Unraid ZFS Calculator
Estimate your usable storage capacity with ZFS on Unraid. An essential tool for planning your server’s storage array.
Calculate Your ZFS Pool Capacity
Enter the total number of physical disks in your ZFS vdev (pool).
Please enter a valid number of disks (minimum 3 for RAIDZ).
Enter the capacity of a single disk in terabytes (TB).
Please enter a valid disk size.
Select the desired fault tolerance level for your pool.
Estimated Usable Capacity
Total Raw Capacity
Parity / Redundancy
Fault Tolerance
Formula: (Total Disks – Parity Disks) * Disk Size * 0.97 (approx. 3% ZFS overhead)
Capacity Breakdown
Configuration Summary
| Parameter | Value |
|---|
What is an Unraid ZFS Calculator?
An unraid zfs calculator is a specialized tool designed to help users plan their server storage by estimating the usable disk space in a ZFS pool. ZFS (Zettabyte File System) is a modern, advanced filesystem and volume manager that provides robust data integrity, built-in RAID functionality (known as RAIDZ), and other powerful features. When you create a ZFS pool in Unraid, not all of the raw disk capacity is available for storing data. Space is allocated for parity (data redundancy) and filesystem overhead. This calculator demystifies the process, providing clear estimates for your specific hardware configuration.
This tool is essential for anyone building or expanding an Unraid server with ZFS pools, from home lab enthusiasts to data hoarders and small business owners. A common misconception is that if you have 5 disks of 10TB in RAIDZ1 (which uses one disk for parity), you get 40TB of usable space. However, the actual usable space is always less due to ZFS’s own metadata, checksums, and other structural requirements. Our unraid zfs calculator accounts for this overhead to give you a much more realistic figure.
Unraid ZFS Calculator Formula and Mathematical Explanation
The calculation for ZFS usable capacity involves several steps. While the exact on-disk format is incredibly complex, we can use a reliable formula for estimation purposes. The core of this unraid zfs calculator is based on this logic.
- Calculate Total Raw Capacity: This is the simplest step. It’s the total number of disks multiplied by the individual disk size.
Formula: Raw Capacity = Number of Disks × Disk Size - Determine Parity Disks: The number of disks dedicated to parity is determined by the chosen RAIDZ level.
- RAIDZ1: 1 parity disk
- RAIDZ2: 2 parity disks
- RAIDZ3: 3 parity disks
- Calculate Pre-Overhead Capacity: Subtract the parity capacity from the raw capacity.
Formula: Pre-Overhead Capacity = (Number of Disks – Parity Disks) × Disk Size - Apply ZFS Overhead: ZFS itself consumes space for metadata, checksums, slop space, and other internal operations. This overhead is not a fixed percentage but generally falls between 2-5% for large files. Our unraid zfs calculator uses a conservative estimate of 3% for general use cases.
Formula: Estimated Usable Capacity = Pre-Overhead Capacity × (1 – ZFS Overhead %)
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Number of Disks | Total physical drives in the vdev | Count | 3 – 12 |
| Disk Size | Capacity of a single drive | Terabytes (TB) | 2 – 22 |
| Parity Disks | Disks used for data redundancy | Count | 1, 2, or 3 |
| ZFS Overhead | Space used by the filesystem itself | Percentage (%) | 2% – 5% |
Practical Examples (Real-World Use Cases)
Example 1: Balanced Home Media Server
A user wants to build a reliable server for their Plex library. They have 6 x 12TB hard drives and want good fault tolerance without sacrificing too much capacity. They use the unraid zfs calculator to compare options.
- Inputs: 6 Disks, 12 TB Size, RAIDZ2
- Calculation:
- Raw Capacity: 6 * 12 TB = 72 TB
- Parity Disks: 2
- Pre-Overhead Capacity: (6 – 2) * 12 TB = 48 TB
- Estimated Usable Capacity: 48 TB * 0.97 ≈ 46.56 TB
- Interpretation: The user will have approximately 46.56 TB of usable space and the pool can withstand the failure of any two disks, which is great for data security.
Example 2: High-Capacity Archive Build
A data hoarder is building a large archive and has acquired 10 x 20TB drives. Their priority is maximizing storage space while still having single-disk redundancy. The unraid zfs calculator helps them see the final numbers.
- Inputs: 10 Disks, 20 TB Size, RAIDZ1
- Calculation:
- Raw Capacity: 10 * 20 TB = 200 TB
- Parity Disks: 1
- Pre-Overhead Capacity: (10 – 1) * 20 TB = 180 TB
- Estimated Usable Capacity: 180 TB * 0.97 ≈ 174.6 TB
- Interpretation: They get a massive 174.6 TB of space, but the pool can only tolerate a single drive failure. For an archive of this size, RAIDZ2 would generally be recommended for better protection during a rebuild.
