rsync doesn't copy *only* modifications -

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i'm using rsync backup files. choose rysnc because (should) use modification times determine if changes have been made , if files need updated.

i started backup (from computer system (debian) portable external hard drive) command:

rsync -avz --update --delete --stats --progress --exclude-from=/home/user/scripts/exclusionrsync --backup --backup-dir=/media/user/hdd/backups/deleted-files /home/user/ /media/user/hdd/backups/backup_user

it worked , took lot of time. believed second time quick (since didn't modify files). unfortunately, 2nd, 3th, 4th, ... times took long first one. still see files being copied if these files in portable hard drive.

i don't understand why rsync doesn't copy modifications (rsync known efficient , copy changes , specificly call --update option).

a side effect of problem files moved backup dir (deleted-filed) transfered. indeed, rsync delete previous file before copy same file during each update...

i found solution reading an answer on serverfault.se. fat filesystem messing timestamps:

fat doesn't track modification times on files precisely as, ext3 (fat precise within 2 second window). leads particularly nasty behavior rsync decide original files newer or older backup file enough needs re-copy data or @ least re-check hashes. in all, makes poor performance on backups. if must stick fat, rsync's --size-only , --modify-window flags workarounds.


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