Seagate CEO Dave Mosley hinted at disk drives with more than two actuators in the company’s latest results earnings call. What might that mean, and how might it be implemented?
Currently, Seagate is shipping an Exos 2X14 dual-actuator drive with eight platters and a 14TB capacity. Shipping started in 2019 and, since then, Seagate has introduced 16TB and 18TB single-actuator Exos drives, with 20TB ones on their way. These use a nine-platter design, with the 18TB drive boasting a capacity of 2TB per platter. Should we then expect 16TB, 18TB and eventually 20TB dual-actuator drives?
Actually, no. The additional actuator mechanisms take up space inside the drive enclosure, and capacity has to be sacrificed for that — the second actuator causes a platter space to be used up.
Serial vs parallel
Because Seagate’s Mach.2 dual-actuator tech has two actuators, the big deal is that this doubles the IOPS the drive can sustain and nearly doubles the throughput — a 1.8x to 1.9x increase. The Exos 2X14 increases IOPS from the 80 or so of the single-actuator Exos 14TB drive to around 160, while the throughput on large sequential IO changes from 261MB/sec to about 483MB/sec. How so?
The thing is that, while a drive has multiple platters with a pair of read/write heads per platter (one per surface), nothing — apart from platter spinning and read/write head slider movement — happens in parallel. Only one read/write head can write or read data at a time. A multi-platter spinning disk is a serial device.
A theoretical way to make a multi-platter disk operate in parallel would be have to an actuator per platter — nine actuators for a nine-platter drive. But that would send both the power draw and cost of disk drives higher, and may not be physically possible inside the fixed 3.5-inch drive format.
A look at the drive’s components shows why.
Drive actuator mechanism
All the read/write heads on their slider arms move across the disk’s surfaces by rotating around a shared pivot axis. The sliders are mounted on a single E-block (possibly so called because of its shape). This E-block rotation is caused and controlled by a voice coil motor (VCM), which has an electrically charged coil moving between two magnets on the other side of the pivot from the E-block.
We’ve labelled a Mach.2 image to show these components;
It is obvious after studying Seagate’s Mach.2 image above that there must be two actuators: one for the upper E-block and its slider arms, and a second for the lower E-block. A look at a Seagate patent diagram for a multi-actuator drive shows this clearly.
The second actuator needs a second VCM, which increases the component cost and the electricity needed to operate the drive — sending that up by 26 per cent.
The diagram shows a gap between the upper actuator’s VCM and E-block and the lower actuator’s VCM and E-block. This enables them to rotate independently around the pivot axis without touching one another.
Seagate’s Exos 2X14 couldn’t be a 2X18, because the 18TB Exos is a 9-platter drive and the dual actuator mechanism needs space between the upper and lower actuators — labelled E-block gap in the diagram above — and this causes a platter to be lost.
Actuator additions
Knowing this, let’s hypothetically add another actuator, or even two more. Does that mean that we lose a platter per additional actuator?
It could mean that, but we don’t actually know how much of the space needed for a disk platter is actually used up by an additional actuator. It could be all of it, or just some of it — 20 per cent, say. In the eight-platter Exos 2X14, enough of the space is taken up to prevent it being a nine-platter drive.
Therefore, were Seagate to build a dual-actuator drive based on its nine-platter, 18TB Exos product, with a 2TB per platter capacity, the result would be an eight-platter drive with 16TB capacity.
Now let’s add a third and a fourth actuator.
If each one takes up a one-platter space, then a triple-actuator product would be a seven-platter 14TB drive with about 240IOPS and 705MB/sec throughput. The quad-actuator model would be a six-platter 12TB drive with roughly 320 IOPS and 927MB/sec throughput.
The question is whether the gain in IOPS and sequential throughput would be worth the lost capacity, the increased power draw, and component cost.
Suppose each additional actuator takes up half of the platter space, and that four actuators means two platters are lost. Then a quad-actuator drive based on the nine-platter Exos 18TB product would be a seven-platter, 14TB drive. But if each additional actuator took up a third of a platter space, then the quad-actuator product would be an eight-platter drive with 16TB overall capacity.
The E-block gap is a crucial parameter affecting the platter count in a drive with anything more than two actuators.
Logical Unit Number size and component simplicity
The Exos 2X14 is treated by host software as two logical drives, each with 7TB capacity. In effect, there are two 7TB LUNs. If a dual-actuator mechanism was added to the 18TB, nine-platter Exos drive base, the result would be two 8TB logical drives. Adding a triple actuator mechanism that caused two platters to be lost would result in a seven-platter drive with 14TB capacity overall but two unequal logical drives — a four-platter 8TB one and a three-platter 6TB one.
Core multi-actuator drive customers — namely hyperscaler businesses — might be concerned that they then had unequal capacity logical drives.
It may be simpler, from an architectural and component point of view, to double the dual-actuator drive mechanism to get a quad-actuator drive stack — if the four actuators in total only caused the sacrifice of a single platter. That way you would get minimal lost capacity and maximal increase in IOPS and throughput.
Prognosis
Seagate is not alone in developing this technology. Western Digital demonstrated multi-actuator technology at the OCP summit in March 2019.
Together with Seagate hinting at having more than two actuators, Blocks & Files thinks that the HDD industry — Seagate and Western Digital certainly and Toshiba maybe — will bring out 16 or 18TB dual-actuator drives by the end of 2022. Seagate may demonstrate a quad-actuator drive by then as well.
