ZFS doing background writes?

[Dirk E]

> On Mon, 16 Mar 2015, Steven Hartland wrote:
>
>> yes exactly that its defined by vfs.zfs.txg.timeout.
>
> Well, but what data (modulo atimes, which I presume OP had turned off before)
> are written?

I indeed have atime=off for the whole pool. But even with atime enabled, this behaviour should not happen, since no files are being accessed. Surely not in single user mode; the only processes outside of the kernel were '/bin/sh' and 'top'. Or zpool iostat when i ran it.

I left the machine on and after a bunch of hours it is still doing periodical writes. The total ARC usage remains small: 5300K and changes slightly over time - again with no host writes.

Even unmounting all ZFS filesystems will NOT cause this behaviour to go away. It seems to originate from the ZFS kernel itself, doing some autonomous maintenance on the background, or other feature.'

Using top to sort processes with the longest CPU TIME, i can see:

idle: 27.9H
kernel: 6:48
intr: 4:34
zfskern: 0:33
geom: 0:29
syncer: 0:16
rand_harv 0:11
cam: 0:09
pf purge: 0:08
powerd: 0:03

This machine has done very little than boot and import the pool. As can be seen, the kernel and ZFS are pretty active in terms of CPU power.

Anyone have a clue?