Planning Historical Digitisation Projects
Backup
Backing up data is a basic precautionary step that everybody working
with computers should take. Backup copies are an insurance policy
against the possibility of your data being lost, damaged or destroyed.
Requirements for a Good Backup Policy
A good backup policy will protect your data from a large range
of mishaps. The range of events that you should consider when planning
how to backup your data includes:
- Accidental changes to data
- Accidental deletion of data
- Loss of data due to media or software faults
- Virus infections and interference by hackers
- Catastrophic events (fire, flood etc.)
A good backup policy should provide protection against all of these
threats.
Frequency of Backup
Backups should be made regularly to ensure that they remain up-to-date.
The more frequently data is being changed the more frequently backups
should be made. If your data is changing significantly every day
you should consider a daily backup, but if you are prepared and
can afford to redo a longer period of work then less frequent backup
may be appropriate.
As well as backing up frequently, you should keep several backup
copies made at different dates. Doing this guards against the danger
that your backup copy will incorporate a recent, but as yet undiscovered
problem, from your working copy.
Multiple Backup Copies
A backup copy may suffer the same mishaps as the working copy of
your data, so it is a good idea to spread the risk by maintaining
several backup copies. A minimum of two backup copies should be
maintained in addition to your working copy of the data.
Offsite Backups
More serious events, such as a fire in the office, will destroy
both the working copy of the data and any backup copies stored at
the same location. Some backup copies should be stored 'offsite'
(offsite is a relative term, dependent on the level of protection
you want).
As well as storing some copies offsite, it is useful to keep a
backup copy onsite. This copy can be quickly retrieved and work
recommenced if there is a minor mishap, such as the accidental deletion
of an important file.
Media
Backup copies should be made on new media. Do not continue to use
media once they start to develop faults. Specifically, floppy disks
are not a good media for backup copies. If they are used, they
should be replaced often.
Store backup copies on multiple media (e.g. zip disk and CD-ROM)
to avoid all your backup copies becoming corrupted by the same drive
or disk fault.
Multiple Formats
Store backup copies in both the software formats that you are using
and in exported formats (many spreadsheets and database packages
can exported to delimited text for example). This will help protect
you from subtle faults that can sometimes develop in complicated
data formats (such as database file formats) that may not become
apparent until after they have been included in both the working
copy and the backup copies.
Institutional Backup Policy
Projects should never assume that their institution's policies
will be appropriate to their needs. Always check.
- Institutions may maintain backups for a limited period
- Institutions may only provide backups to protect against complete
loss of data, and not individual users losing data
- Institutions may not backup all data held on their network
Many organisations advise their users to make their own backups
of critical data. This is good advice and should be followed.
Check Your Backup!
A backup that does not actually work is of no use at all. Always
test your backup procedures to ensure that your backup can be retrieved
and is useable.
Backup is not Preservation
A backup copy is an exact copy of the version of the data you are
working on. If your working copy becomes unuseable, you should be
able to start using your backup copy immediately, on the same computers,
using the same software.
In contrast, a preservation version of the data is designed to
mitigate the effects of rapid technology change that might otherwise
make the data unuseable within a few years.
Next: Preservation
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