Zfs Pool Calculator

An expert tool for calculating usable storage space in ZFS.

Calculate Your ZFS Pool Storage

Comparison of different RAID-Z levels with your current disk configuration.

RAID Level	Usable Capacity	Efficiency	Fault Tolerance	Minimum Disks

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What is a ZFS Pool Calculator?

A zfs pool calculator is a specialized tool designed to help system administrators, storage enthusiasts, and IT professionals estimate the amount of usable storage space available in a ZFS (Zettabyte File System) storage pool. Unlike simple disk addition, ZFS combines disks into resilient and high-performance arrays called “vdevs” using different redundancy strategies like RAID-Z. This calculator simplifies the complex math involved in determining the final capacity after accounting for data parity and redundancy.

Who Should Use a ZFS Pool Calculator?

Anyone planning to build or expand a server, Network Attached Storage (NAS), or data archive using ZFS should use a zfs pool calculator. This includes home lab users building their first TrueNAS or Proxmox server, as well as enterprise administrators designing large-scale storage systems. It’s an indispensable planning tool to balance cost, performance, and data safety. Planning your array with a zfs usable capacity calculator ensures you purchase the right number of disks and avoid surprises.

Common Misconceptions

A frequent misconception is that total storage is simply the sum of all disk capacities. However, ZFS uses some disk space for parity (in RAID-Z) or for copies (in mirrors) to protect against disk failure. Therefore, the usable capacity is always less than the total raw capacity. Another point of confusion is thinking RAID-Z is the same as traditional RAID. While similar, RAID-Z from ZFS eliminates the “RAID write hole,” providing superior data integrity.

ZFS Pool Formula and Mathematical Explanation

The core function of a zfs pool calculator is to apply the correct formula based on the chosen redundancy level. The calculations vary significantly between different RAID-Z levels and mirrored setups.

Step-by-Step Derivation

• RAID-Z1 (Single Parity): This level dedicates the capacity of one disk to parity data. It can withstand the failure of a single disk. The formula is: Usable Capacity = (N – 1) * S, where N is the number of disks and S is the size of a single disk.
• RAID-Z2 (Double Parity): This level uses the capacity of two disks for parity, allowing it to survive the failure of up to two disks. The formula is: Usable Capacity = (N – 2) * S.
• RAID-Z3 (Triple Parity): The most resilient RAID-Z level, it uses three disks’ worth of capacity for parity and can withstand three simultaneous disk failures. The formula is: Usable Capacity = (N – 3) * S.
• Mirrored VDEV: In a simple mirror, every disk is duplicated. This offers high performance but only 50% storage efficiency. For an N-disk mirror, the formula is: Usable Capacity = (N / 2) * S. This calculator assumes you are creating N/2 pairs.

Variables Table

Variable	Meaning	Unit	Typical Range
N	Total number of disks in the vdev	Count	2 – 24+
S	Storage capacity 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 (Real-World Use Cases)

Using a zfs pool calculator helps visualize trade-offs. Let’s explore two common scenarios.

Example 1: Balanced Home NAS (RAID-Z2)

A user wants to build a reliable home media server and decides on a good balance of safety and capacity.

• Inputs: 6 Disks, 8 TB each, RAID-Z2
• Calculation: (6 – 2) * 8 TB = 32 TB
• Outputs:
  • Usable Capacity: 32.0 TB
  • Raw Capacity: 48.0 TB
  • Efficiency: 66.7%
  • Fault Tolerance: 2 Disks
• Interpretation: This setup provides a robust 32 TB of usable space. The user can lose any two disks without losing data, which is excellent for protecting valuable family photos and media. This demonstrates a common use for a raid-z calculator.

Example 2: High-Performance Video Editing Array (Mirror)

A video editor needs maximum read/write performance for editing 4K video files and prioritizes speed and redundancy over raw capacity.

• Inputs: 4 Disks, 10 TB each, Mirrored VDEV
• Calculation: (4 / 2) * 10 TB = 20 TB
• Outputs:
  • Usable Capacity: 20.0 TB
  • Raw Capacity: 40.0 TB
  • Efficiency: 50.0%
  • Fault Tolerance: 1 disk per pair
• Interpretation: While only half the raw capacity is usable, the mirrored setup offers superior I/O performance, which is critical for smooth video editing. This is a key consideration in discussions of raid-z vs mirror configurations.

How to Use This ZFS Pool Calculator

Our zfs pool calculator is designed for simplicity and accuracy. Follow these steps to plan your storage array.

