Shingled magnetic recording (SMR) is a magnetic storage data recording technology used in hard disk drives (HDDs) to increase storage density and overall per-drive storage capacity.[1] Conventional hard disk drives record data by writing non-overlapping magnetic tracks parallel to each other (perpendicular recording), while shingled recording writes new tracks that overlap part of the previously written magnetic track, leaving the previous track narrower and allowing for higher track density. Thus, the tracks partially overlap similar to roof shingles. This approach was selected because physical limitations prevent recording magnetic heads from having the same width as reading heads, leaving recording heads wider.[2][3][4]: 7–9
The overlapping-tracks architecture complicates the writing process since writing to one track also overwrites an adjacent track. If adjacent tracks contain valid data, they must be rewritten as well. As a result, SMR drives are divided into many append-only (sequential) zones that need to be rewritten entirely when full, resembling flash blocks in solid state drives. Device-managed SMR devices hide this complexity by managing it in the firmware, presenting an interface like any other hard disk. Other SMR devices are host-managed and depend on the operating system to know how to handle the drive, and only write sequentially to certain regions of the drive. [4]: 11 ff. [5]
Seagate has been shipping device-managed SMR hard drives since September 2013, while referring to an increase in overall hard disk drive capacity of about 25%, compared to non-shingled storage.[1] In September 2014, HGST announced a 10 TB drive filled with helium that uses host-managed shingled magnetic recording,[6] although in December 2015 it followed this with a 10 TB helium-filled drive that uses conventional non-shingled perpendicular recording.[7] In November 2018, HGST introduced 14- and 15-TB drives.[8]
Western Digital, Seagate and Toshiba have begun selling SMR drives without labeling them as such, thus revealing a large controversy, as SMR drives are much slower in some circumstances than PMR drives.[9] These practices were used to be in both data storage-dedicated (for servers, NASes and cold storage) and consumer-centric HDDs.
There are three different ways that data can be managed on an SMR drive:[10][11]
A Device Managed drive appears to the host identically to a non-shingled drive. It is not necessary for the host to follow any special protocols. All handling of data, as it relates to the shingled nature of the storage, is managed by the device. In addition, the host is unaware that the storage is shingled.[4]
This type of SMR drives are often not labelled by the manufacturer. Its firmware-controlled operation can be compared to solid state drives, as LBA addresses do not correlate much to on-disk structure. The append-only zones are very slow for random writing, so writes are first sent to a PMR (CMR) cache, and the disk moves these data to SMR parts when idle. RAID resilvering tends to overload the cache, sending SMR drives into minutes-long pauses that often get interpreted by the RAID controller as drive failure.[12]
The zoned nature of SMR also means that the disk suffers from write amplification, although for hard drives the main problem with writes is speed instead of longevity. Some SMR hard drives support TRIM for this reason.[13]
A Host Managed device requires strict adherence to a special protocol by the host. Since the host manages the shingled nature of the storage, it is required to write sequentially so as to not destroy existing data. The drive will refuse to execute commands which violate this protocol.[4]
Host Aware is a combination of Drive Managed and Host Managed. The drive is capable of managing the shingled nature of the storage and will execute any command the host gives it, regardless of if it is sequential or not. However, the host is aware that the drive is shingled, and able to query the drive for fill levels. This allows the host to optimize writes for the shingled nature, while also allowing the drive to be flexible and backwards-compatible.[4]
While for traditional SMR models each zone is assigned a type at manufacture time, dynamic hybrid SMR drives allow to reconfigure the zone type from shingled to conventional and back by the customer.[14][15]