NAS and Unrecoverable Read Errors (URE)

Contemporary NAS systems commonly employ RAID5 technology to ensure fault tolerance, safeguarding users against potential data loss in the event of a disk failure.

In 2007, Robin Harris published an article raising concerns about the reliability of RAID5 as a storage technology. The key issue highlighted was the elevated risk of encountering Unrecoverable Read Errors (URE) on a disk after one of the disks has already failed. While it's essential to approach Harris's article with a discerning eye, acknowledging its inconsistencies, the notion that such scenarios might occur in real-life situations cannot be entirely dismissed. It's important to clarify that this doesn't necessarily imply a complete failure of two disks but rather a situation where one member disk has failed, and the other exhibits bad sector(s), making a successful rebuild challenging.

Given that NAS devices are frequently utilized as archives, with users primarily writing data rather than reading, the occurrence of URE becomes a plausible concern. To mitigate this risk and avoid overlooking the timely replacement of a disk, it is advisable to configure your NAS for automatic reading of disks from start to finish at regular intervals. This process, referred to as "scrub," "read patrol," or simply "disk test" in various NAS systems, aids in the proactive identification of bad sectors, ensuring the timely detection of potential issues.