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Date:      Fri, 10 Jan 2014 13:24:25 -0000
From:      "Steven Hartland" <killing@multiplay.co.uk>
To:        "Attila Nagy" <bra@fsn.hu>, "Dmitry Morozovsky" <marck@rinet.ru>
Cc:        'freebsd-fs' <freebsd-fs@freebsd.org>
Subject:   Re: What does ZFS write when nothing should write there?
Message-ID:  <29BFE2939CF14317A149D6C2D68D9A8E@multiplay.co.uk>
References:  <52CFA0B6.7090109@fsn.hu> <alpine.BSF.2.00.1401101707430.24613@woozle.rinet.ru> <52CFF18F.5040809@fsn.hu>

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----- Original Message ----- 
From: "Attila Nagy" <bra@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>

    Regards
    Steve

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