Unraid Zfs Calculator

Plan your storage array with precision. This Unraid ZFS calculator helps you determine usable capacity based on your disk configuration and ZFS redundancy level.

What is an Unraid ZFS Calculator?

An Unraid ZFS Calculator is a specialized tool designed to help users plan and configure their ZFS storage pools within the Unraid operating system. Since Unraid 6.12, ZFS has been integrated, offering powerful features like data integrity protection against bitrot, snapshots, and various levels of redundancy. This calculator simplifies the complex task of determining how much actual, usable storage space you will get from a set of disks after accounting for the capacity lost to parity (redundancy) data required by your chosen ZFS RAID level.

Unlike Unraid’s traditional array, ZFS pools data in groups called “vdevs.” The redundancy is handled within each vdev. This calculator focuses on a single vdev to help you understand the fundamental trade-offs between storage capacity, performance, and data safety for your next Unraid ZFS setup.

The Unraid ZFS Calculator Formula

The calculation for usable space in ZFS depends on the chosen RAID level. ZFS reserves a certain number of disks’ worth of capacity for parity data, which is used to reconstruct data if a drive fails. This calculator uses the following simplified formulas:

• RAID-Z1: Usable Space = (Total Number of Disks – 1) * Size of Smallest Disk
• RAID-Z2: Usable Space = (Total Number of Disks – 2) * Size of Smallest Disk
• RAID-Z3: Usable Space = (Total Number of Disks – 3) * Size of Smallest Disk
• Mirror: Usable Space = (Total Number of Disks / 2) * Size of Smallest Disk

Note: These formulas provide a close estimate. The actual space reported by `zfs list` may be slightly less due to metadata overhead, known as the “slop space” allocation.

Variables Used in ZFS Calculations

Variable	Meaning	Unit	Typical Range
Disk Count	The total number of physical hard drives in the ZFS vdev.	Drives (integer)	3 – 20+
Disk Size	The storage capacity of a single drive in the vdev.	TB or GB	1 – 22 TB
Parity Disks	The number of disks’ worth of capacity reserved for data redundancy.	Disks (integer)	1 (RAID-Z1), 2 (RAID-Z2), 3 (RAID-Z3)
Usable Capacity	The final storage space available for your data.	TB or GB	Varies based on inputs

Practical Examples

Example 1: Balanced Home Media Server

A user wants to build a media server with a good balance of storage and protection for their valuable files. They have 6 disks of 8 TB each and are deciding between RAID-Z1 and RAID-Z2.

• Inputs (RAID-Z1):
  • Number of Disks: 6
  • Disk Size: 8 TB
  • ZFS RAID Level: RAID-Z1
• Result: (6 – 1) * 8 TB = 40 TB usable space with 1-disk fault tolerance.

• Inputs (RAID-Z2):
  • Number of Disks: 6
  • Disk Size: 8 TB
  • ZFS RAID Level: RAID-Z2
• Result: (6 – 2) * 8 TB = 32 TB usable space with 2-disk fault tolerance. The user sacrifices 8 TB of space for the ability to withstand two simultaneous drive failures. For more details on RAID levels, you can review ZFS RAID Levels explained.

Example 2: High Redundancy Archival Pool

A user is creating a ZFS pool for critical backups that must not be lost. They are using 10 disks of 4 TB each and opt for RAID-Z3 for maximum data safety.

• Inputs:
  • Number of Disks: 10
  • Disk Size: 4 TB
  • ZFS RAID Level: RAID-Z3
• Result: (10 – 3) * 4 TB = 28 TB usable space. This configuration can survive the failure of any three disks, making it extremely resilient. The trade-off is that 12 TB (3 disks * 4 TB) is dedicated to parity. If you want to know more about the pros and cons of using ZFS, check out this guide.

