Why does 'zfs scrub' write to disk every 5 seconds?

My NAS is running zfs and does a scrub now and then. When it runs it read all data (as expected), but it also writes a litte during the run (around 0.5%): ~~~ Device rkB/s wkB/s %util sda 13628.80 69.60 58.04 sdb 13625.60 73.60 59.48 sdc 13625.60 70.80 58.96 sdd 13625.60 68.40 58.24 sde 13625.60 69.60 60.00 ~~~ This could be explained by other systems using it. But there are no systems using the NAS, and no user processes running on the server, that would access the NAS. Also the rate is constant for hours: Full read for 4.5 seconds followed by 0.5 seconds write. It is a fixed rythm: silence for 4.5 seconds, and then the DRRDRRD sound of seeking when it is writing. But why does it write *anything* when there are no clients and nothing to correct? I was under the impression that as long as all data are clean, consistent, and unchanged since last scrub, there would be no need to write any data.