The reason for that is pretty simple... Rsync isn't reading the existing file to find out that it is the same. Doing so would normally be a waste of rsync's time. A better question is why are your files changing timestamps when the data is the same.
On 12/18/19 6:47 PM, Wayne Piekarski via rsync wrote:
> I am using rsync with --backup --backup-dir to keep copies of files
> which have changed as part of an incremental backup system. However, if
> only the timestamp has changed, it creates a copy of the file in
> --backup-dir, and if thousands of large files have their timestamps
> changed, this can waste a lot of disk space on something which hasn't
> really changed.
>
> Interestingly, if you use --checksum, rsync will not create a file in
> --backup-dir unless the contents are truly different, but it will fix up
> the timestamp on the remote end to match. This is what I want, but I
> just don't want to pay the performance penalty of running --checksum all
> the time.
>
> Here is an example that shows the problem:
>
> mkdir ./SRC
> echo hello > ./SRC/a
> echo hello > ./SRC/b
> rsync -av ./SRC/ ./DEST/
> touch ./SRC/*
> ls -al --full-time ./SRC/ ./DEST/
> # Creates copies in BACKUP, even though contents are the same
> rsync -av --backup-dir=`pwd`/BACKUP/ ./SRC/ ./DEST/
>
> After this run, the BACKUP directory will contain copies of both a and b
> even though neither actually changed. If you add --checksum, then it
> avoids creating a copy, but still syncs the timestamps correctly.
>
> touch ./SRC/*
> # Does not create any copies in BACKUP since nothing changed
> rsync -av --checksum --backup-dir=`pwd`/BACKUP/ ./SRC/ ./DEST/
>
> The problem with --checksum is that for hundreds of gigabytes of data,
> it can be very slow to run over every file, especially if the timestamps
> are mostly actually the same. But without it, the delta algorithm in
> rsync has already decided to make a backup copy before it realizes later
> that nothing has changed.
>
> Is there a flag I can add to rsync that will tell it to only create a
> backup file if something actually changed, saving lots of wasted backup
> space?
>
> thanks,
> Wayne
>
>
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