CMR vs SMR

I'm building a new server. When I built Trump, I put 2 2TB drives into a Storage Spaces RAID-1 configuration.

That has worked well so I wanted to do the same in the new server.

But it got complicated.

When I started looking at 4TB drives for RAID configurations, I noticed terms that I didn't recognize in the specifications: CMR and SMR.

Then I read the reviews. I got confused. Nobody seemed to like SMR drives. What were CMR and SMR? How were they different?

Conventional Magnetic Recording (CMR) drives write data on a hard disk in tracks that do not overlap. Shingled Magnetic Recording (SMR) allows tracks to overlap, which results in higher data densities, but slower read and write times compared to CMR drives.

Huh?

Just to add to the confusion, CMR is also known as Perpendicular Magnetic Recording (PMR).

This illustration begins to explain it.

The SMR technology increases the density thereby reducing the number of platters needed for a given capacity. Less platters means less cost so you know which way the industry is going.

But the SMR recording technique is accomplished by overlapping the data from one track with the adjacent tracks. Reading one track back is OK but writing one track requires rewriting the adjacent tracks at the same time.

Think about that. To write a track, the drive has to cache the new data. Then it has to read the tracks adjacent to the track needing to be written and cache that data. Then it has to merge the old data with the new data before rewriting all the tracks.

Obviously this makes writes much slower than CMR drives with discrete tracks.

For "normal" usage, these slower writes aren't much of an issue. But in a RAID configuration, they really slow down the throughput.

So how do you know whether a given drive uses CMR or SMR?

Here's Seagate's status (archive.org). Here's Western Digital's status (archive.org). Here's Toshiba's status (archive.org).

It's not easy.