RAID ZFS Calculator

Estimate the usable space and storage efficiency of your ZFS vdev.

What is a RAID ZFS Calculator?

A raid zfs calculator is a specialized tool designed for system administrators, data enthusiasts, and anyone using the ZFS filesystem to determine the practical, usable storage capacity of a ZFS “vdev” (Virtual Device). Unlike simple multiplication, calculating ZFS storage isn’t straightforward due to parity data. ZFS uses a variant of RAID called RAID-Z, which requires a certain number of disks’ worth of space for data redundancy. This calculator helps you plan your storage array by showing you exactly how much space you’ll have for your files versus how much is dedicated to protecting you from disk failure.

This tool is essential for planning a file server, Network Attached Storage (NAS) build (like TrueNAS or Proxmox), or any critical data repository. It removes the guesswork, preventing you from purchasing a set of drives only to discover your usable capacity is much lower than you anticipated. Understanding the output of a raid zfs calculator is the first step in building a reliable and efficient ZFS pool.

The RAID ZFS Formula and Explanation

The core calculation for RAID-Z is beautifully simple, yet powerful. The calculator uses this formula to determine your usable space:

Usable Capacity = (N – P) × S

This formula is the foundation of planning any ZFS vdev. Let’s break down the variables:

Variable	Meaning	Unit	Typical Range
N	Total Number of Disks	Unitless (count)	3 to 12+
P	Parity Disks	Unitless (count)	1 (RAID-Z1), 2 (RAID-Z2), or 3 (RAID-Z3)
S	Size of Smallest Disk	TB or GB	1 TB to 22+ TB

Essentially, you take your total number of disks (N), subtract the number of disks dedicated to parity (P), and multiply the result by the size of a single disk (S). The result is the raw usable space *before* ZFS filesystem overhead is considered. For more detail, you might explore our guide on ZFS best practices, which covers related topics.

Practical Examples

Example 1: Home Media Server (RAID-Z1)

A common home-user setup is a small array for media files, prioritizing capacity over extreme redundancy.

• Inputs: 5 disks, 8 TB each, RAID-Z1 (1 parity disk)
• Calculation: (5 – 1) × 8 TB = 32 TB
• Result: This setup provides 32 TB of usable space and can survive the failure of any single disk. The storage efficiency is 80%.

Example 2: Small Business Critical Data (RAID-Z2)

A small business needs to protect critical client data and can’t afford downtime. They opt for higher redundancy.

• Inputs: 8 disks, 12 TB each, RAID-Z2 (2 parity disks)
• Calculation: (8 – 2) × 12 TB = 72 TB
• Result: This array yields 72 TB of usable space and can withstand the failure of any two disks simultaneously, providing excellent data protection. This is a key difference when comparing RAID-Z vs RAID 10.

How to Use This RAID ZFS Calculator

Using our raid zfs calculator is a simple, three-step process to help you design your storage array:

1. Enter Your Disk Configuration: Start by inputting the total number of physical hard drives in your planned vdev into the “Number of Disks” field. Then, enter the size of a single drive in the “Size of a Single Disk” field and select the correct unit (TB or GB).
2. Select Your RAID-Z Level: Choose your desired level of data protection from the “RAID-Z Level” dropdown. RAID-Z1 is a good baseline, while Z2 and Z3 offer significantly more protection at the cost of usable space.
3. Interpret the Results: The calculator instantly updates. The “Available Usable Space” is your primary result. You can also analyze the “Storage Efficiency” to see what percentage of your raw storage is usable, the “Fault Tolerance” to confirm how many disks can fail, and the “Space Lost to Parity.”

For those new to ZFS, understanding what a vdev is can provide crucial context for these results.

Key Factors That Affect RAID ZFS Calculations

RAID-Z Level

This is the most significant factor. Choosing RAID-Z2 over Z1 doubles your parity overhead, reducing usable space but doubling your fault tolerance. Your choice is a direct trade-off between capacity and security.

Number of Disks

As you add more disks to a vdev, the relative cost of parity decreases. For example, in a 4-disk RAID-Z1, 25% of space is parity. In a 10-disk RAID-Z1, only 10% is parity, making the array more efficient.

Disk Size Consistency

ZFS stripes data across all disks in a vdev. If you use disks of mixed sizes, ZFS will treat all of them as if they are the size of the *smallest* disk in the array. This is why using identical disks is highly recommended to avoid wasting space.

Filesystem Overhead

This calculator shows raw usable capacity. The actual space reported by your operating system will be slightly less (typically 1-5%) due to ZFS’s own metadata, checksums, and management structures. This is a normal part of how ZFS guarantees data integrity.

Ashift Property (Sector Size)

While not an input in this calculator, setting the correct `ashift` value (e.g., `ashift=12` for 4K native drives) is crucial for performance. A mismatch won’t change the calculated capacity but can severely degrade read/write speeds. For more advanced users, our guide to ZFS performance tuning is a must-read.

Reserved Space

It’s a common practice to reserve a small percentage of pool space (e.g., `zfs set reservation…`) to ensure ZFS can perform administrative tasks even when the pool is nearly full. This reserved space is subtracted from the user-available capacity.

Frequently Asked Questions (FAQ)

1. Why can’t I use just two disks for RAID-Z1?

RAID-Z1 requires at least 3 disks: two for data striping and one for parity. With only two disks, you could use a ZFS mirror (like RAID 1), but not RAID-Z.

2. Can I mix disk sizes in a RAID-Z vdev?

You can, but it is highly discouraged. ZFS will only use the capacity of the smallest disk across all disks in the vdev. For example, if you have five 8 TB disks and one 4 TB disk, ZFS will treat all six disks as 4 TB drives, wasting a significant amount of capacity.

3. What is the difference between RAID-Z and traditional RAID-5/6?

The main difference is the elimination of the “RAID write hole,” where a power loss during a write can lead to corrupted parity. ZFS’s copy-on-write nature prevents this entirely, making RAID-Z significantly safer.

4. Why is the usable space from the raid zfs calculator different from what my OS shows?

There are two primary reasons: filesystem overhead (ZFS metadata, checksums) and unit differences (Terabytes vs. Tebibytes). Our calculator uses decimal units (1 TB = 1000^4 bytes), whereas many operating systems use binary units (1 TiB = 1024^4 bytes), which results in a smaller number for the same amount of storage.

5. Is RAID-Z a backup?

No. RAID-Z provides redundancy against hardware failure (disk failure). It does not protect against user error (accidental deletion), malware (ransomware), software bugs, or catastrophic events like fire or theft. You still need a separate backup strategy.

6. Can I add a single disk to an existing RAID-Z vdev to expand it?

No, this is a fundamental limitation. You cannot expand a RAID-Z vdev by adding one disk at a time. To expand capacity, you must either replace each disk one-by-one with a larger disk (and let it resilver each time) or create a new vdev and add it to your ZFS pool. This is a key step in a TrueNAS SCALE setup.

7. Which RAID-Z level should I choose?

For general home use with 3-6 disks, RAID-Z1 is often sufficient. For important data or arrays with 6-8+ disks, RAID-Z2 is strongly recommended. RAID-Z3 is typically reserved for very large arrays (12+ disks) where the chance of multiple simultaneous failures during a long rebuild process is a real concern.

8. How many disks do I need for each level?

The minimums are: 3 disks for RAID-Z1, 4 disks for RAID-Z2, and 5 disks for RAID-Z3. However, using more disks is generally more space-efficient.