RAID-Z2 Capacity Calculator
Determine the usable storage of your ZFS RAID-Z2 array quickly and accurately.
Enter the total number of physical disks in the vdev. Minimum is 4.
Enter the size of a single disk. Assumes all disks are the same size.
Select the unit as advertised by the manufacturer (1 TB = 1000 GB).
Capacity Breakdown
Usable Space
Parity Overhead
What is a RAID-Z2 Capacity Calculator?
A raidz2 capacity calculator is a specialized tool designed to determine the effective storage space available in a ZFS storage pool configured with RAID-Z2. Unlike standard RAID levels, RAID-Z2 is an integral part of the ZFS filesystem, offering enhanced data protection through a double-parity scheme. This calculator simplifies the complex task of figuring out exactly how much of your total raw disk space you can actually use for your data after accounting for this redundancy.
This tool is essential for system administrators, IT professionals, and data hoarders who are planning to build a NAS (Network Attached Storage) or server using TrueNAS, FreeNAS, or any other ZFS-based system. By inputting the number and size of your disks, you can accurately plan your storage needs, understand the overhead, and make informed purchasing decisions. For those evaluating different storage setups, a zfs storage calculator provides a broader view of various ZFS configurations.
The RAID-Z2 Capacity Formula
The calculation for RAID-Z2 is straightforward and analogous to traditional RAID 6. The system reserves the capacity of two full disks for parity information, which allows the array to withstand the failure of any two disks without data loss. The formula is:
Usable Capacity = (N – 2) × S
Where:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| N | Total Number of Disks | Count (unitless) | 4 or more |
| S | Size of the Smallest Disk | TB or GB | 1 TB – 24 TB+ |
| Usable Capacity | The final storage space available for data. | TB or GB | Depends on N and S |
It’s critical to note that ZFS uses the size of the smallest disk in the vdev for this calculation. If you mix disks of different sizes, the capacity of the larger disks will be underutilized. Therefore, using identically sized disks is highly recommended for any raidz2 capacity calculator or real-world setup.
Practical Examples
Example 1: A Small Home NAS
Imagine you are building a small, robust NAS for home use and have purchased 5 disks.
- Inputs: 5 disks, each 4 TB in size.
- Calculation: (5 – 2) × 4 TB = 12 TB
- Results: You will have 12 TB of usable storage, with 8 TB (2 disks × 4 TB) dedicated to parity for data protection. The total raw capacity is 20 TB.
Example 2: A Business Storage Array
A small business needs a larger storage pool for file sharing and backups. They decide on a setup with 12 disks.
- Inputs: 12 disks, each 10 TB in size.
- Calculation: (12 – 2) × 10 TB = 100 TB
- Results: The business will have 100 TB of usable space. The array can survive any two disks failing simultaneously, making it a very reliable solution. Understanding what is raidz2 in detail helps in appreciating this level of protection.
How to Use This RAID-Z2 Capacity Calculator
Using our calculator is simple and intuitive. Follow these steps to get an accurate estimate of your storage potential:
- Enter the Number of Disks: Input the total count of drives you plan to use in your RAID-Z2 vdev. Remember, the minimum for RAID-Z2 is 4 disks.
- Enter the Disk Size: Provide the capacity of a single drive. For best results and efficiency, all drives should be the same size.
- Select the Unit: Choose whether the disk size you entered is in Terabytes (TB) or Gigabytes (GB). The calculator uses the standard manufacturer definition (1 TB = 1000 GB).
- Review the Results: The calculator will instantly display four key metrics:
- Usable Capacity: The primary result showing how much space you have for your files.
- Total Raw Capacity: The sum of the capacities of all disks.
- Parity / Lost Space: The amount of space reserved for data redundancy (always equal to the size of two disks).
- Storage Efficiency: The percentage of raw capacity that is usable.
For more advanced planning, especially when comparing different RAID levels, you might find a raid 6 capacity calculator useful, as RAID 6 is the closest traditional RAID equivalent to RAID-Z2.
