What does ZFS write when nothing should write there?

[Attila Nagy]

On 01/10/14 14:24, Steven Hartland wrote:
> ----- Original Message ----- From: "Attila Nagy" <bra at fsn.hu>
>
>> On 01/10/14 14:08, Dmitry Morozovsky wrote:
>>> On Fri, 10 Jan 2014, Attila Nagy 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>
I owe you a beer if you can explain why this matters on an empty file 
system on a machine, which has four processes running: init, getty, sshd 
and csh. :)