Replacing Atime With Relatime in the Kernel

eldavojohn writes "Our friend Jeremy at the Kernal Trap has dug up some interesting criticism of atime from Linus Torvalds. As Linus submitted patches to improve relatime he noted: 'I cannot over-emphasize how much of a deal it is in practice. Atime updates are by far the biggest IO performance deficiency that Linux has today. Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, _combined_.' And later severely beat atime about the head with a pointed stick: 'It's also perhaps the most stupid Unix design idea of all times. Unix is really nice and well done, but think about this a bit: 'For every file that is read from the disk, lets do a ... write to the disk! And, for every file that is already cached and which we read from the cache ... do a write to the disk!'" Well, I guess I can expect my Linux machine to become a little bit faster!"

Comments

Linus did not say that!

[JbirdUAH]

if the poster had read the article they would have noticed that Linus did not say those things that are quoted - Ingo Molnar did.

Re:Ummm..

[ArcherB]

In your /etc/fstab, add noatime to your options portion of you drive line. For example:

/media/sda1 ext3 defaults,noatime 1 1 atime logs when a file has been written or read. So every time a file is used, it has to write an entry on the HDD, slowing performance, but it can have uses, like in forensics, security or backups (if a file has not been read in three years, it's probably safe to archive and move off the drive). I don't care for it, so I have the noatime line in my fstab.

Re:Summary please?

[mpeg4codec]

File access timestamps, by default, are updated each time a file is read, and this occurs a write to the disk. Even if a file resides in cache, a write must be performed every time it is opened [even though the cache prevents a read from the disk]. A few people found that by disabling this, performance increases dramatically. The long and short of it: use noatime when you're mounting a file system [or in the fstab] if you want to try for yourself.

Re:Ummm..

[Compholio]

Most professional Unixes (Solaris springs to mind) have the option of disabling the access timestamps. I haven't read the article yet, so I'm not quite sure why Linux hasn't followed suit by now.

Linux has had the capability to turn off access timestamps for a long time. "man mount" has details, but all you really have to do is edit /etc/fstab:
/dev/hda3 / ext3 defaults 0 1
becomes
/dev/hda3 / ext3 defaults,noatime,nodiratime 0 1

latest relatime patch

[Ingo Molnar]

Hey, Slashdot posted an article about me! [ They also renamed me to Linus - what more can a geek ask for? ;-) ]

In any case, the latest version of the better-relatime patch can be picked up from:

http://redhat.com/~mingo/relatime-patches/

Apply it, build it, reboot into the new kernel and enjoy a faster (and lower latency) desktop. (no fstab twiddling needed)

Re:latest relatime patch

[Ingo Molnar]

I agree - modifying a text file is a messy complicated business only suitable for the elite super hackers out there. It's much simpler for me to apply the patch and recompile the kernel.

I kid, I kid.

ok, you are kidding, but i'll still bite :-)

Firstly, the patch is mainly about modifying relatime behavior to make it more compatible and more usable.

The fact that you dont have to change fstab is no big deal, provided you have the right util-linux package installed, with the relatime user-space patch applied which not even the latest distro devel repositories have included.

If you dont have that then adding "relatime" to your fstab might leave you with a read-only mounted root filesystem and some commandline (or rescue-image) tinkering to do.

People prefer all-in-one kernel patches that just turns on the feature they are interested in. You'd be surprised how many people are willing to try almost arbitrary kernel patches but loathe to touch their user-space environment in any way.

And ... it's also kind of ironic that this relatively small patch often brings more practical benefits to the desktop than all the "big" desktop interactivity/latency features (cfs, swap-prefetch, -rt kernel) combined.

Ingo^H^H^H^H Linus

Re:Laptop power issue.

[GoRK]

Just a handy tip for you (if you don't already use it) or others who are looking at disabling atime more for the power savings than the IO -- on laptops you should also be using noflushd (non-journaling filesystems only) or (ideally) Laptop Mode Tools. Also do not forget to configure syslog so that it doesn't constantly sync writes.

It's also worth mentioning that you *can* have atime enabled with properly configured laptop-mode and laptop-mode-tools and still see almost as much power savings -- The atime writes will still happen, but at least they will be buffered for when the HDD actually needs to spin up and do a lot of other more pressing IO.

As a flash fs writer...

[EmbeddedJanitor]

atime really hurts some file systems, particularly flash file systems. Many/most flash file systems don't support atime for that reason. So, even if you're running atime in the kernel you will often not be getting atime in the fs.

If you're using a desktop system with a hard disk you'll hardly notice any difference unless you hammer the system really hard.

Remember though that most Linux systems are either embedded (using mainly flash) or servers. In both these cases atime updates can be very damaging to performance and should be avoided unless there's a very good reason to turn it on.