Key Factors That Affect RAID-Z2 Capacity
Several factors influence the final usable space and overall performance of a RAID-Z2 array. Considering them is crucial for effective storage planning.
- Number of Disks (N)
- This is the most direct factor. While the minimum is 4, using more disks increases storage efficiency. For example, a 4-disk array has 50% efficiency ((4-2)/4), but a 10-disk array has 80% efficiency ((10-2)/10).
- Individual Disk Size (S)
- Larger disks provide more raw and, consequently, more usable capacity. However, they also lead to much longer rebuild times if a disk fails, which increases the risk of a second failure during the rebuild process.
- Uniformity of Disk Sizes
- ZFS creates vdevs based on the smallest disk in the group. If you have one 4 TB disk and five 8 TB disks in a RAID-Z2, the system will treat all six disks as 4 TB, wasting a significant amount of space.
- ZFS Filesystem Overhead
- Beyond the parity data, ZFS itself reserves a small amount of space for metadata and other filesystem structures (often called “slop space”). Our calculator focuses on the primary RAID calculation, but be aware that the space reported by your OS might be slightly less.
- RAID-Z2 vs. Other Levels
- Choosing RAID-Z2 means sacrificing two disks’ worth of capacity for parity. RAID-Z1 (like RAID 5) only sacrifices one, offering more space but less protection. RAID-Z3 sacrifices three, offering maximum protection but less space. Comparing options in a zfs performance calculator can clarify these trade-offs.
- Expansion Limitations
- A significant consideration with ZFS is that you cannot easily add a single disk to an existing RAID-Z vdev to expand its capacity. Expansion typically involves replacing each disk with a larger one or adding a completely new vdev to the pool.
Frequently Asked Questions (FAQ)
- 1. What is the minimum number of disks for RAID-Z2?
- The absolute minimum number of disks required to create a RAID-Z2 vdev is four. With four disks, you get the capacity of two, providing 50% storage efficiency.
- 2. How many disk failures can RAID-Z2 tolerate?
- RAID-Z2 is designed to withstand the failure of any two disks within the vdev without any data loss.
- 3. What’s the difference between RAID-Z2 and RAID 6?
- Functionally, both offer double parity and can sustain two disk failures. The main difference is that RAID-Z2 is integrated into the ZFS filesystem, which helps prevent issues like the “write hole” common in hardware RAID 6. ZFS’s copy-on-write nature ensures data consistency.
- 4. Should I use disks of different sizes in a RAID-Z2 pool?
- It is strongly discouraged. ZFS will default to the size of the smallest disk in the vdev, meaning the extra capacity on any larger disks will be wasted. Always use identical drives for optimal capacity.
- 5. Is RAID-Z2 a good backup solution?
- No. RAID of any kind, including RAID-Z2, is a system for providing high availability and redundancy against hardware failure. It is not a backup. It will not protect you from file deletion, corruption, or a ransomware attack. Always follow the 3-2-1 backup rule. See our guide on RAID vs. Backup for more info.
- 6. What is “storage efficiency” in the calculator?
- Storage efficiency is the percentage of the total raw disk space that is available for you to use. It’s calculated as (Usable Capacity / Total Raw Capacity) × 100. The more disks you add, the higher the efficiency.
- 7. Why does the calculator use TB instead of TiB?
- Our calculator uses Terabytes (TB), as this is how disk manufacturers market their drives (1 TB = 1,000,000,000,000 bytes). Operating systems often report capacity in Tebibytes (TiB) (1 TiB = 1,099,511,627,776 bytes), which can cause confusion. A 4 TB drive is approximately 3.64 TiB. We use TB for consistency with purchasing decisions.
- 8. How do I expand a RAID-Z2 pool?
- You cannot add a single drive to expand a RAID-Z2 vdev. To increase capacity, you must either replace every drive in the vdev one-by-one with a larger drive (letting it rebuild each time) or add another vdev (e.g., a second RAID-Z2 array) to your ZFS pool.