For a while now, data storage enthusiasts have been wondering why some of their WD Red NAS drives have been failing with their RAID/ZFS arrays and/or exhibiting signs of slower performance. It turns out that many of these HDDs actually use shingled magnetic recording (SMR) instead of perpendicular (conventional) magnetic recording (PMR/CMR) technology – a fact that Western Digital never bothered listing in its specification sheets.
“All our WD Red drives are designed meet or exceed the performance requirements and specifications for common small business/home NAS workloads,” said Western Digital. “We work closely with major NAS providers to ensure WD Red HDDs (and SSDs) at all capacities have broad compatibility with host systems. Currently, Western Digital’s WD Red 2TB-6TB drives are device-managed SMR (DMSMR). WD Red 8TB-14TB are CMR-based.”
While SMR allows for greater areal density (i.e., higher drive capacity), it’s inherently inferior to PMR/CMR based on the way it writes data. As Synology explains, “rather than writing each magnetic track without overlapping, SMR overlaps each new track with part of the previously written track, much like shingles on a roof.” The result is slower performance and, as evidenced by recurring complaints on r/DataHoarders, problems with RAID arrays.
Tom’s Hardware equates this snafu to an SSD manufacturer selling a drive with cheaper NAND (e.g., QLC) and not disclosing it. It isn’t clear whether Western Digital will update its marketing materials to show that its 2 TB – 6 TB Red NAS are SMR drives.