How exactly does rsync determine that the copy has the incorrect timestamp and not the source file? Does it assume that the copy must be incorrect or are there other criteria that have to be considered?


Quoting Kevin Korb via rsync <rsync@lists.samba.org>:

Whenever you use --times (included in --archive) rsync will fix
incorrect time stamps.  The only thing --size-only is doing is keeping
the incorrect data instead of replacing it.

The purpose of these options is to "fix" a copy done in a way that did
not preserve timestamps but the data is known to have not changed.
These options allow rsync to correct the incorrect timestamps without
even looking inside of the files.  If you are not 100% sure your file
data matches you should not be using --size-only.

On 06/11/2017 09:28 AM, max.power--- via rsync wrote:
When a file of same length already exists at the destination then the
command 'rsync --archive --size-only' (--archive is same as -rlptgoD)
may change the modification time of the destination file even if no
modification was made.

Type the following commands in a terminal in order to reproduce the
problem:

$ mkdir source
$ mkdir target
$ echo "file one" > source/file
$ echo "file two" > target/file
$ srcstamp=201111110000.11
$ tgtstamp=200001010000.00
$ touch -t $srcstamp source/file
$ touch -t $tgtstamp target/file
$ stat -c "%y %s" source/file
$ stat -c "%y %s" target/file

Notice, that although the file in source/ and target/ folder contain
different content they do not  differ in size. Thus, there following
command should not make any changes:

$ rsync -rlptgoD --size-only source/ target

There was indeed no backup performed, which can be verified by viewing
the contents of the files:

$ cat source/file
$ cat target/file

However, the modification time of the target file was updated to the
same modification time as file in source/:

$ stat -c "%y %s" source/file
$ stat -c "%y %s" target/file

Since the file was not modified the modification time should not be
changed. Omitting the '-t' option in above rsync does not solve the
problem because when a backup is actually performed then the
modification time will simply be set to the current system time instead
of the modification time of the source file.

The same behaviour can be observed when using the flags -rlptgoD (or
--archive) with the --checksum flag. Although it is highly unlikely that
the files will differ if the checksums are the same, the modficaition
should still not be changed for consistency reasons.




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