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View from the Hill:- Introducing WORM Hard Disk Drives

hard drives - with a write disable switch for specified zones

by Zsolt Kerekes, editor - - February 28, 2005

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WORM Hard Disk Drives will be coming to the market in a year or so. Remember where you heard it first. The same place which predicted the convergence of services, online content and storage which became embodied as Apple's iPod and iTunes.
Words and their connotations can change their meanings over time...

I first heard the term "WORM" (Write Once Read Many times) back in 1980 - used in the context of one time programmable non volatile memory.

Back then Intel was trying to convert manufacturers to use EPROMs (Erasable PROgrammable Memories) in low cost OTP windowless plastic packages for production runs instead of the more expensive ceramic packages used for development which had windows enabling the devices to be erased in a UV light box and reused. The plastic packages also had the advantage of being compatible with robotic insertion - unlike the more brittle ceramic.

In 1986 I was hearing the term "WORM" again, this time referring to the new generation of 5.25 inch optical disk drives which were hitting the market as long term archival storage.

Then for over a decade the terms WORM and optical storage were synonymous, although the fashionable technology of the time changed from writable CD, then writable DVD, and now HVD (Holographic Versatile Disc).

Then in 2003 - a new type of WORM wriggled into our consciousness - the idea of one time writable tape backup. Introduced by Sony as a feature in its SAIT drives - the concept seemed bizarre at first. But there were good commercial reasons behind the idea. US laws governing the ways in which data had to be archived created a guaranteed market for any data archiving technology which made accidental erasure of data foolproof. Sony managed this with some flash memory inside the tape cartridge which remembered that it had been configured as a WORM device and prevented over-writes at the low level driver level.

I'm going to introduce a new idea - and remember where you heard it first. Because in a few years time this could be a multi billion dollar segment of the hard disk drive market - which needs all the revenue it can get - as it's under constant pressure from two main trends:-
  • a capacity versus price technology curve which at times has much in common with the type of cliff that lemmings (or venture capitalists wedded to learning curve pricing) like to throw themselves off
  • serious threat of obsolescence at the entry level capacity end - by lower priced, lower power and more resilient flash disks.
My new WORM concept - which you may already have anticipated by this point - is WORM HDDs.

There are many reasons why WORM HDDs will come and are inevitable.

The main one is that if hard disk drive technology is to become the real replacement for tape storage - then cost per bit competitiveness - which some vendors claim has already been established - is not enough. HDDs will have to become available with true WORM capability - which means they will have to include a mechanical switch which enables them to be configured as WORM devices - and which over-rides driver level over-writes of the files on the disk when the directory is full. (Later - some industrial SSD makers now offer this feature.)

This will require standards to be set up so that drives from different manufacturers understand how they are supposed to operate. That process could be managed by a current trade body like IDEMA. Or it could be managed by a new body set up for the purpose.

WORM HDDs will have to have some other characteristics - such as the ability to power down - and power up by remote control in a predictable way - otherwise the power consumption of Petabyte disk archives will be horrendous. Some drives already do this. For WORM applications - low cost, low power and high MTBF will be more important than high performance. But we're already seeing many of these capabilities in newer serially connected hard disk drives.

What about reliability? How will hard disk drives compare to optical media or tape - when you come to read the data off them in 5 to 10 years?

Well - tape isn't exactly maintenance free.

If you don't rewind and rewrite the data from time to time it's not guaranteed to stay there. And tape media can stick and jam. Similarly optical media is subject to aging effects. If you take the unit of a hard disk drive WORM repository as being a network connected RAID system rather than a single disk drive - then the problem with long term data viability is easily resolved. At some stage in the 5 to 20 year cycle - when it's no longer economic to buy replacement drives - you just move the data onto another newer replicated WORM HDD system.

WORM HHDs will have a number of advantages compared to write many times disks. For example - the risk from viruses or administrator error deleting large chunks of replicated data (which does already happen with conventional RAID based disk to disk backup systems) will be eliminated. And compared to WORM tape - the WORM HHDs will be faster to read and write.

They will also have a FUD (Fear Uncertainty and Doubt) advantage compared to newer optical storage technologies - in being proven. It sometimes takes several years for the long term reliability of new storage media to be proven - and often the extrapolated data permanence and error rates in past technologies have been revised downwards following a few years of actual product experience.

And finally, WORM HDDs have the same superficial advantage that almost guarantees that the concept will be adopted. Like Internet SCSI, Serial Attached SCSI and similar concepts - we're already familiar with the words.

Laura's Worm Custard

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