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Date: Wed, 26 Nov 2014 13:06:33 -0500
From: Marcus Reid <marcus@blazingdot.com>
To: freebsd-current@freebsd.org, freebsd-fs@freebsd.org
Subject: Delayed atime updates ("lazytime")
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Hi,

Looks like Linux is about to grow another solution to handling atime
updates differently:

  http://lwn.net/SubscriberLink/621046/e59938475fd3e874/

In short, it will only write out atime changes periodically (daily), or
if there is another reason to write out the inode, or if the inode is
about to be pushed out of cache.  This seems like a pretty good
compromise.

Currently, the ZFS configuration that results from using bsdinstall
disables atime on all but /var/mail, which is the only example of
disabling atime by default that I'm aware of outside of Gentoo Linux.
I can't seem to find any information that talks about the rationale
behind that, though a couple things come to mind:

  - some additional IO generated (but that's always been the case)
  - additional wear on SSD devices (enough to compel the change?)
  - zfs snapshot growth (but the snapshot stops growing after one
    full set of inode updates)
  - wake up otherwise idle spinning media on a laptop (the actual reason
    that was cited as motivation for the change)

Something like lazytime would address most of those concerns, and people
who are even more OCD than that could disable atime completely on their
machine.

Marcus