Changing the default for ZFS atime to off?

[Steven Hartland]

----- Original Message ----- 
From: "Damien Fleuriot" <ml at my.gd>
To: "Steven Hartland" <smh at freebsd.org>
Cc: <fs at freebsd.org>
Sent: Sunday, June 09, 2013 11:39 AM
Subject: Re: Changing the default for ZFS atime to off?


> 
> On 8 Jun 2013, at 20:54, "Steven Hartland" <smh at freebsd.org> wrote:
> 
>> One of the first changes we make here when installing machines
>> here to changing atime=off on all ZFS pool roots.
>> 
>> I know there are a few apps which can rely on atime updates
>> such as qmail and possibly postfix, but those seem like special
>> cases for which admins should enable atime instead of the other
>> way round.
>> 
>> This is going to of particular interest for flash based storage
>> which should avoid unnessacary writes to reduce wear, but it will
>> also help improve performance in general.
>> 
>> So what do people think is it worth considering changing the
>> default from atime=on to atime=off moving forward?
>> 
>> If so what about UFS, same change?
>
> I strongly oppose the change for reasons already raised by many
> people regarding the mbox file.
> 
> Besides, if atime should default to off on 2 filesystems and on
> on all others, that would definitely create confusion.

A very valid point.

> Last, I believe it should be the admin's decision to turn atime
> off, just like it is his decision to turn compression on.

Trying to play devils advocate here; compression is off by default
because it uses resources and doesn't give a benefit for all cases.

Is that not the same as atime, and it should be an admins decision
to turn it on where it's wanted?

> Don't mistake me, we turn atime=off on every box, every
> filesystem, even on Mac's HFS.
> Yet I believe defaulting it to off is a mistake.

That's what prompted me to start this discussion. If a large portion
of users either disable atime already or would disable atime if they
knew about it, does that bring into question the current default?

Potentially a better solution would be to make atime an option
in the installer, as that helps educate admins that the option
exists, which is potentially the biggest issue here?

    Regards
    Steve

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