ZFS RAID Calculator
Estimate storage capacity, fault tolerance, and pool efficiency for ZFS configurations.
27.32 TiB
Capacity Distribution Map
■ Parity
■ Overhead
*Formula: Usable ≈ (Raw – Parity) * 0.984 (TiB conversion) – 1.5% ZFS reservation.
What is a ZFS RAID Calculator?
A zfs raid calculator is an essential tool for system administrators and storage engineers to predict the actual usable capacity of a ZFS pool. Unlike traditional hardware RAID, ZFS introduces unique data structures, copy-on-write overhead, and metadata requirements that consume space. Using a zfs raid calculator helps you account for “slop space” and the binary conversion (TB to TiB) that often leaves users wondering where their storage went.
Who should use it? Anyone building a TrueNAS server, Proxmox storage node, or custom Ubuntu ZFS array. A common misconception is that a 10TB drive provides 10TB of usable space. In reality, ZFS pools are measured in Tebibytes (TiB), and the zfs raid calculator accounts for the ~9% loss during the decimal-to-binary conversion alone.
ZFS RAID Calculator Formula and Mathematical Explanation
Calculating ZFS capacity involves several layers of subtraction. The zfs raid calculator follows this general logic path:
- Raw Capacity: Drive Count × Drive Size.
- RAID Level Subtraction: Minus the parity drives (n-1 for RAID-Z1, n-2 for RAID-Z2, etc.).
- Binary Conversion: Multiply by (10^12 / 2^40) ≈ 0.9095 to convert TB to TiB.
- ZFS Slop Space: ZFS reserves approximately 1/64th (1.56%) of the pool capacity to ensure the pool doesn’t lock up when full.
- Metadata Overhead: Additional space (usually 0.5% to 1%) for the ZFS intent log and metadata.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| N | Number of Drives | Count | 2 – 255 |
| S | Individual Drive Capacity | TB (Decimal) | 1 – 22 TB |
| P | Parity Level | Drives | 0 (Stripe) to 3 (RAID-Z3) |
| Ashift | VDEV Alignment (2^n) | Bits | 9 (512B) or 12 (4K) |
Practical Examples (Real-World Use Cases)
Example 1: The Small Business Server
A user sets up a 6-drive RAID-Z2 array using 12TB hard drives. Using the zfs raid calculator, the raw capacity is 72TB. However, RAID-Z2 consumes 2 drives for parity, leaving 48TB raw usable. After TiB conversion (43.66 TiB) and the 1.5% slop space reservation, the final usable capacity is approximately 43 TiB. This configuration allows for 2 simultaneous drive failures without data loss.
Example 2: High-Performance SSD Mirror
A developer uses 4 x 2TB NVMe drives in a “Striped Mirror” (VDEV 1: 2 drives, VDEV 2: 2 drives). The zfs raid calculator shows that with a 50% redundancy ratio, the raw 8TB becomes 4TB raw usable. Converting to TiB results in ~3.64 TiB. This setup provides maximum IOPS but lower capacity efficiency.
How to Use This ZFS RAID Calculator
Follow these steps to get the most accurate results for your storage planning:
- Select RAID Level: Choose RAID-Z1 for basic home use, RAID-Z2 for production data, or Mirror for high performance.
- Input Drive Count: Enter the total number of disks you intend to put into a single VDEV.
- Input Drive Size: Use the manufacturer’s labeled capacity in Terabytes (TB).
- Adjust Ashift: For most modern drives (Advanced Format), leave this at Ashift 12 (4K). Older enterprise drives might use Ashift 9.
- Review the Chart: The visual breakdown shows exactly how much space is lost to parity versus ZFS management overhead.
Key Factors That Affect ZFS RAID Calculator Results
Several technical factors can drastically change your available space beyond basic math:
- Decimal vs. Binary: Manufacturers sell “1 TB” (1,000,000,000,000 bytes), but ZFS sees “0.909 TiB”. This is the biggest source of confusion in zfs raid calculator outputs.
- Ashift Settings: Incorrect Ashift settings (e.g., using 512B alignment on 4K drives) can lead to massive “write amplification” and lost capacity.
- VDEV Padding: RAID-Z has a minimum allocation unit. With small files, parity overhead can actually be higher than the theoretical 1/n or 2/n.
- Pool Reservation: ZFS performs poorly when it exceeds 80-90% capacity. This calculator accounts for the hard “slop space” limit but not your personal quota.
- Compression: Using LZ4 or ZSTD compression can actually increase your “effective” usable space, though the zfs raid calculator only shows the physical capacity.
- Snapshots: While not a direct pool overhead, snapshots consume space over time as data changes, reducing available capacity for new files.
Frequently Asked Questions (FAQ)
ZFS reserves 1.5% of the pool as “slop space” to prevent the file system from becoming read-only when completely full. Additionally, file system metadata takes up space.
For optimal performance with 4K sectors, many experts suggest using a drive count that is a power of 2 plus the parity drives (e.g., 4+2=6 or 8+2=10 drives).
Historically, you could only add new VDEVs to a pool. However, recent ZFS versions are introducing “RAID-Z Expansion,” though it is still recommended to plan your capacity ahead using a zfs raid calculator.
RAID-Z3 is used for very large arrays (12+ drives) or ultra-critical data where three simultaneous drive failures must be survived.
Operating systems usually report in TiB (Binary), while marketing materials use TB (Decimal). Our zfs raid calculator provides both for clarity.
Ashift 12 ensures that ZFS aligns data to 4KB blocks. This prevents “read-modify-write” cycles on modern drives, improving both speed and longevity.
You can, but ZFS will treat every drive as if it were the size of the smallest drive in the VDEV. Use the zfs raid calculator based on the smallest drive.
While ZFS supports many, it is generally recommended to keep VDEVs under 12-15 drives to prevent extremely long rebuild times.
Related Tools and Internal Resources
- RAID Rebuild Time Calculator – Estimate how long it will take to replace a failed disk based on pool size.
- Storage Density Calculator – Calculate TB per rack unit for data center planning.
- NAS Hard Drive Buying Guide – Learn which drives are best for your next ZFS build.
- SSD Endurance Calculator – Determine if your SSDs can handle ZFS write loads.
- Server Power Calculator – Estimate the electricity costs of spinning your ZFS array.
- Latency Impact Calculator – See how RAID levels affect your storage response times.