Re: ext3 IO latency measurements (was: Linux 2.6.29)

Linus Torvalds
2009-03-26T16:28:36Z

On Thu, 26 Mar 2009, Theodore Tso wrote:
>
> Most distributions are putting relatime into /etc/fstab by
> default, but we haven't changed the mount option.

I don't think this is true. Fedora certainly does not. Not in F10, not in
F11.

And quite frankly, even if you then _manually_ put 'relatime' in
/etc/fstab, the default Fedora install will totally ignore it. Why?
Because it mounts the root partition while using initrd, and totally
ignores /etc/fstab.

In other words, not only do distributions not do it, but you can't even do
it by hand afterwards the sane way in the most common distro!

There really is reason for the kernel to just say "user space has sh*t for
brains, and we'd better change the default - and if some distro really
_thinks_ about it, and decides that they really want old-fashioned atime,
let them do that".

Because right now, I do not believe for a moment that any distro that
defaults to "atime" has spent lots of effort thinking about it. Quite the
reverse. They probably default to "atime" because they spent no time AT
ALL thinking about it.

> It wouldn't be hard to add an "atime" option to turn on atime updates,
> and make either "noatime" or "relatime" the default. This is a simple
> patch to fs/namespace.c

Yes. I think we have to.

> No argument here. I use noatime, myself. It actually saves a lot
> more than relatime, and unless you are using mutt with local Maildir
> delivery, relatime isn't really that helpful, and the benefit of
> noatime is roughly double that of relatime vs normal atime update, in
> my measurements:
>
> http://thunk.org/tytso/blog/2009/03/01/ssds-journaling-and-noatimerelatime/

I do agree that "noatime" is better, but with "relatime" you at least are
likely to not break anything. A program has to be _really_ odd to care
about the "relatime" vs "atime" behavior.

Linus