Western Digital has pushed the capacity of its spinning disk hard disk drives (HDDs) by 2TB (terabytes) with the addition of an 11th platter. That has manifested in two new drives, the standard recording 26TB DC H590 and the new 32TB SMR (shingled magnetic recording) DCH HC690.
SMR drives overlap recording tracks – think clinker boat construction – and each new track slightly masks the previous one. It can be read, but rewriting is hampered a little compared to conventional recording with the gain of higher density.
This means SMR drives are best used for sequential writes, where, when full, they are completely erased and replaced by new writes, somewhat similarly to tape. By contrast, on standard drives you can write and erase at will.
Such HDDs usually spin at 7,200rpm and connect to hosts at 6Gbps via SATA or 12Gbps via SAS. But spinning disk hard drives fall far short of saturating available bandwidth, which can be between 600MBps and 1.2GBps. Having said that, the HC590 allows writes of 302MBps and the HC690 269.5MBps.
That’s a lot less performant than solid-state drives (SSDs), which can saturate SATA and SAS bandwidth – but at a cost, with flash prices often more than double per gigabyte that of HDD.
Western Digital soon to overtake Seagate?
While SMR drives add to recording density by eliminating lost space between tracks, Western Digital also gets higher density of data per track by using energy-assisted perpendicular nagnetic recording (ePMR), which it has used since 2020.
ePMR acts via two electric charges at the write head, which help avoid polarisation of existing bits on the platter surface by newly written bits. That occurs when bits are written closely together, especially as manufacturers seek ever-greater densities per track. Using a second charge allows for a more precise field at the write head.
Seagate has adopted an alternative to ePMR, namely heat-assisted magnetic recording (HAMR). That consists of using materials that are not polarisable until they are softened by heating. That occurs briefly at the moment of writing, just before the next is written. The magnetic field of the write head is in the form of a sphere which only modifies the bits required and not those that remain solid nearby.
Seagate seems to have succeeded at pushing the capabilities of HAMR further than Western Digital has with ePMR. By the end of the year, it will productise standard HDDs that achieve 28TB or 30TB, and SMR drives that get to 32TB or 34TB, and all with 10-platter drives.
A solid market: Spinning disk going nowhere
Toshiba has demoed 32TB and 31TB hard disk drives that use heat- and microwave-assisted magnetic recording (HAMR and MAMR) plus shingled drive tracks to boost capacity over existing products by more than 40%.
Currently, Toshiba’s largest capacity drive is a 22TB non-shingled MAMR model. Hard drive manufacturer competitor Seagate has 30TB and 32TB HAMR conventional and shingled drives out for certification with customers.
Spinning disk hard drives continue to find plentiful use as storage for the big cloud providers, where they are used as a more economical media compared to flash for use in durable services such as object storage – AWS S3, for example. Recent studies estimate that around 30 million hard drives are sold per quarter.