Genealogy: Digital media life expectancy

Genealogy: Digital media life expentancyA lot of ink has been spilled over the issue of media longevity. Media consumers and producers have placed a great deal of emphasis on seeking and promoting high longevity media. Ultimately, however, for a great many reasons, longevity is overrated as a desirable media characteristic.

  • Media life expectancy claims are statistical averages based on accelerated aging tests and can only provide a rough estimate of how long any particular piece of media will last under certain storage and handling.
  • Longevity provides no protection against many media threats, including theft, natural disasters, infrastructure failures, and accidental handling damage.
  • Media technology changes so rapidly that high longevity media is likely to be threatened by obsolescence before its useful life is over.

Suggested Preservation Action
A better strategy is to take steps to maximize the intrinsic longevity of standard longevity media.

  • Adhere to good storage and handling practices.
  • Buy quality media.
  • Take note of media manufacturer and batch numbers so performance and quality trends can be tracked.
  • Don’t overbuy.
  • Remember that some unrecorded media has a shorter shelf life than recorded media (optical & magneto-optical in particular).
  • Buy media designed for the speed and capacity of the drives in which it will be used.

Suggested Preservation Action
All media needs periodic testing to confirm data integrity. At a minimum this should include procedures to:

  • Confirm fidelity of all media immediately after recording.
  • Once recorded, read samples (by batch code and manufacturer and/or storage location) of entire media and samples of files from several media on a periodic basis.
  • Determine indications of problems with specific batches, manufacturers, or storage conditions and more extensive testing within those categories.
  • Test blank media (can be expensive and time-consuming).
  • Monitor use of error correction and replace media before errors become uncorrectable.

Specific Media Issues

Hard disk drives

  • Highly commoditized. Expect to pay more for quality drives with above average reliability.
  • Don’t buy excess capacity. Prices and technology change extremely rapidly.
  • Don’t expect more than five years use from any hard disk drive.
  • High temperature can dramatically reduce life expectancy. Control the environment and make sure fans are working and not clogged with dust.

Magnetic tape

  • The most commonly used backup medium.
  • New technologies offer extremely high density storage.
  • High density cartridge technologies (SDLT, LTO, AIT) considered the most reliable.
  • New generations of tape formats appear regularly. Backward compatibility is usually only offered for 1-2 generations.

Optical (CD/DVD variants)

  • • These media have several possible failure modes
    • Dye layer (for recordable media)
      • best stability/compatibility compromise is metal-stabilized cyanine (blue-green)
      • phthalocyanine (pale blue) and metalized azo (deep blue) also good
      • color can be deceiving—don’t use it as definitive sign of a dye layer
    • o Reflective layer
      • gold most stable
      • silver and alloys becoming more common (because they’re cheaper, not because they’re better)
      • Rewritable media uses aluminum; recordable media can’t use aluminum due to reactivity with dye layer
    • Substrate separation
  • Unrecorded media has 5-10 year shelf life
  • Top surface (label area) of CDs requires extra care, as it is more vulnerable than the bottom layer
  • CD/DVD cleaning should be done axially (i.e. outer edge to inner edge), not radially or along the tracks
  • DVDs are more vulnerable to flexing damage due to closer track spacing. Use special DVD carriers to minimize flexing upon removal

Disaster Recovery
The methods and procedures already mentioned are designed to minimize casual loss of data and maximize media longevity. However, even if you had perfect storage conditions and impeccable handling protocols, some media would still fail. Therefore, valuable data must be stored redundantly, that is, backed up, on more than one piece of media. In addition, backups and disaster recovery plans are needed to avoid catastrophic media loss from causes such as:

  • Sabotage (theft, vandalism, malicious modification/erasure, viruses, terrorist attack etc)
  • Natural disaster (fire, flood, earthquake, hurricane, tornado, infrastructure failure)

A disaster recovery plan that deals specifically with information technology infrastructure is needed. Developing such a plan is not a one-time process; it has to be tested and modified as changing circumstances dictate. Revisit the disaster recovery plan for events such as new staff, new or reorganized physical plant, or new equipment. Once a disaster recovery plan is in place, take steps to prevent catastrophes and minimize damage from them.

The maintenance of redundant copies of valued digital content is an essential component of any digital preservation program, and a key element in the prevention of catastrophic loss. A great variety of backup solutions are available. Which to use depends on:

  • Quantity of data
  • Rate of change
  • Degree of automation desired
  • Available budget

In addition to backing up data files, application software and operating systems may also need backup. In some cases, it may be necessary to purchase additional licenses or obtain special permission from the software vendor in order to back up applications.

In addition to testing backup media periodically to ensure the data is still readable and has not been altered, restore procedures should also be tested to ensure that the hardware, software and any outside vendors involved in maintaining backups are all functioning as expected.

A prudent backup strategy places at least one copy of all critical data at a sufficient distance from the main data store so that it is not likely to succumb to the same disaster. This is called off-site storage. Institutions should check with regulatory agencies for their records retention requirements. Medical and financial records may have more rigorous requirements for the distance of the off-site storage facility from the main facility. Establishing a reciprocal storage arrangement with a cooperating institution may be a low-cost way to manage off-site storage. If outsourcing, make sure you are getting true data management, not just warehousing. Generic storage facilities are unlikely to know how to properly store and handle digital media. Environmental controls and handling protocols should be at least as rigorous as those at the main facility.

An important consideration in selecting a backup strategy is the possibility of loss of the entire primary facility (all equipment and data) to a disaster. In such a situation, it will be necessary to replace the entire IT infrastructure and to restore the secondary data store to new equipment. If new equipment that can handle the backup media, the restoration software, and the applications software and operating environment needed to access the data cannot be purchased, it may be time to rethink the backup plan.