What does ZFS write when nothing should write there?

[Ronald Klop]

On Fri, 10 Jan 2014 15:18:43 +0100, Attila Nagy <bra at fsn.hu> wrote:

> On 01/10/14 14:29, Dmitry Morozovsky wrote:
>> On Fri, 10 Jan 2014, Steven Hartland wrote:
>>
>>>>>> I've created 6 zpools, each of them with zpool create -m /data/A  
>>>>>> dataA
>>>>>> mirror
>>>>>> daX daY.
>>>>>> The machine has nothing running except sshd and my shell.
>>>>>>
>>>>>> Yet, I see this in gstat:
>>>>> [snip]
>>>>>
>>>>>>       0     88      0      0    0.0     82    573    4.1 9.0  da5
>>>>>>       0     89      0      0    0.0     83    573    4.8 9.8  da6
>>>>>>       0     87      0      0    0.0     81    573    2.6 5.7  da9
>>>>>>       0     89      0      0    0.0     84    573    3.0 6.7  da10
>>>>> Did you turn off atime?
>>>>>
>>>> No, but how does it matter?
>>>> The process list is the following: init, getty, sshd, csh and the  
>>>> pool is
>>>> completely empty.
>>> With atime on each time you access a file it will update its "atime"
>>> hence causing writes.
>>>
>>> We use atime=off at the pool level on all machines to avoid that
>>> zfs set atime=off <pool>
>> BTW, it seems that ZFS updates atime of some inodes (root one?) on  
>> every kernel
>> update thread invocation even when completely empty -- is it correct  
>> behaviour?
>>
> Because there are no files, it must (?) be the root. But at this  
> frequency?

Some dtrace commands might give enlightenment.

Ronald