RAID-Z2 Storage Capacity Calculator
A RAID-Z2 array uses the capacity of two disks for parity (redundancy), allowing the array to survive the failure of any two disks.
What is a RAID-Z2 Calculator?
A raidz2 calculator is a specialized tool designed to determine the usable storage space in a ZFS (Zettabyte File System) storage pool configured with RAID-Z2. Unlike a generic storage calculator, a RAID-Z2 calculator specifically accounts for the double-parity mechanism inherent to this RAID level. It helps system administrators, IT professionals, and home NAS enthusiasts accurately plan their storage arrays before purchasing hardware. By inputting the number of disks and their individual capacities, the calculator computes the total raw capacity, the space consumed by parity data, and the final usable capacity you’ll have for your files.
This is crucial because RAID-Z2 always dedicates the equivalent of two full disks to data redundancy. Failing to account for this can lead to a significant overestimation of available space. This tool is essential for anyone building a reliable storage solution with ZFS, from a small home media server to a large enterprise file server where data integrity is paramount. A good calculator, like the one on this page, also helps visualize the storage efficiency of your chosen configuration.
The RAID-Z2 Formula and Explanation
The calculation for RAID-Z2 usable capacity is straightforward but critical to understand. ZFS achieves its double-disk fault tolerance by striping data and two sets of parity information across all disks in the virtual device (vdev).
The core formula is:
Usable Capacity = (N - P) * S
Where:
- N is the total number of disks in the vdev.
- P is the number of parity disks (which is always 2 for RAID-Z2).
- S is the size of the smallest disk in the vdev.
This formula highlights that no matter how many disks you add to a RAID-Z2 vdev, the capacity of exactly two of those disks is used for redundancy. Our raidz2 calculator automates this for you.
| Variable | Meaning | Unit (Auto-Inferred) | Typical Range |
|---|---|---|---|
| N | Total number of disks | Unitless (count) | 4 to 12 disks is common |
| P | Parity Disks | Unitless (count) | Fixed at 2 for RAID-Z2 |
| S | Size of a single disk | TB, GB, etc. | 1 TB to 22 TB+ |
For more detailed planning, consider exploring zfs performance tuning to understand how array configuration impacts speed.
Practical RAID-Z2 Calculator Examples
Example 1: Small Home NAS Setup
A user is building a home NAS for media storage and backups. They want a good balance of protection and capacity and have purchased 5 disks.
- Inputs: 5 disks
- Units: 8 TB per disk
- Calculation: (5 – 2) * 8 TB = 24 TB
- Results: The user will have 24 TB of usable space, with 16 TB dedicated to parity, and the array can withstand the failure of any two disks.
Example 2: Prosumer/Small Business Array
A small business needs a robust file server. They opt for a server with 10 drive bays to maximize capacity and efficiency.
- Inputs: 10 disks
- Units: 16 TB per disk
- Calculation: (10 – 2) * 16 TB = 128 TB
- Results: The business will have 128 TB of usable space. The storage efficiency is 80% ((128 TB / 160 TB) * 100), which is much better than the 60% in the first example. This demonstrates how RAID-Z2 becomes more space-efficient with more drives. To learn more, compare raidz1 vs raidz2 trade-offs.
How to Use This RAID-Z2 Calculator
Using our calculator is simple and intuitive. Follow these steps to accurately plan your ZFS storage pool:
- Enter the Number of Disks: In the “Number of Disks in vdev” field, input the total count of drives you plan to use in your RAID-Z2 array.
- Enter the Disk Size: In the “Size per Disk” field, type the capacity of a single drive. Remember to use identical-sized drives for a vdev.
- Select the Correct Unit: Use the dropdown menu to choose the unit of your disk size (TB or GB). The calculator assumes these are manufacturer-advertised sizes (power-of-10). The results will automatically be converted to both TB and their binary equivalent (TiB) for technical accuracy.
- Interpret the Results: The calculator instantly updates, showing your “Usable Storage Capacity” as the primary result. You can also view intermediate values like Total Raw Capacity, the amount of space used for Parity, and your overall Storage Efficiency percentage.
- Analyze the Chart: The bar chart provides a quick visual comparison of how much of your total raw capacity is usable versus how much is reserved for fault tolerance.
Key Factors That Affect RAID-Z2 Calculations
While the basic formula is simple, several factors can influence your real-world storage capacity and performance.
- Number of Disks: This is the most significant factor. More disks in a RAID-Z2 vdev increase storage efficiency because the two-disk parity overhead is spread across a larger base. An array with 4 disks has 50% efficiency, while an array with 10 disks has 80% efficiency.
- Disk Size: Larger individual disks provide more raw and usable capacity. However, they also lead to much longer resilvering (rebuild) times when a disk fails, increasing the risk of a second failure during the rebuild. See our guide on zfs resilvering time for more info.
- TB vs. TiB: Disk manufacturers advertise capacity in terabytes (TB, 10^12 bytes), but operating systems measure capacity in tebibytes (TiB, 2^40 bytes). A 10 TB drive is only about 9.1 TiB. Our calculator shows both values to avoid confusion.
- ZFS Metadata Overhead (“Slop Space”): ZFS reserves a small percentage of the pool’s total space (about 3%) to prevent fragmentation and manage metadata. This is not accounted for in simple capacity formulas but will slightly reduce your final usable space.
- Number of Vdevs: For very large arrays, you might create a pool with multiple RAID-Z2 vdevs striped together. This improves performance but complicates capacity calculation. Our calculator focuses on a single vdev for clarity.
- Use Case (Record Size): For specialized workloads like databases or virtualization, the ZFS record size can affect performance and how space is used. For general file storage, the default (128K) is usually fine. This is a key part of enterprise storage solutions.
Frequently Asked Questions (FAQ)
-
1. What is the minimum number of disks for RAID-Z2?
Technically, RAID-Z2 can be created with 3 disks, but this is impractical as you’d have only one disk’s worth of usable space. The practical and recommended minimum is 4 disks. -
2. Can I mix disk sizes in a RAID-Z2 vdev?
While ZFS allows it, it is strongly discouraged. The vdev will treat all disks as if they are the size of the smallest disk in the array, wasting capacity on all larger disks. -
3. What’s the difference between RAID-Z2 and RAID-6?
Both offer double-disk fault tolerance. However, RAID-Z2 is a software implementation within ZFS that solves the “RAID write hole,” making it more resilient against data corruption during unexpected power loss compared to most hardware RAID-6 controllers. For a deeper dive, read our comparison of data redundancy strategies. -
4. What happens if three disks fail in my RAID-Z2 array?
If three disks fail simultaneously, the entire storage pool will be lost. RAID-Z2 is designed to protect against a maximum of two concurrent disk failures. -
5. Is RAID-Z2 a backup?
No. RAID of any kind provides redundancy against hardware failure; it is not a backup. It will not protect you from file deletion, ransomware, or catastrophic data corruption. You must always maintain separate backups. -
6. How many disks are best for RAID-Z2 efficiency?
Efficiency improves with more disks. A sweet spot for performance and efficiency is often considered to be between 6 and 10 disks per vdev. -
7. Should I use TB or TiB for my calculation?
Use the unit your disk manufacturer provides (usually TB). Our raidz2 calculator will convert this to TiB for you so you can see what the operating system will report. -
8. Why does the calculator show less space than I expected?
This is typically due to the TB vs. TiB conversion. One Terabyte (1,000,000,000,000 bytes) is about 931 Gibibytes (GiB) or 0.91 Tebibytes (TiB). The calculator accurately reflects this difference.