ZFS ashift Interactive Guide & Calculator

The ZFS `ashift` Calculator

Don't guess. The `ashift` property is the most critical, permanent setting for your ZFS pool. This guide will help you find the optimal value for your specific hardware to maximize performance and reliability.

Step 1: Select your Drive Type

Choose the type of storage media you are adding to your ZFS pool.

Step 2: Get Physical Sector Size

Run the following command on your Linux system (replacing `/dev/sdX` or `/dev/nvmeXn1` with your drive's path) and paste the relevant output line below.

Paste the key output line here:

The Cardinal Sin: Read-Modify-Write

Setting `ashift` too low is the most catastrophic mistake in ZFS. It forces your drive into a vicious cycle called Read-Modify-Write (RMW), crippling performance and accelerating wear on SSDs. This section explains how this happens.

How a Single Write Becomes Three Operations

When ZFS wants to write a 512-byte block (because of a wrong `ashift=9` setting) to a drive that physically uses 4096-byte sectors, the drive's controller must perform this inefficient sequence. Hover over each step to learn more.

1.

READ

The drive reads the entire 4096-byte physical sector into its cache, even though ZFS only asked to write 512 bytes.

→

2.

MODIFY

It modifies the 512-byte portion of the data within its cache, leaving the other 3584 bytes untouched.

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3.

WRITE

It writes the entire, modified 4096-byte block back to the physical media, causing unnecessary write amplification.

The Impact: This process transforms a single intended write operation into a sequence of a read, a modification, and a write. For a 4KiB file, this can mean **8 reads and 8 writes** instead of just one, severely degrading performance.

Hardware Deep Dive

The optimal `ashift` value is a direct reflection of your drive's physical architecture. Learn the differences between drive types and why the "512e lie" is so dangerous for ZFS.

Hard Drive Sector Types

Drive Type	Physical Sector	Reported (Logical) Sector	ZFS Auto-Detect `ashift`
512-native (512n)	512 bytes	512 bytes	ashift=9 (Correct)
512-emulation (512e)	4096 bytes (4KiB)	512 bytes	ashift=9 (Incorrect!)
4K-native (4Kn)	4096 bytes (4KiB)	4096 bytes (4KiB)	ashift=12 (Correct)

The 512e Lie: Most modern HDDs are 512e. They lie to the OS for backward compatibility, causing ZFS auto-detection to choose the wrong `ashift`, leading to severe performance loss. This is why you must always set `ashift` manually.

Flash Media (SSDs & NVMe)

Flash drives use a different architecture (pages and blocks) than HDDs. While early advice suggested trying to match the drive's internal page size (e.g., `ashift=13` for 8K pages), modern evidence points to a clear winner.

The Modern Consensus: Use `ashift=12`. Drive controllers are heavily optimized for the standard 4KiB I/O used by all modern operating systems. Benchmarks consistently show that `ashift=12` provides the best, or at least safest, performance for nearly all SSDs and NVMe drives.

Attempting to guess an internal page size can be counter-productive. Stick with the industry standard that drive firmware is designed to handle efficiently.

The Great Trade-Off

Choosing the correct `ashift` involves balancing performance and space efficiency. While `ashift=12` is vital for performance, it can lead to wasted space in RAID-Z configurations due to a "padding penalty."

RAID-Z3 Performance vs. Capacity

This chart visualizes a benchmark on a 24-drive RAID-Z3 array. Choosing the correct `ashift=12` yields much higher performance, but at the cost of 5TiB of usable capacity compared to the misaligned `ashift=9`.

RAID-Z2 Padding Penalty (Interactive)

Select a vdev width to see how the space overhead from padding changes dramatically between `ashift=9` and `ashift=12` for a small-block workload (recordsize=16k).

The Verdict: The modern consensus is clear: always prioritize performance. The "horrible" write speeds and slow resilver times from a misaligned `ashift` are not worth the extra capacity. Accept the padding penalty as a cost of using RAID-Z with modern hardware.

Final Recommendations & Syntax

Synthesizing all the principles, here are clear, scenario-based recommendations and the exact command syntax you need to create a properly configured, future-proof ZFS pool.

Rule #1: Future-Proof Your Pool

The `ashift` property is immutable. A pool created with `ashift=9` cannot be repaired with modern 4K drives when a legacy disk fails. Therefore, the unbreakable rule is:

Always use `ashift=12` as the absolute minimum.

`zpool create` Command Example

This example shows how to create a high-performance mirrored pool with all the recommended settings for performance and longevity. Hover over the parameters for an explanation.

sudo zpool create \
    -o ashift=12 \
    -O compression=lz4 \
    -O atime=off \
    -O xattr=sa \
    mypool mirror /dev/disk/by-id/DRIVE1-SERIAL /dev/disk/by-id/DRIVE2-SERIAL

Pro Tip: Always use stable device paths from `/dev/disk/by-id/`. Never use `/dev/sdX` names, as they can change between reboots and prevent your pool from importing.