ZFS Pool Calculator
Usable Capacity (TiB)
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Calculated using the zfs pool calculator logic.
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Storage Distribution Visualization
Green: Usable | Red: Parity | Gray: Overhead/Reserved
What is a zfs pool calculator?
A zfs pool calculator is an essential tool for system administrators and storage enthusiasts using the ZFS file system. Unlike traditional hardware RAID, ZFS handles data management and redundancy at the software level, introducing unique overheads such as metadata reservation, slop space, and specific RAID-Z parity requirements. By using a zfs pool calculator, you can accurately predict how much actual space will be available for your files before purchasing expensive hardware.
Anyone building a TrueNAS server, a Proxmox host, or a DIY Linux storage box should use a zfs pool calculator. A common misconception is that a RAID-Z2 pool of ten 10TB drives will provide 80TB of space. In reality, after binary conversion (TB to TiB) and ZFS metadata overhead, the usable space is significantly less. This zfs pool calculator accounts for those hidden losses.
zfs pool calculator Formula and Mathematical Explanation
The mathematical derivation used by this zfs pool calculator follows several steps to reach the final usable capacity:
- Raw Capacity: Total Drives × Drive Size.
- Decimal to Binary Conversion: Storage manufacturers use decimal (1 TB = 10^12 bytes), but ZFS uses binary (1 TiB = 2^40 bytes). The conversion factor is approximately 0.9095.
- Parity Deduction: Depending on the RAID-Z level, 1, 2, or 3 drives worth of capacity are removed.
- ZFS Slop Space: ZFS reserves about 1/64th of the pool to ensure it never completely runs out of space, which could lead to pool failure.
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| N | Number of physical drives | Integer | 1 – 255 |
| S | Single drive advertised size | TB (Decimal) | 1 – 22 TB |
| P | Parity drives based on RAID-Z | Integer | 0, 1, 2, or 3 |
| C | Binary Conversion Factor | Ratio | 0.90949 |
Practical Examples (Real-World Use Cases)
Example 1: Home Media Server
A user has 6 drives of 14TB each in RAID-Z2. Using the zfs pool calculator, the raw capacity is 84TB. RAID-Z2 removes 28TB for parity, leaving 56TB. Converting to TiB (56 * 0.9095) results in ~50.93 TiB. After ZFS slop and metadata, the zfs pool calculator shows a usable capacity of approximately 48.5 TiB.
Example 2: Enterprise Backup Node
An enterprise uses 12 drives of 20TB in RAID-Z3. The zfs pool calculator subtracts 3 drives for parity (60TB loss). The remaining 180TB decimal becomes ~163.7 TiB. ZFS overhead further reduces this to about 158 TiB usable.
How to Use This zfs pool calculator
- Enter the Number of Drives you plan to install in your VDEV.
- Input the Drive Size (TB) as labeled by the manufacturer.
- Select your desired RAID Level (Stripe for speed, RAID-Z2 for balanced safety).
- The zfs pool calculator will instantly update the primary “Usable Capacity” result.
- Review the chart to see the ratio of data to parity.
- Use the “Copy Results” button to save your configuration for planning documents.
Key Factors That Affect zfs pool calculator Results
Several critical factors influence the final output of the zfs pool calculator and your real-world performance:
- Ashift Settings: Using an incorrect ashift (e.g., ashift=9 for 4k drives) can lead to massive padding overhead not fully captured by a simple zfs pool calculator.
- Swap Partitions: OS installers like TrueNAS may take 2GB from every drive for swap, reducing the available size for the zfs pool calculator inputs.
- Reservation for Metadata: ZFS stores multiple copies of metadata to prevent corruption, which consumes a small percentage of the pool.
- The 80% Rule: For performance reasons, it is highly recommended to never fill a ZFS pool beyond 80%. This zfs pool calculator shows absolute usable space, but you should aim for lower utilization.
- Recordsize: Large files benefit from large recordsizes, while small files can lead to “slack space” where blocks are not fully utilized.
- Quotas and Reservations: If you set dataset quotas, they will limit the usable space regardless of what the zfs pool calculator indicates for the physical pool.
Frequently Asked Questions (FAQ)
Why is usable space lower than raw space in the zfs pool calculator?
Because of parity drives, decimal-to-binary conversion, and mandatory file system overhead like the “slop space” required for operation.
Does the zfs pool calculator account for L2ARC or SLOG?
No, L2ARC and SLOG are cache devices and do not increase the persistent storage capacity of your ZFS pool.
What is the difference between RAID-Z1 and RAID-Z2?
RAID-Z1 allows for 1 drive failure, while RAID-Z2 allows for 2. The zfs pool calculator shows how RAID-Z2 consumes more parity space for safety.
Is RAID 10 better than RAID-Z2?
ZFS mirrors (similar to RAID 10) offer better IOPS, but RAID-Z2 often provides better capacity efficiency in a zfs pool calculator.
Can I add drives to an existing RAID-Z vdev later?
Historically no, but recent ZFS updates have introduced RAID-Z expansion. However, your zfs pool calculator estimate should be based on your final intended configuration.
What is ‘Slop Space’?
It is a small portion of the pool (usually 1/64th) that ZFS forbids users from filling so that the system always has room for administrative tasks.
How accurate is this zfs pool calculator?
It provides a very high-confidence estimate based on standard ZFS pool behaviors, though specific hardware block sizes may cause slight variations.
Should I use a zfs pool calculator for SSDs?
Yes, the same principles of parity and binary conversion apply to SSD-based ZFS pools.
Related Tools and Internal Resources
- zfs raid levels: A deep dive into the pros and cons of RAID-Z levels vs mirrors.
- zfs performance: How to optimize your pool for high-speed data transfer.
- zfs storage pool: Understanding the fundamental architecture of ZFS pools and VDEVs.
- raidz capacity: Detailed technical breakdown of how parity blocks are distributed.
- zfs metadata overhead: Why metadata takes up more space than you might expect.
- zfs ashift: The ultimate guide to alignment shift and drive sector sizes.