raidz2 calculator

Calculate usable ZFS storage capacity, parity overhead, and fault tolerance for RAIDZ2 configurations.

What is a RAIDZ2 Calculator?

A RAIDZ2 Calculator is an essential tool for system administrators and storage enthusiasts planning a ZFS-based storage pool. RAIDZ2 is a data software-defined RAID level provided by the ZFS file system that offers double-parity protection. This means your data remains safe even if two drives in a single vdev fail simultaneously.

Who should use it? Anyone building a TrueNAS server, a Proxmox storage node, or a DIY Linux NAS. Using a RAIDZ2 Calculator helps you avoid the common pitfall of overestimating how much space you will actually have after accounting for parity, binary-to-decimal conversion, and ZFS-specific overhead like "slop space" and metadata.

Common misconceptions include the idea that RAIDZ2 is exactly like RAID 6. While they share the "two-drive parity" concept, ZFS handles data blocks differently, meaning the usable space can vary based on your ashift settings and the number of drives in your vdev.

RAIDZ2 Calculator Formula and Mathematical Explanation

The math behind a RAIDZ2 Calculator involves several layers of subtraction from the raw capacity. Here is the step-by-step derivation used in our tool:

1. Raw Capacity: Total Drives × Drive Size.
2. Parity Deduction: (Total Drives – 2) × Drive Size.
3. Binary Conversion (TiB vs TB): Manufacturers sell drives in decimal TB (10^12 bytes), but ZFS calculates in binary TiB (2^40 bytes). This accounts for a ~9% difference.
4. ZFS Slop Space: ZFS reserves about 1/32nd (3.125%) of the pool to ensure it can always perform operations even when "full".
5. Padding Overhead: Depending on the ashift (sector size), small blocks may require extra padding, which can consume additional space.

Variable	Meaning	Unit	Typical Range
N	Number of Physical Drives	Count	4 – 12 per vdev
C	Advertised Drive Capacity	TB	1TB – 24TB
P	Parity Drives	Count	Fixed at 2 for RAIDZ2
Ashift	ZFS Block Alignment	Power of 2	9 (512B) or 12 (4K)

Practical Examples (Real-World Use Cases)

Example 1: The Home Media Server
A user has 6 drives, each 12TB. Using the RAIDZ2 Calculator, the raw capacity is 72TB. After subtracting 2 drives for parity (24TB), the theoretical usable is 48TB. However, after ZFS slop space and the 10% recommended free space buffer, the actual usable capacity for files is approximately 41.5 TiB. This setup allows for high reliability for a movie collection.

Example 2: Enterprise Backup Node
An admin uses 12 drives of 18TB each. The RAIDZ2 Calculator shows a raw capacity of 216TB. Parity takes 36TB. Because the drive count is higher, the efficiency improves. The usable space is roughly 160 TiB. This configuration is ideal for backups where capacity is prioritized but two-drive failure protection is mandatory.

How to Use This RAIDZ2 Calculator

Follow these steps to plan your storage pool accurately:

• Step 1: Enter the total number of drives you plan to put in a single vdev.
• Step 2: Input the capacity of a single drive in Terabytes (TB).
• Step 3: Select the correct ashift. Use 12 for almost all modern SATA/SAS drives.
• Step 4: Adjust the "Reserved Space" percentage. We recommend 10% to maintain ZFS performance.
• Step 5: Review the "Estimated Usable Capacity" and the distribution chart to see where your storage is going.

Key Factors That Affect RAIDZ2 Calculator Results

1. Drive Count (Vdev Width): RAIDZ2 efficiency increases as you add more drives to a vdev, but rebuild times also increase, raising the risk of a third drive failure during recovery.
2. Ashift Settings: Setting an incorrect ashift (e.g., using 9 for 4K native drives) can lead to massive "write amplification" and lost capacity.
3. Decimal vs. Binary: Remember that a "10TB" drive is only 9.09 TiB in the eyes of the ZFS operating system.
4. Recordsize: Larger recordsizes (e.g., 1M for media) generally result in better space efficiency than small recordsizes (e.g., 16K for databases).
5. Metadata Overhead: ZFS stores multiple copies of metadata to ensure file system integrity, which consumes a small portion of usable space.
6. Swap Partitions: Some OS installers (like TrueNAS) take a small amount of space (2GB) from every drive for swap, slightly reducing the available capacity for the pool.

Frequently Asked Questions (FAQ)

Can I add more drives to a RAIDZ2 vdev later?

Traditionally, no. You would have to add a whole new vdev. However, ZFS RAIDZ expansion is a feature recently added to OpenZFS, though it may not be available in all distributions yet.

Is RAIDZ2 better than RAID 10?

RAID 10 provides better IOPS (performance), but RAIDZ2 provides better space efficiency and can survive any two drive failures, whereas RAID 10 can only survive two failures if they are in different mirror pairs.

What is the minimum number of drives for RAIDZ2?

The absolute minimum is 3 drives, but this would behave like a 3-way mirror in terms of capacity. 4 drives is the practical minimum for RAIDZ2.

How does the RAIDZ2 Calculator handle SSDs?

The math is the same for SSDs, but you should be even more careful with the "Reserved Space" to allow for SSD over-provisioning and wear leveling.

What is 'Slop Space' in ZFS?

Slop space is a small amount of capacity (usually 1/32nd of the pool) that ZFS prevents the user from filling so that administrative tasks like deleting files can still function when the pool is "full".

Does RAIDZ2 protect against bit rot?

Yes, ZFS uses checksums to detect bit rot, and RAIDZ2 uses the parity data to automatically repair corrupted blocks.

Why is my usable space lower than the calculator says?

Check if your OS created swap partitions on the drives or if you are using a very small recordsize with a high ashift, which causes padding overhead.

Should I use RAIDZ2 for 4 drives?

For 4 drives, RAIDZ2 gives you the capacity of 2 drives. Many prefer a "Mirrored Vdev" (RAID 10 style) for 4 drives due to better performance.

Related Tools and Internal Resources

• ZFS Storage Guide – A comprehensive manual for ZFS beginners.
• RAIDZ1 vs RAIDZ2 – Choosing the right parity level for your data.
• Disk Failure Protection – Understanding MTBF and pool reliability.
• Usable Storage Capacity – Deep dive into binary vs decimal storage.
• ZFS Overhead – Why ZFS uses more space than traditional RAID.
• Storage Pool Planning – Best practices for vdev and pool layout.