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Date: Fri, 10 Jan 2014 17:29:21 +0400 (MSK)
From: Dmitry Morozovsky <marck@rinet.ru>
To: Steven Hartland <killing@multiplay.co.uk>
Subject: Re: What does ZFS write when nothing should write there?
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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?

-- 
Sincerely,
D.Marck                                     [DM5020, MCK-RIPE, DM3-RIPN]
[ FreeBSD committer:                                 marck@FreeBSD.org ]
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*** Dmitry Morozovsky --- D.Marck --- Wild Woozle --- marck@rinet.ru ***
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