Linux ZFS Pool Degraded Resilver — What It Means & How to Fix It

Fix ZFS storage pool degraded state by replacing failed disks and understanding the resilvering process for data recovery. This guide covers everything you need to know about this topic, including common causes, step-by-step solutions, and answers to frequently asked questions.

Here are the key things to understand: A degraded ZFS pool means one or more disks in a redundant VDEV have failed but data is still accessible. Resilvering is ZFS's process of rebuilding data onto a replacement disk from the remaining copies. Unlike traditional RAID rebuilds, ZFS resilvers only copy used blocks, which is often faster. A degraded pool is at risk: if another disk in the same VDEV fails before resilver completes, data is lost. Understanding these fundamentals will help you diagnose and resolve this issue more effectively.

The most common reasons this occurs include: Physical disk failure (bad sectors, drive death) causing the pool to lose a member. Disk temporarily disconnected (loose cable, USB disconnection) marked as faulted. Too many checksum errors on a disk causing ZFS to fault it out of the pool. Controller failure causing communication loss with one or more disks. Identifying the root cause is the first step toward finding the right solution.

To resolve this, follow these recommended steps: Check pool status: zpool status to identify which disk is faulted and which VDEV is degraded. Replace the failed disk: zpool replace poolname /dev/old-disk /dev/new-disk. Monitor resilver progress: zpool status shows estimated time remaining. After resilver completes, verify: zpool scrub poolname and check for zero errors in zpool status. If these steps do not resolve the issue, consider consulting additional resources or a qualified professional.

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