How to Use This Unraid ZFS Calculator

1. Enter Disk Count: Input the total number of physical disks you plan to use in your ZFS vdev.
2. Provide Disk Size: Enter the capacity of a single disk. If your disks are different sizes, enter the size of the smallest one, as ZFS will treat all disks in the vdev as being that size.
3. Select Units: Choose whether the disk size you entered is in Terabytes (TB) or Gigabytes (GB). The calculator will adjust its output accordingly.
4. Choose ZFS RAID Level: Select from RAID-Z1 (1 parity disk), RAID-Z2 (2 parity disks), RAID-Z3 (3 parity disks), or Mirror. This is the most critical factor for determining your final capacity and fault tolerance.
5. Interpret the Results: The calculator instantly shows your usable capacity, raw capacity, the space lost to redundancy, and how many drives can fail before data is lost. Use the visual chart to quickly understand the ratio of usable to reserved space.

Key Factors That Affect ZFS Storage

• RAID Level Choice: This is the biggest factor. RAID-Z1 offers the most space but the least protection, while RAID-Z3 offers the least space but the most protection.
• Number of Disks: More disks generally mean more capacity, but also a higher probability of one failing. The optimal number depends on your chosen RAID level.
• Disk Size Uniformity: ZFS vdevs work best with identical disks. If you mix sizes, the vdev’s capacity is limited by the smallest disk in the set.
• Vdev Layout: A ZFS pool can be made of multiple vdevs. For example, a 12-disk pool could be a single 12-disk RAID-Z2 vdev or two 6-disk RAID-Z1 vdevs. This affects performance and expansion flexibility. To understand this better, you can read about the RAID and RAIDZ differences.
• RAM Amount: ZFS uses RAM for its Adaptive Replacement Cache (ARC) to improve performance. The more RAM you have, the better ZFS can perform, especially with frequently accessed data. Insufficient RAM can become a bottleneck.
• Metadata Overhead (Slop Space): ZFS reserves a small fraction of the pool’s space for its own metadata and management, which is not available for user data. This is typically around 3%.

Frequently Asked Questions (FAQ)

1. Can I mix disk sizes in an Unraid ZFS pool?

Yes, but it’s not recommended for a single vdev. Within a vdev (like a RAID-Z1 group), the usable space from each disk is limited to the size of the smallest disk in that vdev. You get more flexibility with ZFS by adding new vdevs of different sizes to an existing pool. This is a key difference compared to the traditional Unraid Array vs ZFS setup.

2. What’s the difference between RAID-Z1 and Unraid’s standard parity?

Both offer single-disk failure protection. However, ZFS RAID-Z1 stripes data across all disks, meaning all disks spin up for reads and writes, offering higher performance. Unraid’s standard array only spins up the required data disk(s) and the parity disk, saving power but offering lower performance. Additionally, ZFS provides self-healing from data corruption (bitrot), which the standard Unraid array does not.

3. How many disks do I need for RAID-Z?

For RAID-Z1, you need a minimum of 3 disks. For RAID-Z2, a minimum of 4 is often recommended. For RAID-Z3, at least 5. A mirror vdev requires an even number of disks, typically starting with 2.

4. Why is my usable capacity less than the calculation?

This calculator provides a very close estimate of raw usable capacity. The final space you see in Unraid will be slightly lower due to ZFS’s internal metadata, filesystem overhead, and the difference between manufacturer-advertised capacity (TB) and the actual binary capacity (TiB). Some calculators, like the ZFS Capacity Calculator, go into deeper detail on this.

5. Is RAID-Z2 worth the extra lost space over RAID-Z1?

For many, yes. When a disk in a RAID-Z1 array fails, the array is vulnerable—a second failure during the rebuild (which can take days) would cause total data loss. RAID-Z2 protects against this scenario by allowing two disks to fail simultaneously, making it significantly safer for large arrays or critical data.

6. Can I expand a ZFS vdev by adding one disk?

No, this is a major limitation of ZFS. You cannot add a single disk to an existing RAID-Z vdev to increase its size. You must either replace each disk one-by-one with a larger capacity drive or expand the pool by adding an entirely new vdev.

7. How much RAM does ZFS need?

A common rule of thumb is 1GB of RAM for every 1TB of storage, but this is a loose guideline. For basic use, 8GB or 16GB of RAM can be sufficient. More RAM allows the ZFS cache (ARC) to be more effective, boosting read performance. For heavy use or deduplication, much more is needed.

8. What are the main drawbacks of ZFS?

The primary drawbacks are its high RAM requirements for optimal performance, the inflexibility of expanding existing vdevs, and higher power consumption as all disks in a vdev are active during I/O operations.