Changing the default for ZFS atime to off?

[Steven Hartland]

----- Original Message ----- 
From: <kpneal at pobox.com>


> On Sat, Jun 08, 2013 at 07:54:04PM +0100, Steven Hartland 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.
> 
> I believe mutt also uses them. Basically, any mail program using mbox mail
> folders uses them to correctly report which mailboxes have not been read
> yet.
> 
> There are probably other cases as well. I don't think they should be
> discounted simply because nobody here who bothers to speak up runs into
> them.
> 
> Turning off atime creates surprises for users.
> 
>> 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?
> 
> I vote no. At least, don't change it unless the filesystem is actually on
> a flash device. Otherwise we risk breakage down the road because something
> that used to work doesn't work on a fresh FreeBSD install.

I don't think having different defaults for different disks would be a good
thing as that would just cause confusion.

Would updating the installers to enable atime on the volumes that require
it be an acceptable solution?

> Has anyone done any kind of study to see exactly how much I/O is caused
> by having atime updates be enabled? Does it _really_ make that much of
> a difference to performance, and would it _really_ help prolong the life
> of flash devices?

I've just done some a very basic tests here on an 8.3-RELEASE machine:-
1. make buildkernel # atime=on adds 2k writes totalling 27MB
2. find /usr/src # atime=on adds 100 writes totaling 3MB

    Regards
    Steve


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