HDD - SMR versus CMR

Datatalk.

Some newer drives use SMR instead of CMR recording. Unfortunately, manufacturers are not clear when they are using SMR technology.

SMR mostly affects the writing speed, may decrease lifespan, and greatly hampers (if not completely blocks) recovery of data upon OS failures, drive failures, or power failures. It should be cheaper (to manufacture)...

Links

Because these people know better (this blog entry is mostly for my own reference).

• Storage review

• PC Inside

• https://www.truenas.com/community/resources/list-of-known-smr-drives.141/

• https://nascompares.com/answer/list-of-wd-cmr-and-smr-hard-drives-hdd/

• Seagate https://www.seagate.com/nl/nl/products/cmr-smr-list/

• Western Digital https://support-en.wd.com/app/answers/detailweb/a_id/50697/related/1

SMR drives - Shingled Magnetic Recording

Data is recorded 'overlapping', like the shingles on a roof, thus allowing more data on a drive. (Reading requires less space than writing.)

• All SMR drives have larger cache to compensate (somewhat)
• Some (all?) have additional CMR tracks to buffer larger writes
• Drives do 'invisible track and data' management in the background, reorganizing data in SMR tracks, which effectively means a. read all data from a block of sectors, modify that data, the writing all that data back.
• OS and tools think they read / write from a specific sector, but the drive is handling all that shuffling around and hiding it from the OS.

The good

• Whilst reading these drives perform just as well as CMR drives.
• When doing incidental writes (less than half cache size, and giving the drive enough time to process the new data) they should also perform well.
• Drives are suited for large, continuous file storage with little changes (a collection of movies, music, or a data archive).
• Drives may be cheaper to manufacture

The bad

• When data exceeds (half) the cache size, the incoming data may interfere with the hidden 'reshuffle' process, and the drive will slow down.
• If the drive is busy reshuffling data, and the power to the machine fails, then there will be a much higher chance of data loss.
• Any errors will affect a larger number of blocks, not a single sector. Recovery will be more difficult if not impossible.
• Because writing is very slow, these drives will not work properly in NAS stations (causing failures during rebuilds).
• Completely unsuitable for random, changing data base usage.

Conclusion

In general it is better to use CMR than SMR, unless the price difference starts playing a role. When doing large file storage (write once, read many) like movies or music tracks SMR will do.

Examples

• WD 4 TB - WD40EZRZ - CMR
• WD 4 TB - WD40EZAZ - SMR