Use this RaidZ2 Calculator to accurately determine the usable storage capacity of your ZFS pool, taking into account the two required parity drives and potential formatting overhead.
RaidZ2 Capacity Calculator
RaidZ2 Capacity Formula
The calculation for RaidZ2 is straightforward as it always reserves two disks for parity, regardless of the total number of drives in the pool.
Usable Capacity (TB) = (N - 2) * S * (1 - L)
Formula Sources: Wikipedia: ZFS RAID-Z, RAID-Z Info Guide
Variables Explained
- Total Number of Disks (N): The total count of physical storage devices (HDDs or SSDs) in your vdev. RaidZ2 requires a minimum of 4 disks.
- Individual Disk Size (S): The nominal capacity of a single drive, typically measured in Terabytes (TB). This should be uniform across all disks.
- Parity Disks: Fixed at 2. These disks store redundancy data, allowing the pool to survive the failure of any two drives.
- Overhead/Loss (L): An optional percentage (0-20%) to account for filesystem overhead, formatting losses, or manufacturer differences (e.g., TB vs. TiB).
Related Calculators
Explore other related storage and return calculators:
- RAID-Z1 Capacity Estimator
- RAID-6 Array Size Tool
- NAS Power Consumption Calculator
- Disk Failure Rate Predictor
What is RaidZ2?
RaidZ2 is a high-reliability data redundancy level within the ZFS filesystem, requiring a minimum of four disks to function. It reserves two disks’ worth of space for parity information, meaning that you lose the capacity of two drives, but gain the ability to withstand the simultaneous failure of any two disks in the pool without data loss. This level of protection is highly recommended for pools using many large hard drives.
Unlike traditional RAID-6, which uses a fixed block allocation, RaidZ2 uses dynamic stripe width, which helps mitigate the “write hole” risk. This makes it a superior choice for critical data storage where downtime and data integrity are paramount. The reliability gained often outweighs the storage capacity sacrifice.
How to Calculate RaidZ2 Capacity (Example)
Let’s calculate the usable capacity for a pool with 8 disks, each 10 TB in size, and assume a 3% overhead for formatting.
- Determine Data Disks: Subtract the 2 parity disks from the total. $8 – 2 = 6$ data disks.
- Calculate Nominal Usable Capacity: Multiply the number of data disks by the disk size. $6 \times 10 \text{ TB} = 60 \text{ TB}$.
- Apply Overhead: Convert the overhead percentage (3%) to a decimal: $1 – 0.03 = 0.97$.
- Find Final Usable Capacity: Multiply the nominal capacity by the overhead factor. $60 \text{ TB} \times 0.97 = 58.2 \text{ TB}$.
- The final usable capacity for the RaidZ2 pool is 58.2 TB.
Frequently Asked Questions (FAQ)
A: The minimum number of disks required to create a RaidZ2 vdev (virtual device) in ZFS is four (4). This allows for two data disks and two parity disks.
Q: How many drive failures can RaidZ2 survive?A: RaidZ2 can safely survive the simultaneous failure of any two disks within the vdev without losing any data. This provides a significant safety margin during long resilver operations.
Q: Is RaidZ2 faster than RaidZ1 or RAID-5?A: RaidZ2 generally has slower write performance compared to RaidZ1 (which uses single parity) because it requires more parity calculations and writes. However, its increased reliability often makes the slight performance hit acceptable for most archival and home lab environments.
Q: Should I use TB or TiB for my disk size input?A: For best accuracy, you should use the actual capacity reported by your operating system, which is usually in TiB (tebibytes, where 1 TiB = 1024 GB). If you use the manufacturer’s listed TB (terabytes, where 1 TB = 1000 GB) you should apply a larger overhead percentage (5-10%) to account for the difference.