What keeps one side of an rsync so busy?

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http://unix.stackexchange.com – I've got a debian machine in my lan serving as a backup server for the others. It has four HDDs combined into a software RAID 5 md device, on that an lvm, and on that a btrfs. Backups are made using rsync, and for a large filesystem take over an hour. For a long time I had thought there would be little I could do about this. Recently, however, I noticed that the HDD activity was very different on both ends of the transfer. While the sending side, running Gentoo and mostly using ext4, had hardly any disk IO at all, the receiving side was constantly busy. As most data wouldn't change between tr (HowTos)