An exact copy repeats the source's mistakes too. Getting yesterday's state back takes something else.
A mirror holds a single state: whatever the source had at the last run. Delete a file from the source by mistake, and the next rsync --delete deletes it from the mirror as well. Break the configuration, and the mirror gets the broken one.
A past state comes back from saved versions: a nightly archive with the date in its name, or a program like restic that keeps dated versions of the data. How many versions to keep and when to drop old ones is set by the retention rules.
A mirror is the tool for an exact copy as of now: moving data to another disk, or a standby server with the same files.
Yesterday afternoon someone deleted orders.csv from source by mistake, and overnight rsync -a --delete source/ mirror/ ran. Where do you get the file now?
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mirror lacks the file too: a reverse run brings back to source exactly what it already has.
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The overnight run with
--deletehas already removedorders.csvfrom mirror:ls mirrorwill not show it. -
A plan compares the two current trees, and the file is in neither: the output will be empty.
The mirror repeated the deletion, so the file survives only in a copy saved before it. Those copies and how long they are kept are set up separately from the sync.
Key points
- A mirror holds one, latest state of the source and repeats its mistakes, deletions included.
- A past state comes back from saved versions: dated archives or versions kept by a backup program.