ZFS Pool Calculator
Estimate usable capacity, efficiency, and fault tolerance for your ZFS storage array.
Configuration
Capacity Breakdown
A visual comparison of usable storage versus capacity lost to parity/mirroring.
RAID Level Comparison
| RAID Level | Usable Capacity | Efficiency | Fault Tolerance | Min. Disks |
|---|
Comparative analysis for the current disk configuration.
What is a ZFS Pool Calculator?
A zfs pool calculator is an essential tool for system administrators, storage enthusiasts, and anyone planning to deploy a ZFS-based storage solution. It helps you estimate the amount of usable disk space you will have after accounting for redundancy (parity or mirroring). Unlike traditional filesystems, ZFS integrates the volume manager, meaning the way you group your disks (your “pool” configuration) directly impacts capacity, performance, and data safety. This calculator simplifies the complex planning process by providing clear estimates for different RAID levels.
Anyone building a Network Attached Storage (NAS) device, a file server, or a backup system with ZFS should use a zfs pool calculator. A common misconception is that if you buy 10 disks of 4TB each, you get 40TB of storage. This is only true for non-redundant arrays (like RAID 0 or “stripe”), which offer no protection against disk failure. A proper zfs pool calculator shows the trade-off between raw capacity and the fault tolerance provided by RAID-Z1, RAID-Z2, RAID-Z3, or mirroring. For more information on ZFS basics, see our guide on what ZFS is.
ZFS Pool Calculator Formula and Mathematical Explanation
The core calculation for a zfs pool calculator depends on the chosen RAID level. The formulas provide a close approximation, though actual usable space can be slightly less due to metadata overhead (known as “slop space,” typically around 3%).
- RAID-Z1 (Single Parity): Requires a minimum of 2, but practically 3, disks. It uses the capacity of one disk for parity information. The formula is:
Usable Capacity = (N - 1) * S - RAID-Z2 (Double Parity): Requires a minimum of 3, but practically 4, disks. It uses two disks’ worth of capacity for parity, allowing it to survive two simultaneous disk failures. The formula is:
Usable Capacity = (N - 2) * S - RAID-Z3 (Triple Parity): Requires a minimum of 4, but practically 5, disks. It uses three disks’ worth of capacity for parity, providing extreme fault tolerance. The formula is:
Usable Capacity = (N - 3) * S - Mirror (Striped Pairs): This configuration pairs disks up into mirrors. For every pair, the capacity is that of a single disk. The formula assumes an even number of disks for maximum efficiency.
Usable Capacity = (N / 2) * S - Stripe (RAID 0): This level offers no redundancy and simply combines all disks.
Usable Capacity = N * S
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| N | Total number of disks in the vdev | Count | 2 – 24+ |
| S | Size of a single disk | Terabytes (TB) | 1 – 22+ |
| P | Number of parity disks (1 for Z1, 2 for Z2, 3 for Z3) | Count | 1, 2, or 3 |
Practical Examples of ZFS Pool Calculations
Example 1: Home Media Server (Balanced)
An enthusiast is building a home NAS for media streaming and backups. They choose 5 disks of 8TB each and opt for RAID-Z2 for a good balance of capacity and protection against two drive failures.
- Inputs: N=5, S=8TB, Level=RAID-Z2
- Raw Capacity: 5 * 8 TB = 40 TB
- Calculation: (5 – 2) * 8 TB = 24 TB
- Output: The user gets approximately 24 TB of usable space. This is a robust setup for home use. A proper zfs raid calculator helps confirm this choice.
Example 2: Small Business File Server (High Redundancy)
A small business needs a highly reliable file server. They purchase 10 disks of 4TB each and decide to use striped mirrors for high I/O performance and simple recovery.
- Inputs: N=10, S=4TB, Level=Mirror
- Raw Capacity: 10 * 4 TB = 40 TB
- Calculation: (10 / 2) * 4 TB = 20 TB
- Output: The business gets 20 TB of very fast and redundant storage. Each of the 5 mirrored pairs can lose one drive without data loss. Planning this with a zfs pool calculator is crucial for budget approval.
How to Use This ZFS Pool Calculator
Using this zfs pool calculator is straightforward. Follow these steps to plan your storage array effectively.
- Enter Number of Disks: Input the total count of physical hard drives or SSDs you intend to use in a single virtual device (vdev).
- Enter Single Disk Size: Provide the capacity of one of your disks in Terabytes (TB). ZFS pools work best when all disks in a vdev are the same size.
- Select RAID Level: Choose the desired redundancy level from the dropdown menu. This is the most critical decision, affecting capacity, performance, and fault tolerance.
