[ale] OT shingled magnetic recording can double a hdd capacity

[Ron Frazier (ALE)]

Hi all,

I heard about this on the This Week In Computer Hardware podcast and 
thought you might find it interesting.

http://twit.tv/twich

www.pdl.cmu.edu/PDL-FTP/FS/CMU-PDL-12-105.pdf

http://www.google.com/search?q=Shingled+Magnetic+Recording+%28SMR%29&btnG=Search&hl=en&biw=&bih=&gbv=1

It's a technique called shingled magnetic recording.  It seems that the 
modern hard drives write tracks much wider than is necessary for the 
read head to read from.  Normally, each track has a distinctive (but 
very tiny) gap between it and the next track.  This gap is dictated by 
the minimum width of space the write head needs to get in there and 
rewrite sectors at random.

However, if you give up the ability to do totally random writes, you can 
double the capacity of the drive by writing overlapping tracks, like 
shingles on a roof.

Let's say you write track 1, then you step the head 1/2 track over, 
rather than a full track.  Then, you write track 2.  Then, you step 1/2 
track over again, where track 2 would have been, and you write track 3.  
Then you do track 4 the same way.  You've written 4 tracks in the space 
that would have previously held 2.  The read head can still read them 
with minimal modifications.  So, with just a small mechanical, 
electrical, firmware tweak, you've doubled the capacity of the drive.

The catch is that if you try to rewrite any track, you'll erase part of 
the adjacent track.  So, you have to rewrite the entire drive at once to 
rewrite anything.

There is a way around that, and that is to write the tracks in bands.  
Each band is written in shingled mode, so each band has to be rewritten 
all at once.  However, the bands have a normal track gap between them.  
So, you can rewrite one band at a time rather than the whole disk.

The tracks on a modern drive are mind bogglingly small.  According to this:

http://storageeffect.media.seagate.com/2011/11/storage-effect/the-period-at-the-end-of-a-sentence-is-1000000-nanometers-wide/

There are 340 THOUSAND tracks per inch which means they're only 75 
NANOMETERS wide.  That's somewhere between the size of a virus and an 
anti body.

Let's say there are 340,000 tracks on a drive platter.  Let's say you 
divide that into 1000 bands of data, which are all shingled.  Let's say 
you give up a track each time you leave a gap between bands, so you give 
up 1000 tracks.  You still have 339,000 tracks to play with.  Since the 
shingled tracks are half width tracks, you've essentially doubled the 
drive.  If you have 1000 data bands on what is now an 8 TB drive, you'd 
have to rewrite data in 8 GB segments to rewrite a data band.

These numbers are made up, except for TPI, but you get the idea.

We're probably not going to see applications of this any time soon since 
there would need to be hardware and software standards and OS and 
controller awareness.  However, to me, it's a fascinating concept.  I 
wish I could figure out how to implement it on my on in a file system 
driver or something.

Sincerely,

Ron


-- 

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Ron Frazier
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