Hello everybody!
Here is a way to build a hard disk twice as fast as now: build it with two arms!
The arms are at opposite locations with respect to the platters. Each platter track holds data blocks: #0 2 4 6... and 180° apart: #1 3 5 7... Both arms read or write the same track as soon as the data is over a block long.
So when the OS requests to read or write blocks #0 1 2 3, one arm reads #0 2 and the other #1 3, and since both the blocks and the arms are 180° apart, data is read simultaneously, thus doubling the contiguous read/write throughput. Get 240MB/s with current technology!
Even better, the platter's rotational latency is halved. That is, if reading blocks #0 2 by the North head and #1 3 by the South head requires to wait more than 180°, just swap the roles and read #0 2 by the South and #1 3 by the North. A 7200/min disk then has the same rotational latency as a 15000/min!
What does not improve with two arms is the time needed to move a head from one track to another. So with usual disks with a seek time of 8.5ms and a rotational latency of 4.2ms, random access time sinks from 8.5+4.2=12.7ms to 8.5+4.2/2=10.6ms, and this is the figure HdTach or HdTune will show - less impressive. Drop the innermost area of the platter to improve these benchmarks.
One should understand, however, that this time is very pessimistic because files needed by an OS or an applications aren't just scattered randomly over the disk. Since Windows 98, the files each application loads are observed by the defragmenter who then places them next to another on the disk.
Then, as the arm moves very little and hence quickly, disk rotation is the main latency of a disk. This is the performance measured by CrystalDiskMark for instance, and it will double with two arms.
XP and followers improve hard disk operation further by issuing parallel read commands for all files XP or an application need (keyword Prefetch), so that the disk can reoptimise the access sequence (keyword NCQ) depending on the files' positions on the platters. This is the performance measured by Atto for instance, and it doubles as well with two arms.
Optimizing multiple file accesses with two arms requires a special controller onboard the disk; two independent Sata connexions on a separate Raid-0 adapter wouldn't be quite as good. It's about time to issue the Sata-3 standard, we saturate Sata-2 here!
A two-armed disk has much better performance than a raid-0, because it halfs the average rotational latency instead of multiplying it by 4/3 - this is even better than spindle synchronization, and is compatible with adaptive formatting. Very small stripe sizes would make sense with Windows 2000 Pro and before, where parallel read commands are uncommon - another advantage over Raid. In servers and XP, bigger stripes make sense thanks to simultaneous commands.
Two arms need a longer disk case, and this is a huge drawback for 2.5" and 1.8" disks, where Compact Flash and SSD are likely to exterminate hard disks. Put a 1.8" platter in a 2.5" case? But for 3.5" disks, things look very good. Many PCs have room for a longer disk case, and any PC enthousiast would adapt a disk tray to double the disk performance. As for a file server, people build the computer around the disks anyway.
This means that a two-armed disk can reuse existing platters and existing arms and heads - the difficult technology in a disk. Only the controller and the case must be adapted. All OS and applications can remain the same. The improved performance should crush Flash speed; combined with terabyte capacity, this would ensure some years 3.5" hard disk supremacy over Flash competitors.
Of course, a 10,000 or 15,000 /min disk can be built with two arms as well. And more than two arms would improve performance further (organize blocks accordingly); this needs a faster interface, and a disk case broader as well - not a significant drawback for a file server.
I hope to be the first inventor of the multi-armed disk, or at least of the corresponding block organization that reduces the rotational latency. I have no patent on it and it is public from now on, so please feel free (but check existing patents before, I didn't) to use this idea to provide me with faster hard disks.
Marc Schaefer, aka Enthalpy