Raidz2 Capacity Calculator

This raidz2 capacity calculator is a specialized tool for planning your ZFS storage array. Enter your disk specifications below to get a precise estimate of usable storage space, accounting for double-parity overhead.

What is a RAID-Z2 Capacity Calculator?

A raidz2 capacity calculator is a specialized utility designed to determine the actual usable storage space in a ZFS storage pool configured with RAID-Z2. Unlike generic RAID calculators, it specifically accounts for the double-parity mechanism inherent to RAID-Z2, where the capacity of two full drives is dedicated to data redundancy. This allows the storage array to withstand the failure of any two disks without losing data.

This tool is essential for system administrators, data hoarders, and IT professionals who are planning to build a ZFS-based server or NAS (Network Attached Storage). By inputting the number of drives and their capacity, users can instantly see how much space will be available for their files versus how much will be used for fault tolerance. This foresight is critical for budget planning and ensuring the final storage pool meets capacity requirements. A reliable raidz2 capacity calculator is the first step in proper ZFS storage pool design.

Common Misconceptions

A frequent misunderstanding is that RAID-Z2 is a backup. It is not. RAID-Z2 is a form of redundancy or fault tolerance; it protects your data from hardware failure (disk failure), not from file corruption, accidental deletion, malware, or catastrophic events like fire or theft. A robust data protection strategy always includes a separate, and preferably offsite, backup solution in addition to any RAID configuration.

RAID-Z2 Formula and Mathematical Explanation

The core calculation for a raidz2 capacity calculator is straightforward and effective. It follows a simple subtraction principle to account for the double-parity data protection scheme.

The formula is:

Usable Capacity = (N - 2) × D

This formula provides a clear picture of the trade-off between storage space and data security. For every RAID-Z2 vdev (virtual device), you sacrifice the capacity of two disks to gain protection against two drive failures. This is a fundamental aspect of its design, differing from RAID-Z1 (one parity disk) and RAID-Z3 (three parity disks).

Table of Variables for the RAID-Z2 Capacity Formula

Variable	Meaning	Unit	Typical Range
N	Total Number of Disks	Count (integer)	4 – 24
D	Capacity of Smallest Disk	TB or GB	1 TB – 24 TB
Usable Capacity	Final storage space available	TB or GB	Varies based on inputs
Parity Capacity	Space reserved for redundancy (2 × D)	TB or GB	Varies based on D

Practical Examples (Real-World Use Cases)

Example 1: Home Media Server

A user is building a home NAS for storing media files, documents, and personal backups. They have purchased 6 hard drives, each with a capacity of 8 TB.

• Inputs for raidz2 capacity calculator:
  • Number of Disks (N): 6
  • Disk Size (D): 8 TB
• Calculation:
  • Usable Capacity = (6 – 2) × 8 TB = 4 × 8 TB = 32 TB
  • Raw Capacity = 6 × 8 TB = 48 TB
  • Parity Overhead = 2 × 8 TB = 16 TB
• Interpretation: The user will have 32 TB of usable storage space. The system can survive the failure of any two of the six drives, making it a robust solution for protecting valuable family photos and videos. This is a far better choice than RAID-Z1 for arrays of this size, offering more peace of mind. For a deeper dive, comparing RAID 6 vs RAID-Z2 is a worthwhile read.

Example 2: Small Business File Server

A small business needs a reliable central file server for documents, project files, and client data. They opt for a server with 12 drive bays, populated with 16 TB enterprise drives.

• Inputs for raidz2 capacity calculator:
  • Number of Disks (N): 12
  • Disk Size (D): 16 TB
• Calculation:
  • Usable Capacity = (12 – 2) × 16 TB = 10 × 16 TB = 160 TB
  • Raw Capacity = 12 × 16 TB = 192 TB
  • Parity Overhead = 2 × 16 TB = 32 TB
• Interpretation: The business gets a massive 160 TB of highly-redundant storage. The double-parity protection is critical in a business environment where downtime and data loss can have significant financial consequences. The high storage efficiency (over 83%) makes this a cost-effective setup for a large volume of data. Understanding data redundancy explained in this context reinforces the value of RAID-Z2.

How to Use This RAID-Z2 Capacity Calculator

Using this raidz2 capacity calculator is a simple process designed to give you instant and accurate results for your ZFS storage planning.

1. Enter the Number of Disks: Input the total quantity of physical hard drives you plan to use in your RAID-Z2 vdev. The minimum required is 4.
2. Enter the Disk Size: Type in the capacity of a single drive. If you are using drives of different sizes, you MUST enter the size of the smallest drive in the set, as ZFS will treat all drives as if they are that size.
3. Select the Unit: Choose whether the disk size you entered is in Terabytes (TB) or Gigabytes (GB).
4. Review the Results: The calculator will instantly update.
   • Total Usable Capacity: This is the primary result, showing the actual storage space you will have for your data.
   • Total Raw Capacity: The sum of the capacities of all disks combined.
   • Parity/Redundancy: The amount of space (equal to two disks) reserved for fault tolerance.
   • Storage Efficiency: The percentage of raw capacity that is usable for data storage.
5. Analyze the Chart: The dynamic chart provides a quick visual representation of how your total storage is divided between usable space and parity overhead.

This tool is invaluable during the initial phases of a FreeNAS setup guide or TrueNAS deployment, ensuring you make informed decisions before purchasing hardware.

