Shingled Magnetic Recording Basics aka SMR Drives

Shingled Magnetic Recording (SMR) is a hard drive technology that allows for increased capacity from the same physical platters, often at a lower cost per terabyte. However, SMR is a complex technology that is often misunderstood. To understand how SMR works, one must first understand the fundamental asymmetry between read and write heads in conventional hard drives.

Write heads require a stronger magnetic field to flip the magnetic polarity of a spot on a platter, while read heads can sense the polarity with a much narrower field. CMR drives deal with this by spacing tracks apart wide enough to accommodate the full footprint of the write head.

SMR, on the other hand, throws away this spacing by writing each new track so it partially overlaps the track before it. This results in a much tighter track pitch, as the data read back is only the sliver of each track that hasn't been paved over by its neighbor. By packing tracks tighter, more can fit on the same platter, effectively increasing capacity.

The CMR model has tracks that stand alone, with each track being a fully independent lane. Rewriting any single track does not disturb the tracks on either side of it. However, the SMR model works differently, with tracks overlapping like shingles. Each track is laid down partially on top of the last one, which shrinks the pitch and buys density. But this also means writing to a track destroys the part of the neighboring track it overlaps, unless something rewrites that whole overlapping run in order.

The key to understanding SMR lies in the concept of 'managed'. Managed refers to who is responsible for respecting the order those overlapping tracks have to be written in. There are three ways an SMR drive can be managed: DM-SMR (Drive-Managed), HA-SMR (Host-Aware), and HM-SMR (Host-Managed). DM-SMR hides the shingling entirely, presenting an ordinary block device that accepts random writes anywhere.

HA-SMR exposes zone information and accepts explicit zone commands from a cooperative host. HM-SMR enforces zone rules at the command level and rejects anything that violates them.

Checking which type of SMR a drive uses can be tricky, as the retail box rarely indicates it. On Linux systems, the `lsblk -o NAME,ZONED` command can report whether a drive is host-managed or none, and `smartctl -a /dev/sdX` can usually surface the zoned device model. If neither indicates a zoned device and the drive is a spinning drive shipped after roughly 2015, it's not advisable to assume it's SMR, as many drive-managed models never advertise SMR at all.