• name the backup sub dir with suffix "NOT COMPLETED"
• remove the suffix after last file

generally:

• sort files to backup from resent modified time to oldest modified time (Idea: the newest files can't exist in any backups created in the past)

resume

• resume backup if last sub dir has the "NOT COMPLETED" suffix
• rename last dir with suffix to current timestamp
• look in database for OLDEST_FILE_TIMESTAMP
• read source files
• sort by timestamp (newest to oldest)
• split file list by OLDEST_FILE_TIMESTAMP -> OLD_FILES, NEW_FILES
• backup OLD_FILES first
• compare NEW_FILES only by filesize/mtime
• remove "NOT COMPLETED" suffix from backup sub dir

start new backup

• We can use the same resume strategy on every new backup run to save many times