Perpendicular magnetic recording (PMR) and shingled magnetic recording (SMR) are technologies used to physically store bits of data onto a hard disk drive (HDD). For HDD manufacturers, one of the most important concerns is areal density, which is the number of bits of data that can be recorded onto a magnetic disk. Areal density is measured by calculating the number of bits or gigabits per square inch, with the higher values allowing for greater storage while using the same amount of disk space. SMR offers larger drive capacity than the traditional PMR because SMR technology achieves greater areal density.

This article provides an overview of PMR and SMR technologies and highlights key differences between them.

Perpendicular Magnetic Recording (PMR)

PMR, also known as conventional magnetic recording (CMR), works by aligning the poles of the magnetic elements, which represent bits of data, perpendicularly to the surface of the disk. Magnetic tracks are written side-by-side, without overlapping. Because the write head is normally larger than the read head, HDD manufacturers try to shrink the size of the write head as much as possible.

Shingled Magnetic Recording (SMR)

SMR is an extension of PMR and offers improved areal density. 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. By overlapping the tracks, write heads become thinner, thus expanding areal density.

Differences between PMR and SMR

Regardless of whether an HDD uses PMR or SMR, the read head only requires a small portion of the written magnetic track. When new data is written on an SMR drive, the tracks are fully readable without impacting performance.

However, when the data on an SMR drive is edited or overwritten, the write head will not overwrite the data onto the existing magnetic track. Instead, the new data will be written onto an empty area of the disk, while the original track with the old data will temporarily remain. When the HDD becomes idle, it will enter a reorganization mode, where the old bits of data on the original track will be erased and made fully available for future use.

This reorganization mode must occur to completely delete tracks, making the idle time essential for an SMR drive. If an SMR drive is being used heavily for read and writes, it will not have enough time to reorganize the magnetic tracks, causing the tracks with the old data to stay put temporarily. As a result, the SMR drive may need to write new data and reorganize the old track at the same time, resulting in a negative impact on the overall read/write performance. To combat this, SMR drive manufacturers have developed firmware that optimizes read/write performance while data is being overwritten.