I currently use
rsync to backup virtual disks. But this program is very slow on big files because it tries to find similarities around the place it should drops data.
cp take 10 minutes, while
rsync take more than 2 hours.
Worse, rsync sometimes hangs. A serverfault question talk about that, and I gave the answer, but this does not speed-up rsync.
I don't use
cp because the underlying filesystem is ZFS and a snapshot is made after each backup. This allow to have some kind of file deduplication, and to retain backups during months.
So it is essential to write only modified data.
I'm looking for a program similar to a dumb
rsync: only comparing data located at the same place in the source and the destination file.
This can be achieved by comparing fixed-size block if needed.
I tested programs similar to rsync, but they are either unable to handle very big files, either slower. Related question