, two disks’ worth of capacity from each vdev is dedicated to storing parity information. This overhead is constant regardless of the number of data disks beyond the minimum required.
- Usable Capacity: This is the total raw capacity minus the parity overhead.
Variables Explained:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
N |
Total number of disks in the vdev. | Count | ≥ 5 (4 minimum, but 5+ is practical for 2+ data disks) |
S |
Individual disk size (usable capacity). | GiB | 128 – 20,000+ |
P |
Number of parity disks. | Count | 2 (for RAIDZ2) |
Total Raw Capacity |
Sum of all disk sizes. | GiB | N * S |
Parity Overhead |
Capacity lost to parity. | GiB | P * S |
Usable Capacity |
Final capacity available for data. | GiB | (N - P) * S |
Practical Examples (Real-World Use Cases)
Example 1: Small Home Server
Scenario: A user is building a home media server and wants robust protection against drive failure. They plan to use 6 hard drives.
Inputs:
- Individual Disk Size: 4000 GiB (approx. 4TB)
- Total Number of Data Disks: 6
Calculation:
N= 6S= 4000 GiBP= 2- Total Raw Capacity = 6 * 4000 = 24,000 GiB
- Parity Overhead = 2 * 4000 = 8,000 GiB
- Usable Capacity = (6 – 2) * 4000 = 4 * 4000 = 16,000 GiB
Result: 16,000 GiB (approx. 14.5 TiB) of usable storage.
Interpretation: With 6 x 4TB disks in RAIDZ2, the user gets a significant amount of storage space while being able to tolerate two simultaneous drive failures. This is ideal for storing large media files where data loss would be inconvenient.
Example 2: Professional Workstation Storage
Scenario: A video editor needs a ZFS pool for their workstation that can handle multiple drive failures without data loss, as project files are critical.
Inputs:
- Individual Disk Size: 18000 GiB (approx. 18TB)
- Total Number of Data Disks: 10
Calculation:
N= 10S= 18000 GiBP= 2- Total Raw Capacity = 10 * 18000 = 180,000 GiB
- Parity Overhead = 2 * 18000 = 36,000 GiB
- Usable Capacity = (10 – 2) * 18000 = 8 * 18000 = 144,000 GiB
Result: 144,000 GiB (approx. 131 TiB) of usable storage.
Interpretation: This configuration provides a massive amount of storage with high redundancy. The 144 TiB usable space can hold numerous large video projects, and the RAIDZ2 level protects against the failure of up to two drives, minimizing downtime and risk of data loss during intensive editing work.
How to Use This RAIDZ2 Calculator
Using the RAIDZ2 calculator is designed to be simple and intuitive. Follow these steps:
- Input Disk Size: Enter the usable capacity of a single physical disk in the pool into the “Individual Disk Size (GiB)” field. Use whole numbers (e.g., 4000, 8000, 18000). Remember that advertised drive sizes (like 4TB) are often decimal (terabytes), and the actual usable capacity in GiB will be slightly less after formatting. This calculator uses the GiB value you provide.
- Input Number of Disks: Enter the total number of physical disks that will make up your RAIDZ2 virtual device (vdev). Remember, RAIDZ2 requires a minimum of 4 disks, but practically, you’ll want at least 5 disks to have at least 3 disks for data and 2 for parity.
- Calculate: Click the “Calculate RAIDZ2” button.
How to read results:
- Usable Capacity: This is the primary result – the total amount of storage space you will have available for your files after accounting for parity.
- Total Raw Capacity: The sum of all disk capacities before any redundancy is considered.
- Parity Overhead: The amount of storage space consumed by the parity data required for RAIDZ2’s dual-disk failure protection.
- Number of Parity Disks: This will always be 2 for RAIDZ2.
- Table Details: The table provides a breakdown of all key metrics, including the number of data disks (Total Disks – 2), failure tolerance, and capacity figures in both GiB and a more understandable unit (TiB).
- Chart: The chart visually represents the proportion of disk space used for data versus parity.
Decision-making guidance: Use the results to determine if your planned configuration meets your storage needs and redundancy requirements. If the usable capacity is too low, consider using more disks, larger disks, or a different ZFS redundancy level (like RAIDZ3 for triple parity, or mirroring for simplicity but less capacity efficiency).
Key Factors That Affect RAIDZ2 Results
While the core formula is simple, several factors influence the practical application and perceived value of a RAIDZ2 configuration:
- Disk Size (S): This is the most direct factor. Larger individual disks mean higher total and usable capacity, assuming the number of disks remains constant. The calculator uses the smallest disk’s size in a mixed-size pool, so always use the smallest disk size for calculations if mixing.
- Number of Disks (N): More disks in a vdev increase total capacity but also increase the *absolute* parity overhead (in GiB or TB). However, the *percentage* of capacity lost to parity decreases as N increases. For example, 5 disks lose 2/5 (40%) to parity, while 10 disks lose 2/10 (20%).
- RAID Level Choice (Parity Disks, P): RAIDZ2 uses 2 parity disks. RAIDZ1 uses 1 (less redundancy, more capacity), and RAIDZ3 uses 3 (more redundancy, less capacity). The calculator is specific to RAIDZ2.
- ZFS Overhead (Metadata, Snapshots): ZFS itself consumes some space for metadata, internal structures, and potentially ZFS snapshots. The calculator provides raw usable capacity, not accounting for these ZFS-specific overheads, which can be a few percent.
- Record Size: While not directly impacting raw capacity calculation, ZFS record size can affect performance and how efficiently small files are stored. Larger record sizes are generally better for large files (like videos, VM images) and performance in RAIDZ setups.
- Pool Allocation Class: ZFS pools can be configured for `all`, `striped`, or `raidz` (which includes RAIDZ1/2/3) allocation classes. This calculator assumes the `raidz` class, where data is written across all disks in a vdev with parity.
- Drive Formatting & Overhead: The advertised capacity of a drive (e.g., 4TB) is usually a decimal TB (10^12 bytes). Operating systems and ZFS work with binary GiB (2^30 bytes). Additionally, firmware, bad block management, and ZFS structures consume a small portion of this. The calculator relies on the user inputting the actual *usable* GiB.
Frequently Asked Questions (FAQ)
Q: What is the minimum number of disks for RAIDZ2?
Q: Can I mix different sized drives in a RAIDZ2 vdev?
Q: How much usable space do I lose to parity?
2 * S capacity, resulting in (N - 2) * S usable capacity.Q: Is RAIDZ2 suitable for operating system drives?
Q: What happens if more than two disks fail in a RAIDZ2 pool?
Q: How does ZFS RAIDZ2 compare to hardware RAID 6?
Q: Does the calculator account for ZFS specific overheads like ARC, L2ARC, or SLOG?
Q: Should I use RAIDZ2 or mirroring for my ZFS pool?