How to Use This Unraid ZFS Calculator
Using this unraid zfs calculator is straightforward. Follow these steps to accurately plan your storage pool:
- Enter the Number of Disks: Input the total count of drives you will be using for this specific ZFS pool.
- Enter the Disk Size: Provide the capacity of an individual drive in Terabytes (TB). This assumes all drives in the vdev are the same size.
- Select the RAIDZ Level: Choose between RAIDZ1, RAIDZ2, or RAIDZ3 from the dropdown menu. Your choice depends on your desired balance between capacity and data redundancy.
- Review the Results: The calculator will instantly update, showing the ‘Estimated Usable Capacity’ as the primary result. You can also see the ‘Total Raw Capacity’, ‘Parity/Redundancy’ allocation, and ‘Fault Tolerance’ (the number of disks that can fail without data loss).
- Analyze the Chart and Table: Use the visual chart and summary table to better understand how your storage is allocated. This is a key feature of our unraid zfs calculator. For more insights on Unraid, check out our Unraid setup guide.
Key Factors That Affect Unraid ZFS Results
The results from any unraid zfs calculator are influenced by several critical factors. Understanding them is key to effective storage planning.
- RAIDZ Level: This is the most significant factor. RAIDZ1 provides the most space but least protection. RAIDZ3 provides the most protection but sacrifices the most space to parity.
- Number of Disks: More disks mean more raw capacity, but also affect the ratio of data-to-parity disks. A 4-disk RAIDZ2 has 50% data efficiency, while a 10-disk RAIDZ2 has 80% data efficiency.
- Disk Size: Larger drives provide more capacity overall, but they also mean longer rebuild (resilvering) times when a disk fails, increasing the risk of a second failure during the process.
- ZFS Overhead: As mentioned, ZFS reserves space for its own operations. This includes metadata, which describes the filesystem structure, and checksums for data integrity. The overhead is higher for pools storing many small files versus large media files. See our ZFS vs BTRFS comparison for more detail.
- Record Size (Block Size): While not an input in this simplified unraid zfs calculator, the `recordsize` property in ZFS can impact efficiency. A mismatch between the record size and the size of files being stored can lead to wasted space.
- Slop Space: ZFS reserves a small amount of space (around 3%) called “slop space” to prevent the pool from becoming 100% full, which would cause major performance issues. This is factored into our overhead estimate.
Frequently Asked Questions (FAQ)
1. Why is the usable capacity not simply (Total Disks – Parity Disks) * Size?
Because ZFS, like any advanced filesystem, requires its own space for metadata, checksums, and management data structures. This overhead ensures data integrity and enables features like self-healing. Our unraid zfs calculator includes a conservative estimate for this overhead. Thinking about your media? Our guide to setting up a Plex Media Server on Unraid can help.
2. Which RAIDZ level should I choose?
For pools with up to 6 drives, RAIDZ1 is often acceptable. For pools with 6-12 drives, RAIDZ2 is highly recommended to protect against a second drive failure during a long rebuild. RAIDZ3 is typically used for very large arrays (12+ disks) where the risk and time of rebuild are significant.
3. Can I mix and match disk sizes in a ZFS pool?
While Unraid’s main array is famous for allowing mixed disk sizes, a single ZFS vdev (pool) is not as flexible. All disks in a single vdev should be the same size. If you use different sizes, ZFS will treat all disks as if they are the size of the smallest disk in the vdev.
4. What happens if I add more disks later?
You cannot expand a RAIDZ vdev by adding a single disk. To expand your pool’s capacity, you must add another vdev of the same type (e.g., add a second 6-disk RAIDZ2 vdev to your existing one). Planning for this is crucial and a good unraid zfs calculator is your first step.
5. Does this calculator work for mirrored vdevs?
This specific unraid zfs calculator is focused on RAIDZ configurations. For a simple two-disk mirror, your usable capacity would be the size of one disk. For multiple mirrored pairs, it would be the sum of one disk from each pair. To pick the right hardware, see our recommendations for the best NAS HDDs.
6. How accurate is this unraid zfs calculator?
This calculator provides a very close estimate for planning purposes, suitable for most use cases, especially those involving large media files. The actual, byte-for-byte capacity can vary slightly due to factors like record size and the exact version of ZFS being used.
7. What is ‘slop space’?
Slop space is a small percentage of the pool (about 1/32 or ~3%) that ZFS reserves to prevent fragmentation and ensure it can always perform critical writes even when the pool is nearly full. It’s a key reason why usable capacity is lower than expected.
8. Why should I use ZFS on Unraid?
ZFS offers superior data integrity protection (bit rot detection and correction), snapshots, and robust RAID options, making it ideal for cache pools or for dedicated pools for important data like virtual machines or critical documents. Many users are moving to ZFS for their cache pools for better performance and reliability. Join the discussion on our community storage forum.