Seagate to double disk heads to get stronger signal from disk platters
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September 15, 2014
Hard drive maker Seagate is considering doubling up or maybe even tripling the number of
disk read heads on its drives to get a stronger signal from disk platters, with more and
narrower data tracks.
Currently, a disk read/write head has a single read head and a single write head. If the
tracks on a disk platter are placed closer together, providing greater track density, and made
narrower (increased track pitch) then a read head could find its signal from a track is affected
by interference from neighboring tracks.
Seagate said during an analysts' session last week that TDMR, or Two-Dimensional Magnetic Recording,
could potentially help solve that issue.
The idea is that the interference from neighboring tracks is noise and what’s needed is
a better signal-to-noise ratio.
You can simply achieve this by having one read head directly behind the other – in series –
or by having three heads-- one for the target track and two outriders for the track edges either
on either side, in parallel.
The two outriders can be used to subtract adjacent track “noise” from the desired central track signal.
The background to this is that existing perpendicular magnetic recording (PMR) technology is
getting close to the point where the bit area cannot be shrunk any more so as to deliver disk capacity
improvements through having increased areal density.
A new recording technology is needed, and that is likely to be HAMR – heat-assisted magnetic recording –
in which the instability of ever-smaller PMR bits is replaced by similar size HAMR bits which, since
they require localized heating to change, are far more stable.
But HAMR technology is going to require the costly retooling of disk drive platter manufacturing,
as well as developing read/write heads with lasers on them to provide the short-term, highly localised
A practical way to delay such spending is to use existing PMR and have the tracks closer together.
Since you need less track width to read a data signal than you do to write it, tracks can be partially
overlapped to increase track density, with the disadvantage of having to rewrite blocks of tracks at a
data when data re-writes are needed.
This is shingled magnetic recording or SMR, and its downside is a slower write speed. By having
narrower tracks closer together, you can also increase the capacity of disk platters, without having
any write speed penalty, but then reading data from tracks becomes problematic.
TDMR promises to solve that issue, however. At the analyst event, Seagate’s president for operations
and technology, Dave Mosley, said shipping SMR drives that support 1.4 TB per square inch areal
densities, are 20 percent higher than PMR he said, which implies it’s reached 1.12 TB per square inch.
Seagate has shipped nearly three million SMR drives, and says caching and tiering can help
sort out the write issues.
That’s in the lab we think, as Seagate’s 3.5-inch Terascale HDD, announced in July 2013, has just 625 Gbit per sq. inch.
It’s 2.5-inch Enterprise Performance 10K drive, announced last week, has 644.6 Gbit/in2.
Mosley said TDMR can get us to 1.6 TB per square inch, representing a 15 percent increase on
the base recording technology.
He’s looking to this to start appearing from 2016 onwards-- he used the term product integration.
He said HAMR could go from 1.2 TB per square inch to as high as 5.0 TB per square inch.
HAMR's initial product integration would start in 2016 and Mosley thinks that Seagate
will have 20 TB disk drives by 2020.
Overall, HAMR requires heating a 20 nm laser spot whilst travelling at 80 MPH above the platter.
We would guess that Western Digital, HGST and Toshiba are all looking at TDMR as well, and that
2017 should see TDMR drives appearing.
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