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First, let's differentiate data backups from data archives. Backups assume the use of data that is regularly updated and needs to be refreshed, while archives contain static data that most likely will be written once and read occasionally. Typically, archived data is not referenced frequently, so users should avoid maintenance on it. Archival media should incorporate technology and a data writing process that remains stable for long periods of time. It should not require user handling, or a connection to a computer or electrical supply.
Traditional hard disk drives (HDDs) will eventually fail because the technology is not designed to guarantee long-term retention of data. HDDs may also demagnetize over time if not used.
Optical archival media technologies, such as CDs, DVDs or Blu-ray, offer greater storage density and error correction, but their long-term reliability is still being analyzed. Initial concerns regarding DVD reliability focused on DVD-R and DVD+R; DVD+R discs are now considered more reliable. Assuming cold storage of archived data is being considered, manufacturers of archival-grade discs designed for cold storage (such as Verbatim Gold Archival and Panasonic Blu-ray Discs) claim they can be stored for at least 20 years with no maintenance. Keep them in watertight boxes in a cool area with no exposure to light. As with all good archival media practices, create multiple copies of the archived data and periodically rewrite the data to new disks.
Be careful when considering cloud-based archival offerings, as the cloud hosting company may not be around as long as you might like. One option is to rent a private online backup server that offers automatic mirroring/backup of archived data in case of a vendor hardware failure. You own and have access to your data, and hardware failures are the vendor's responsibility. A service-level agreement (SLA) may be advisable here. If the cloud provider goes out of business, you can still get your data back (add that to the contract or an SLA) and store it elsewhere.
An option for long-term data archiving is Amazon Glacier, which provides secure, low-cost data archiving and online backup. Customers can store large or small amounts of data for as little as 1 cent per gigabyte per month. The service provides cold storage of archived data, with the assumption that you will rarely have reason to access it.
Strangely enough, magnetic tape is still a popular archival medium for many organizations. It provides excellent quality, is cheap and readily available, and can be stored in environmentally optimized facilities for years with minimal degradation in quality.
Data archiving media best practices
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