ZFS RAID Calculator
Estimate the usable capacity of your ZFS storage pool based on your disk configuration and RAID-Z level.
The total number of physical disks in a single virtual device (VDEV).
Enter the capacity of one disk. Use the dropdown to select the unit.
TB = 1000^4 bytes, GB = 1000^3 bytes. Calculations convert to TiB/GiB for accuracy.
The redundancy level determines fault tolerance and affects usable capacity.
Capacity Breakdown
RAID Level Comparison
| RAID Level | Usable Capacity | Efficiency | Fault Tolerance |
|---|
What is a ZFS RAID Calculator?
A zfs raid calculator is an essential tool for system administrators, storage enthusiasts, and IT professionals who use the Zettabyte File System (ZFS). Its primary purpose is to estimate the actual usable storage space you will have after configuring a set of disks into a ZFS storage pool. ZFS uses various levels of RAID, known as RAID-Z, which provide different balances of data protection (redundancy), performance, and storage efficiency. This calculator helps demystify the complex calculations involved by accounting for parity data, which is data used to reconstruct information in case of a disk failure.
Unlike traditional hardware RAID, ZFS integrates the file system and volume manager, providing advanced features like data integrity protection, snapshots, and copy-on-write. However, this also means that calculating usable capacity isn’t as simple as just subtracting the capacity of parity disks. Our zfs raid calculator takes these nuances into account to provide a realistic estimate, helping you plan your NAS build or server storage array effectively.
ZFS RAID Formula and Explanation
The core calculation for ZFS usable capacity depends on the number of disks, the size of each disk, and the chosen RAID-Z level. The fundamental idea is to subtract the capacity dedicated to parity from the total raw capacity.
The basic formula is:
Usable Capacity = (Total Disks - Parity Disks) * Size of Single Disk
This provides a good starting point, but the true available space is slightly less due to ZFS’s internal metadata and “slop space” reservations, which this zfs raid calculator approximates for a more accurate figure.
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Total Disks (N) | The number of physical hard drives in the VDEV (Virtual Device). | Unitless Integer | 3 – 20+ |
| Parity Disks (P) | The number of disks’ worth of capacity dedicated to redundancy. | Unitless Integer | RAID-Z1: 1, RAID-Z2: 2, RAID-Z3: 3. For RAID-10, it’s half the total disks. |
| Disk Size (S) | The advertised capacity of a single disk. | TB or GB | 1 TB – 24 TB |
Practical Examples
Understanding how different configurations impact storage is key. Here are two realistic examples using the zfs raid calculator.
Example 1: Home Media Server
A user is building a home NAS for media streaming and backups. They prioritize a good balance of capacity and protection against a double disk failure.
- Inputs: 6 Disks, 8 TB each, RAID-Z2
- Calculation:
- Raw Capacity: 6 disks * 8 TB = 48 TB (~43.66 TiB)
- Parity Disks: 2
- Usable Capacity: (6 – 2) * 8 TB = 32 TB (~29.11 TiB)
- Result: The user gets approximately 29.11 TiB of usable space with the ability to withstand two simultaneous drive failures, making it a robust choice. Explore our general storage calculator for other scenarios.
Example 2: Small Business File Server
A small business needs a high-performance file server. They need good read/write speeds and can tolerate a single disk failure, but want to maximize capacity.
- Inputs: 10 Disks, 4 TB each, RAID-Z1
- Calculation:
- Raw Capacity: 10 disks * 4 TB = 40 TB (~36.38 TiB)
- Parity Disks: 1
- Usable Capacity: (10 – 1) * 4 TB = 36 TB (~32.74 TiB)
- Result: With RAID-Z1, the business achieves a high storage efficiency of 90% while still maintaining protection against a single drive failure. This setup is great for performance and space. Learn more about ZFS vs BTRFS to see if it’s the right choice for you.
