Written by W. Curtis Preston
Thursday, 02 June 2011 00:46
Tape is inherently a more stable magnetic medium than disk when used to store data for long periods of time. This is simply "recording physics 101," according to Joe Jurneke of Applied Engineering Science, Inc.
I had heard rumblings of this before, but it was Joe that finally explained it in almost plain English in a post to this thread from hell on LinkedIn. Here's the core of his argument:
By the way, the time dependent change in magnetization of any magnetic recording is exponentially related to a term known as KuV/kt. This relates the "blocking energy" (KuV) which attempts to keep magnetization stable, driven by particle volume (V) and particle anisotropy (Ku) to the destabilizing force (kt) the temperature in degrees kelvin (t) and Boltzmans constant (k). Modern disk systems have KuV/kt ratios of approximately 45-60. Modern production tape systems have ratios between 80 and 150. As stated earlier, it is exponentially related. The higher the ratio, the longer the magnetization is stable, and the more difficult it is to switch state.....Recording Physics 101....
I had to call him to get more information. He explained how this came about. Disk drives have been pushed for greater and greater densities, which caused their vendors to create a much tighter "areal density." Tape, on the other hand, mainly got longer and fatter to accomodate more data in the same physical space. (Yes, it increased areal density, too, but nowhere near as much as the disk drive folks did.) The result is that the tape folks have more room to play, allowing them to use magnetic particles with a bigger particle volume (the V in the equation). The bigger the particle volume, the more stable the magnetism is, according to the KuV/kt equation. In addition, tapes are generally stored outside of the drive, which means their temperature is lower than disk drives. That means they have a lower k volume (degrees kelvin), which is one of the "bad" numbers in the KuV/kt equation. Having a higher V value and a lower t value is what translates into tape systems having ratios of 80-150, vs disk systems that have ratios of approximately 45-60. While I don't have an exact cite to point to in order to show these exact values, what he's describing makes perfect sense to me.
Add to this the fact that tape drives also have a lower bit error rate than disk. SATA disk is 1:10^14, FC disk is 1:10^15, LTO is 1:10^16, and IBM 3xx0 and Oracle T10000s are 1:10^17.
Add to this the fact that tape drives always do a read after write, where disk drives do not always do this.
Write data more reliably than disk
Read it after they've written it to make sure they did (where disks often don't do that)
Have significantly less "bit rot" or "bit flip" than disk drives over time.
Like I said in a previous post, I think we've put these guys out to pasture a little too soon.