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CMR vs SMR: What It Means for Your NAS

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Get a CMR drive for a NAS, RAID array, or anything with heavy or continuous writes; SMR drives are fine for write-once storage like backups and archives, but they slow drastically during rebuilds.

CMR vs SMR: the short answer

CMR (conventional magnetic recording) is the right choice for a NAS, RAID array, ZFS pool, or any workload with heavy or continuous writes. SMR (shingled magnetic recording) is fine for write-once storage — backups, archives, media libraries — where data is written once and read occasionally.

The reason: SMR drives pack data tighter by overlapping tracks, which saves space but makes rewriting slow. Under sustained random writes, like a RAID rebuild, an SMR drive can slow to a crawl. In ServeTheHome's test, a ZFS rebuild that CMR drives finished in under 17 hours took nearly 230 hours on an SMR drive.

How CMR and SMR drives work

In a CMR drive, tracks sit side by side with small gaps between them, so the heads can rewrite any track without touching its neighbors. This is how hard drives have traditionally worked.

SMR overlaps tracks "like the shingles on a roof," as the Zoned Storage project describes it. That fits more data on each platter, but a track can't be rewritten on its own — changing data means rewriting a whole band, or zone, of tracks.

SMR comes in three kinds. Drive-managed SMR handles everything internally and looks like a normal drive to your computer. Host-aware SMR is backward compatible but also accepts zone commands. Host-managed SMR accepts only sequential writes and needs software that supports zones. Consumer SMR drives are drive-managed.

Why SMR is a problem in a NAS

A drive-managed SMR drive reorganizes its data during idle time, according to Western Digital. Copy some files, leave the drive alone, and it quietly reshuffles data into place and recovers its speed.

A RAID or ZFS rebuild gives it no idle time. The sustained random writes keep coming, the drive falls behind on its internal cleanup, and performance drops sharply. ServeTheHome tested this in May 2020: rebuilding a four-drive ZFS RAIDZ array about 60% full, three CMR drives finished in under 17 hours, while the SMR WD Red took nearly 230 hours — about nine and a half days. A second SMR drive gave the same result.

The WD Red SMR controversy

In April 2020, Western Digital confirmed to Blocks & Files that its WD Red NAS drives from 2TB to 6TB used drive-managed SMR, while the 8TB to 14TB models were CMR — something WD's spec sheets hadn't disclosed. Seagate and Toshiba also confirmed undocumented SMR in some of their drives, though not in drives sold for NAS use.

After the backlash, in June 2020 WD made "WD Red" mean SMR only and renamed its CMR NAS drives "WD Red Plus." WD Red Pro stayed CMR. That naming still holds, which is why the exact line name matters when you shop.

Which drives are CMR and which are SMR?

For a NAS, stick to lines that are CMR across the board:

Known SMR drives include plain WD Red; Seagate BarraCuda 3.5-inch 4TB and 8TB and BarraCuda 2.5-inch 2TB and 4TB; some WD Blue desktop and 2.5-inch models; and Toshiba P300 2TB to 6TB. Makers revise their lineups, so treat any list as a starting point and confirm the exact model with the manufacturer. The NAS hard drive deals page linked below lists only drives whose listing explicitly says CMR.

How to check before you buy

The reliable method is to look up the exact model number with the manufacturer rather than trusting a store listing, which may not mention the recording type at all.

When SMR is fine

SMR's denser tracks often make it cheaper per terabyte, and that is a real advantage for the right job. Seagate says SMR suits sequential work: media libraries, backup tiers, archives, and data written once and read occasionally.

If a drive spends most of its life holding files rather than rewriting them, SMR costs you little. Keep it out of RAID arrays, ZFS pools, and anything with continuous writes. In 2020, Synology's compatibility list flagged SMR models as suitable only for light workloads, warning of slowdowns under continuous writes.

FAQ

Is CMR or SMR better?

For a NAS, RAID, ZFS, or heavy continuous writes, CMR — SMR drives slow sharply under sustained random writes. For write-once storage like backups, archives, and media libraries, SMR is fine and often cheaper per terabyte. Match the drive to the workload.

Can I use an SMR drive in a NAS?

You can, but you shouldn't. In 2020 Synology's compatibility list flagged SMR models as suitable only for light workloads. A RAID rebuild is the worst case: ServeTheHome measured an SMR drive taking nearly 230 hours on a rebuild that CMR drives finished in under 17.

How do I know if my hard drive is SMR?

Look up the exact model number with the manufacturer. Seagate publishes a CMR/SMR list, Western Digital says to check the model's data sheet, and Toshiba lists recording type in its lineup tables. Don't rely on a store listing title — it may not say.

Are WD Red drives SMR?

Plain WD Red drives are SMR. After the 2020 controversy, WD renamed its CMR NAS drives "WD Red Plus," and WD Red Pro has stayed CMR. For a NAS, buy Red Plus or Red Pro rather than plain Red.

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