1. Enter Number of Disks: Input the total quantity of physical drives you intend to use in your virtual device (vdev).
2. Enter Disk Size: Provide the capacity of a single drive in terabytes (TB). The calculator assumes all drives are of identical size.
3. Select Redundancy Level: Choose your desired ZFS redundancy level from the dropdown menu (RAID-Z1, RAID-Z2, RAID-Z3, or Mirror). This is the most critical decision affecting both zfs storage efficiency and data protection.
4. Review the Results: The calculator instantly updates, showing the primary result (Usable Capacity) and key metrics like Total Raw Capacity, Storage Efficiency, and Fault Tolerance.
5. Analyze the Comparison Table and Chart: The tools below the main result help you compare how different RAID levels would perform with your exact disk setup, aiding in making a fully informed decision.

Key Factors That Affect ZFS Pool Results

The results from any zfs pool calculator are influenced by several critical factors. Understanding them is key to designing an effective storage solution.

1. RAID-Z Level: As shown by the calculator, this is the most significant factor. RAID-Z1 offers the most space but least protection (1 disk failure). RAID-Z3 offers the least space but maximum protection (3 disk failures).
2. Number of Disks: The more disks in a RAID-Z vdev, the higher the storage efficiency. For example, in RAID-Z1, a 3-disk array has 66.7% efficiency, while a 9-disk array has 88.9% efficiency.
3. Disk Size: Larger individual disks provide a greater total raw and usable capacity, directly scaling the final numbers.
4. VDEV Layout: This calculator focuses on a single vdev. However, advanced pools can be constructed from multiple vdevs. For example, a pool of two separate 3-disk RAID-Z1 vdevs has different performance and capacity characteristics than one 6-disk RAID-Z1 vdev. Exploring a zfs fault tolerance guide is crucial for complex setups.
5. ZFS Overhead: ZFS reserves a small amount of space (often called “slop space”) to maintain performance and prevent fragmentation. Our calculator provides an estimate, but the exact amount can vary.
6. Compression: ZFS offers transparent compression, which can significantly increase effective storage capacity without a major performance penalty. While this zfs pool calculator shows physical capacity, enabling compression on your datasets can make your usable space go much further.

Frequently Asked Questions (FAQ)

1. What is the minimum number of disks for RAID-Z?

RAID-Z1 requires a minimum of 3 disks. RAID-Z2 requires a minimum of 4 disks. RAID-Z3 requires a minimum of 5 disks. A mirrored vdev requires a minimum of 2 disks.

2. Can I mix disk sizes in a ZFS pool?

While technically possible, it is highly discouraged. ZFS will treat all disks in a vdev as if they are the size of the smallest disk, wasting capacity on the larger drives. Always use identical disks for best results.

3. What happens if a disk fails?

If a disk fails in a redundant RAID-Z or mirror array, the pool will enter a “degraded” state but remain online and accessible. You must replace the failed disk and initiate a “resilver” process to rebuild the data and restore redundancy.

4. RAID-Z1 vs. RAID-Z2: Which is better?

For arrays with many large-capacity disks (e.g., 8+ disks of 10TB+), RAID-Z2 is strongly recommended. The long rebuild time (resilvering) of a large, degraded RAID-Z1 array puts significant stress on the remaining drives, increasing the risk of a second failure before the rebuild completes. The zfs usable capacity might be lower with RAID-Z2, but the data security is much higher.

5. Is RAID-Z the same as hardware RAID?

No. RAID-Z is a software-based solution integrated directly into the ZFS filesystem. This tight integration allows it to solve long-standing issues like the RAID-5 write hole, providing superior data integrity protection.

6. How much usable space do I really get?

Our zfs pool calculator provides a very close estimate of the usable physical space. The actual free space reported by your OS may be slightly different due to metadata, slop space, and whether you are measuring in terabytes (TB) or tebibytes (TiB).

7. Can I add a disk to an existing RAID-Z vdev?

No, you cannot expand a RAID-Z vdev by adding a single disk. You must replace each disk with a larger one and let it resilver, or you can add a whole new vdev to the pool. This is a fundamental aspect of how to build a zfs pool.

8. Why is storage efficiency so low on mirrors?

A mirror provides redundancy by making an exact 1-to-1 copy of your data on another disk. This means for every 2 disks you add, you only get the capacity of 1, resulting in 50% efficiency. The trade-off is much faster performance, especially for writes.