ZFS doing background writes?

[Ronald Klop]

On Tue, 17 Mar 2015 07:30:24 +0100, Dirk E <cipher_nl at hotmail.com> wrote:

>> 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?

It sounds like I have the same problem. I did not get much reply on my  
question:
https://lists.freebsd.org/pipermail/freebsd-fs/2014-September/020004.html

Regards,
Ronald.