- Review the Results: The calculator instantly updates the “Estimated Usable Capacity,” “Total Raw Capacity,” “Storage Efficiency,” and “Fault Tolerance.”
- Analyze the Comparison Table & Chart: Use the dynamic chart and table below the main results to see how different RAID levels would perform with your exact disk setup. This is invaluable for making an informed decision without having to re-enter the data. For a deeper dive, consider our guide on TrueNAS setup which utilizes ZFS.
Key Factors That Affect ZFS Pool Calculator Results
The output of a zfs pool calculator is influenced by several key decisions. Understanding them is vital for designing an optimal storage system.
- 1. RAID Level (Redundancy Method)
- This is the most significant factor. RAID-Z1 offers the most space but only protects against one disk failure. RAID-Z2 and Z3 offer more protection at the cost of capacity. Mirrors offer great performance and simple expansion but have a 50% efficiency. Choosing between raidz vs mirror is a common planning step.
- 2. Number of Disks (Vdev Width)
- More disks generally mean more capacity and potentially more performance. However, for RAID-Z levels, performance doesn’t scale linearly within a single vdev. The number of disks also dictates which RAID-Z levels are possible.
- 3. Disk Size
- This directly scales the total raw and usable capacity. If you use disks of different sizes in the same vdev, ZFS will treat all of them as if they are the size of the smallest disk, wasting space on the larger ones.
- 4. Number of Vdevs
- Advanced ZFS pools can be built by striping multiple vdevs together (e.g., striping three separate RAID-Z1 vdevs). This calculator focuses on a single vdev, but for complex builds, performance and capacity scale with the number of vdevs.
- 5. ZFS Metadata Overhead (Slop Space)
- ZFS reserves a small percentage of the pool’s space for its metadata and to maintain performance as the pool fills up. Our zfs pool calculator provides a close estimate, but expect the final reported OS capacity to be about 2-5% smaller due to this overhead.
- 6. Ashift (Sector Size)
- Using the correct sector size (ashift=12 for 4K native drives) is crucial for performance. While it doesn’t drastically change the total capacity calculation in this high-level zfs pool calculator, misalignment can severely degrade I/O performance.
Frequently Asked Questions (FAQ)
1. Why is the usable capacity from the zfs pool calculator less than my total disk space?
This is due to data redundancy. ZFS reserves space on your disks to store parity data (in RAID-Z) or a full copy of the data (in a mirror). This reserved space is used to reconstruct data if one or more disks fail, which is a core feature of ZFS.
2. Which RAID level is best?
It depends on your needs. RAID-Z1 is okay for home use with 3-5 drives. RAID-Z2 is recommended for 6-12 drives as it protects against two failures. Mirrors are best for performance-critical tasks like running virtual machines. Our ZFS vs BTRFS comparison provides more context on filesystem choices.
3. Can I use disks of different sizes in my ZFS pool?
Yes, but it’s not recommended within the same vdev. The vdev will treat all disks as being the size of the smallest disk, wasting capacity on the larger ones. It’s better to create separate pools or vdevs for different-sized disks.
4. How many disks do I need for RAID-Z?
For RAID-Z1, you need at least 3 disks. For RAID-Z2, at least 4. For RAID-Z3, at least 5. Using more disks than the minimum is generally better for efficiency.
5. Does this zfs pool calculator account for “slop space”?
This calculator provides a high-level estimate based on the parity/mirroring formula. It does not subtract the ~3% overhead from ZFS’s internal metadata (“slop space”), so your final operating system-reported capacity will be slightly lower than the number shown here.
6. What is the difference between a zfs pool calculator and a zfs raid calculator?
The terms are often used interchangeably. A “pool” is ZFS’s term for a storage volume created from one or more vdevs (virtual devices), and those vdevs are typically configured in a RAID-like manner (RAID-Z, mirror, etc.). So, a storage efficiency calculator for ZFS is essentially calculating the properties of a RAID configuration.
7. How do I expand my ZFS pool?
Expanding ZFS pools can be complex. The easiest way is to replace each disk in a vdev one by one with a larger disk, resilvering (rebuilding) the data after each replacement. Once all disks are replaced, the pool’s capacity can be expanded. Alternatively, you can add a new vdev to an existing pool, striping the new vdev with the old one.
8. What happens if a disk fails?
If you have a redundant configuration (RAID-Z or mirror) and the number of failed disks is within your fault tolerance limit, your data remains safe. You must replace the failed disk and trigger a “resilver” process, where ZFS rebuilds the data onto the new disk using the parity information.