mjhjmtu

When rsyncing a directory to a freshly plugged-in external USB flash drive, via

rsync -av /source/ /dest/

all files get transferred (i.e. rewritten) despite no changes in the files.

Note that overwriting the files only takes place once the USB is un- and replugged. Doing the rsync command twice in a row without unplugging the drive in-between does successfully skip the whole directory contents.

Including the -u update option and explicitly adding the -t option did not change anything.

The mount point remains the same (i.e. /media/user/<UUID>, the drive is automouted by xfce, the /dev/sdxy obviously changes)
The hard drive source is ext4, while the USB is vfat with utf8 character encoding.

What could be the reason for this behaviour is it the change in the /dev/ name entry? How can I make rsync run with properly recognizing file changes? My backup should just take seconds without this, while it now is always minutes due to the large amount of data being overwritten repeatedly, nor is the massive writing the best for the flash drive's life time expectancy.

Your FAT drive can store timestamps only to two second accuracy. When you unplug and replug the drive you effectively break all the file times. See the --modify-window option for a workaround.

Secondly, you're never going to see fast backups with rsync like this, because when copying locally it behaves much like cp.

By default, rsync identifies changed files through their modification time and file size. A quick Google search for "vfat timestamp" reveals several problems related to the tz=UTC mount option.

Could it be that the timestamps on external drive differ from the internal drive and rsync therefore always considers the files to be different? If that is the case, you should probably check your mount options.

Alternatively (and actually regardless of whether my theory is right), you could try to run rsync with the -c option, which makes it check for changes through hashes of the file contents. This potentially slows down things, but I'm unsure about the practical impact.

My Environment

Raspberry Pi3 B+,Raspbian Strech(Linux)

I added next 1 line below into /etc/fstab
dev/sda1 /media/pi/TOSHIBA vfat defaults 0 0

Then
sudo umount /media/pi/TOSHIBA

To find device name you can use next command
lsusb

Then I re-inserted the USB MEMORY.

Then USB MEMORY's owner was changed like next 1 line below.
drwxr-xr-x 3 root root 8192 11月 9 23:22 TOSHIBA

Then I have been able to backup with permission,owner and timestamps using next 1 line below.
rsync -alvptgo /home/pi /media/pi/TOSHIBA/pi

which I have really used as DIFF backup command.

But PAY ATTENTION to that you must insert a USB MEMORY in the slot when you reboot

OR

Delete that 1 line in /etc/fstab before you reboot.