What is the difference between a fixity check and a backup for keeping archived files safe?

Fixity checks and backups are both essential to long-term preservation, but they answer different questions. A backup asks, “Do I still have a copy?” A fixity check asks, “Is the copy I have still exactly what it was supposed to be?” Keeping archived files safe usually requires both working together.

What a backup does

A backup is a duplicate copy of your files, stored so that you can recover from loss. Backups protect against the obvious threats: hardware failure, accidental deletion, ransomware, fire, flood, or a corrupted storage system. The core idea is redundancy — having more than one copy, ideally in more than one location and on more than one type of media.

A backup alone, however, does not tell you whether the data inside the copy is still intact. If a file silently corrupts and that corruption is then copied into your backups, you may have many copies of a damaged file and not know it.

What a fixity check does

Fixity refers to the property of a digital file remaining unchanged over time. A fixity check verifies integrity by calculating a checksum or hash (for example, an MD5 or SHA value) when the file is ingested, then recalculating it later. If the two values match, the bits are unchanged. If they differ, something altered the file — bit rot, a failed transfer, or tampering.

Fixity checks do not create copies and cannot recover a lost file. They simply detect change, so you know when and which files need attention.

How they work together

• Backups give you something to restore from.
• Fixity checks tell you whether what you have — and what you restored — is authentic and uncorrupted.

A sound preservation routine runs fixity checks regularly, maintains multiple backup copies, and uses fixity results to decide which copy to trust. If a primary file fails a check, a verified backup copy can replace it. This pairing supports the reliability and authenticity expectations that records standards place on digital records over their full retention period.

Learn more about related practices on the archives and preservation topic hub.