Key Factors That Affect RAID-Z2 Results

While the basic formula is simple, several underlying factors influence the effective capacity and performance of a RAID-Z2 pool. A good raidz2 capacity calculator provides the baseline, but understanding these elements is key for expert-level planning.

1. Number of Disks in the VDEV

The more disks you add to a RAID-Z2 vdev, the higher your storage efficiency becomes. For example, a 4-disk array has 50% efficiency ((4-2)/4), while a 10-disk array has 80% efficiency ((10-2)/10). However, wider vdevs can have longer resilver (rebuild) times and potentially lower IOPS performance for random workloads.

2. Mixed Disk Sizes

ZFS is flexible and allows you to build a vdev with different-sized disks. However, it will treat all disks in the vdev as having the capacity of the *smallest* disk. If you have five 10 TB drives and one 8 TB drive in a RAID-Z2 vdev, ZFS will treat it as an array of six 8 TB drives. This is why using identical drives is highly recommended to avoid wasting capacity.

3. ZFS Metadata Overhead (“Slop Space”)

ZFS reserves a small portion of the pool’s total space (around 3%) for its own metadata and to prevent the pool from becoming 100% full, which would severely degrade performance. This “slop space” is not available for user data, so the actual effective capacity will always be slightly less than what the simple formula suggests. This calculator focuses on the pre-overhead capacity for clear planning.

4. Ashift (Sector Size)

The `ashift` property tells ZFS the physical sector size of the underlying disks (e.g., `ashift=12` for 4K native disks). Using the correct `ashift` value is critical for performance. A mismatch can lead to severe write amplification, hurting both performance and disk endurance. While it doesn’t directly change the calculated capacity, it dramatically impacts the pool’s health and speed. Detailed TrueNAS SCALE performance guides emphasize setting this correctly at pool creation.

5. Record Size (Block Size)

ZFS writes data in blocks, and the `recordsize` (default is 128KB) can have a major impact on storage efficiency, especially with compression. If you store many small files (e.g., a few KBs each), each file will still occupy a full block on disk, leading to wasted space. Tailoring the `recordsize` to your workload can improve both performance and capacity utilization.

6. Compression

One of ZFS’s most powerful features is transparent compression. Enabling compression (e.g., LZ4, which is fast and recommended) can significantly increase your effective storage capacity. If your data is highly compressible (like text documents or logs), you could see your effective capacity increase by 20-50% or more. This gain is not reflected in a basic raidz2 capacity calculator but is a crucial real-world factor.

Frequently Asked Questions (FAQ)

1. What is the absolute minimum number of disks for RAID-Z2?

The minimum number of disks required to create a RAID-Z2 vdev is four. With four disks, two are used for data and two for parity, resulting in 50% storage efficiency but providing excellent protection against two drive failures.

2. Can I add a single disk to an existing RAID-Z2 vdev to expand it?

No, this is a common point of confusion. You cannot expand a RAID-Z vdev by adding a single disk. To increase the capacity of a RAID-Z pool, you must either replace each disk one-by-one with a larger disk (letting it resilver each time) or add a completely new vdev to the pool.

3. RAID-Z2 vs. RAID-Z3: When should I choose three parity disks?

RAID-Z3 provides triple-parity, meaning it can withstand three simultaneous drive failures. It is generally recommended for very wide vdevs (e.g., 12+ disks) or for mission-critical data where the risk of a third drive failing during a long resilver process is a major concern. For most home and small business use cases with 6-10 disks, RAID-Z2 offers an excellent balance of safety and capacity.

4. Why does this raidz2 capacity calculator not ask about vdevs?

This calculator is focused on calculating the capacity of a single RAID-Z2 vdev. If your storage pool consists of multiple vdevs (e.g., two separate 6-disk RAID-Z2 vdevs), you would use the calculator for one vdev and then multiply the results by the number of identical vdevs in your pool.

5. What happens if I use disks of different speeds (e.g., 5400 RPM and 7200 RPM)?

The performance of the entire vdev will generally be limited by the slowest disk in the array. For consistent and predictable performance, it is highly recommended to use disks with the same rotational speed and from the same model line.

6. Is RAID-Z2 better than RAID 6?

Functionally, both RAID-Z2 and RAID 6 offer double parity protection. However, RAID-Z2 is integrated into the ZFS filesystem, which protects it from the “RAID write hole” and provides superior data integrity features like self-healing data scrubbing. For this reason, RAID-Z2 is generally considered more robust than traditional hardware RAID 6.

7. How much performance is lost during a resilver (rebuild)?

During a resilver, the system reads from all healthy disks and writes to the new disk to rebuild the lost data. This is an I/O-intensive process that will degrade the overall performance of the pool, especially for read operations. The impact depends on the system’s CPU, the width of the vdev, and the workload. Using a powerful CPU can help mitigate the performance hit.

8. Should I use a hot spare with RAID-Z2?

A hot spare is a drive that sits idle until another drive in the pool fails, at which point it automatically takes its place and starts the resilver process. With RAID-Z2’s double-parity, the need for an immediate rebuild is less urgent than with single-parity RAID-Z1. Many experts argue that the capacity is better used in a wider vdev or that a “cold spare” (a new drive on a shelf) is sufficient, but this depends on your risk tolerance and how quickly you can physically replace a failed drive.