Can you use the last modified file time on Windows as change indicator for incremental file backups?

Short answer: No.

If your backup software is using the last modified time on Windows as the only indication if a file changed or not the backup software will miss some changed files.

The first issue is that Windows does not update the last modified time until the file is closed by the program writing to it. To work around that your program will need to track which files are open when a shadow copy is created.

The second less known issue is that last modified times are not changed at all if a file is modified by memory mapping it and then modifying it via memory mapping. This isn’t just an edge case – Windows itself creates such files, as well as e.g. Microsoft Exchange and other performance sensitive applications.

UrBackup handles both cases during incremental backups, many other backup software does not.

One thought on “Can you use the last modified file time on Windows as change indicator for incremental file backups?

  1. “Daddy, what have you been doing all your life?” “Well, I’ve been a blogger, son!” “Wow, cool, I want to read your blog! Where is it?” “Sorry, honey, I just ignored backing it up!” It occurred to me when I was searching for the solutions here https://www.baculasystems.com/enterprise-backup-solution-with-bacula-systems/easy-and-scalable-windows-backup for my enterprise and for my personal photos)) Sorry for the off top, just decided to share my thoughts on it. A lot of my friends got stuck with this problem.

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