How to Use This zfs raid calculator
- Enter the Number of Disks: Input the total count of physical disks you plan to use in a single ZFS VDEV.
- Specify Disk Size: Enter the capacity of a single disk. For example, if you have 8TB drives, enter ‘8’.
- Select Units: Choose whether the disk size you entered is in Terabytes (TB) or Gigabytes (GB). The calculator will automatically handle the conversion to Tebibytes (TiB) for accurate space representation.
- Choose RAID-Z Level: Select the desired level of redundancy from the dropdown. RAID-Z2 is a popular choice for its balance of safety and capacity.
- Review the Results: The calculator instantly updates to show your total usable capacity, raw capacity, the amount of space lost to parity, storage efficiency, and fault tolerance.
Key Factors That Affect ZFS Usable Space
- RAID-Z Level: This is the most significant factor. RAID-Z1 uses one disk for parity, Z2 uses two, and Z3 uses three. The more parity disks, the less usable space, but the higher the fault tolerance.
- Number of Disks: The efficiency of RAID-Z improves with more disks in a VDEV. For example, a 5-disk RAID-Z1 (80% efficiency) is more space-efficient than a 3-disk RAID-Z1 (66.7% efficiency).
- VDEV Configuration: ZFS pools can be made of multiple VDEVs. For example, a pool of two 6-disk RAID-Z2 VDEVs will have different performance and capacity characteristics than one 12-disk RAID-Z2 VDEV. This calculator focuses on a single VDEV for simplicity.
- Metadata Overhead (Slop Space): ZFS reserves a small percentage of the total pool space (around 3%) to manage its metadata and prevent the pool from becoming 100% full, which would cripple performance.
- Disk Size (TB vs TiB): Hard drive manufacturers market drive capacity in Terabytes (TB, 10^12 bytes), while operating systems measure capacity in Tebibytes (TiB, 2^40 bytes). A 4 TB drive is actually about 3.64 TiB, and our zfs raid calculator correctly accounts for this discrepancy.
- Record Size: The `recordsize` (or block size) in ZFS can cause minor variations in usable space, especially with very small files. While not a factor in this calculator, it’s a consideration for advanced tuning. Check out our guide on understanding RAID levels for more depth.
Frequently Asked Questions (FAQ)
It depends on your needs. RAID-Z1 is good for home use with 3-5 drives. RAID-Z2 is the recommended standard for 6-12 drives, offering excellent protection against a second drive failing during a rebuild. RAID-Z3 is for critical applications with many drives where data integrity is paramount.
Yes, but ZFS will treat all disks in a VDEV as if they are the size of the *smallest* disk. To avoid wasting capacity, it is highly recommended to use identical disks within a single VDEV.
This is due to two main factors: the TB to TiB conversion (a 1 TB drive is only ~0.909 TiB) and ZFS’s internal overhead, often called “slop space,” which reserves about 1/32nd of the pool for metadata and operational health.
RAID-Z uses parity, allowing for high storage efficiency. RAID-10 (Striped Mirrors) creates exact copies of data across pairs of disks, offering higher performance (especially for random I/O) but with only 50% storage efficiency. Our zfs raid calculator can model both.
The minimum requirements are: 2 disks for RAID-Z1 (though 3+ is recommended), 4 disks for RAID-Z2, and 5 disks for RAID-Z3.
The pool becomes “degraded” but remains online and accessible. ZFS uses the parity data to serve any data requested from the failed drive. You must replace the failed drive and initiate a “resilver” process to rebuild the data and restore redundancy.
It provides a very close estimate for planning purposes. The final, exact `zfs list` command output might vary slightly due to factors like specific ZFS version, record size, and metadata from stored files. However, it’s the best tool for initial data backup strategy planning.
No, this tool calculates capacity for a single VDEV. To find the total capacity for a pool with multiple VDEVs, calculate the usable capacity for one VDEV and then multiply it by the number of identical VDEVs in your pool.