WD improves data density of HDDs by putting metadata on nand memory
Western Digital introduces a new method to increase the data density of hard drive platters. The manufacturer calls it OptiNAND and uses nand memory to store metadata. WD presents a 20TB HDD with the technology.
With OptiNAND, WD can make a hard drive with a capacity of 20TB, using the same epmr technology as the manufacturer now uses to make up to an 18TB HDD. WD already had a 20TB HDD in its range, but that is an SMR variant. With this shingled magnetic recording technique, data tracks on the HDD platters are partly superimposed. This increases the storage capacity, but is detrimental to the performance. The new 20TB HDD with OptiNAND does not have that disadvantage. The epmr technique used is a variant of conventional perpendicular magnetic recording, or cmr.
WD is now supplying the 20TB HDD with OptiNAND to the first customers, but the manufacturer has not given exact product specifications. It is a hard drive with a total of nine platters, with a capacity of 2.2 TB each. WD has increased the capacity of the platters by adding a nand module that stores metadata that normally resides on the HDD itself.
Nand memory for metadata
Hard drives contain gigabytes of metadata. This is partly already there from the factory and during use it is further supplemented. That metadata contains details about data stored on the drive and improves the performance and reliability of HDDs. Putting that metadata in the OptiNAND provides more space for storage capacity and, according to WD, also improves performance because the metadata can be quickly accessed from the flash memory.
With hard drives, data sometimes needs to be refreshed to avoid interference with data on nearby tracks. The closer the data tracks are to each other, the more often they need to be refreshed. With traditional HDDs, this is refreshed per track, but WD says with the OptiNAND HDD when writing the write operations are stored in the NAND memory at sector level. As a result, the refresh also takes place per sector instead of per track. This makes it possible to place the tracks closer together, without losing performance.
WD does not specify how much nand memory the OptiNAND HDD has. The manufacturer does not provide details about this in a technical document either. WD does write in it that it concerns a BGA package that contains a controller with one or more nand memory chips. WD does not give any details about the speed of the memory used. The document does state that various memory chips can be used. These are eMMC 5.1, UFS 2.1, UFS 3.0 and UFS 3.1.
The technique differs from the so-called sshds, which have been around for years. These are traditional HDDs that have a small SSD on board, as it were. In addition, the nand memory is used to store frequently used files, so that they can be read quickly. This speeds up the operation of those HDDs, but does not affect the capacity of the platters. With the OptiNAND technique there are no files on the flash memory, only metadata.
The nand memory can also be used to store ‘more than 100MB’ of write cache data in the event of power loss. With regular HDDs, that is only about 2MB, because they have a minimal amount of cache memory. According to WD, the performance loss of the OptiNAND HDDs with write cache disabled is therefore minimal compared to when write cache is enabled.
Accurate writing head
To be able to read the platters with the higher data density, a very accurate write head is also needed. WD uses its triple stage actuator for this, which uses three pivots to accurately determine the position of the write button. WD already used this technology in its 20TB SMR HDD and in 16TB and 18TB CMR HDDs.
Roadmap of WD with Triple Stage Actuator
WD: OptiNAND HDDs up to 50TB this decade
In its own words, WD can use the OptiNAND technology to further increase the storage capacity of HDDs in the coming years. Sometime in this decade, that should yield 50TB HDDs. The technology can also be used with SMR HDDs to further increase the capacity per platter.
Manufacturers are trying to increase the capacity of hard drives in various ways now that the data density of platters with the traditional pmr technology has reached a ceiling. Seagate mainly focuses on the hamr technique, or heat-assisted magnetic recording. Write heads with lasers are used in order to be able to write smaller bits on material that first has to be heated. A variant of this is mamr, or microwave-assisted magnetic recording. In 2017, WD said it was investing heavily in mamr, but the manufacturer has not supplied such HDDs and is currently using epmr as an intermediate step.
Developments in the market for hard drives are currently largely focused on the business market and data centers. The new techniques are hardly used in consumer HDDs, because data density does not play a major role there and the market has largely switched to SSDs.