View source for HowTo Disable The "relatime" Method For File "atime" Updates

='''Overview'''=

== [[Image:Warning.png‎]] This Artical Applies to Kernels Prior to v2.6.30 ==

* For Kernel v2.6.30 or greater, use the '''mount''' command with a combination of the "'''remount'''", "'''relatime'''", "'''atime'''" and/or the "'''strictatime'''" option for the desired "<i>access</i>" time update method.


This "'''HowTo'''" explains various ways to <i>disable</i> the "'''relatime'''" method in the [http://www.kernel.org/ Linux Kernel] for updating the "'''atime'''" field for a file when a process "'''read'''" occurs. One should consider only <i>disabling</i> the "'''relatime'''" method for audit or debugging purposes. A significant disk I/O performance hit may occur for an active '''Linux''' server if this method is <i>disabled</i>.    

'''NST''' has the "'''relatime'''" [http://www.kernel.org/ Kernel] update "'''atime'''" method enabled by default.

=='''atime'''==

The "'''Access time'''" field for a file within a given file system is know as: "'''atime'''". When a process reads a file, the "'''atime'''" field is updated with the current date/time for this access. Completely <i>disabling</i> "'''atime'''" updates using the "'''noatime'''" mount flag is probably one of the biggest performance tweaks that a '''Linux''' administrator can make. An active '''Linux''' server is continually reading files which generates a significant number of "'''atime'''" updates. This translates to <i>numerous</i> metadata updates made to the file system by writes to the physical disk which can lead to poor I/O performance.

=='''relatime'''==
The [http://www.kernel.org/ Linux Kernel] supports the "'''Relative atime'''" ('''relatime''') update method.  A "'''relatime'''" update will only write the "'''atime'''" field if the previous "'''atime'''" is older than the "'''mtime'''" (modify time) or "'''ctime'''" (change status time) for the file. Therefore, only "'''One'''" "'''atime'''" field update will occur after either a "'''mtime'''" or "'''ctime'''" change regardless of the number of subsequent file read access attempts that occur. The "'''relatime'''" method for <i>updating</i> the "'''atime'''" field indicates that at least <u>one</u> or <u>more</u> process read attempts have been made on the file. 

This technique can avoid a significant number of metadata "'''atime'''" updates. It's similar to the "'''noatime'''" mount option, but useful for applications like "[http://www.mutt.org/ mutt]" that need to know when a file has been read since it was last modified.

='''relatime''' Method: Interval Adjustment=
The '''interval''' update period for the "'''atime'''" field using the "'''relatime'''" last access time stamp method for a file can be ''adjusted'' using the [http://www.kernel.org/ Kernel] parameter: 
"'''relatime_interval'''". This value is in "'''seconds'''" and is defaulted to: "'''86400 secs'''" (Number of seconds in a day). This makes the "'''relatime'''" update method a compatible solution for [http://en.wikipedia.org/wiki/Hierarchical_storage_management '''HSM'''], '''mailer-notification''' and [http://gentoo-wiki.com/HOWTO_clean_/tmp '''tmpwatch'''] applications.

The example below uses the "'''sysctl'''" command to adjust the "'''relatime_interval'''" parameter to: "'''10 minutes'''" for the "'''/'''" ('''root''') file system. One needs to also use the "'''mount'''" command to "'''remount'''" the file system for the change to take effect.

<div class="screen">
 <div class="userInput"><span class="prompt">[root@probe ~]# </span>/sbin/sysctl -w fs.relatime_interval=600
 </div>
 <pre class="computerOutput">
fs.relatime_interval = 600</pre>
 <div class="userInput"><span class="prompt">[root@probe ~]# </span>/bin/mount -v -o remount /dev/sda1
 </div>
 <pre class="computerOutput">
/dev/sda1 on / type ext3 (rw)
[root@probe ~]#</pre>
</div>


='''relatime''' Method: Disable Online=
The [http://www.kernel.org/ Kernel] parameter: "'''default_relatime'''" controls whether the "'''relatime'''" method for "'''atime'''" updates is enabled or not. Set the value to: "'''1'''" (Default for '''NST''') to enable the method or set it to: "'''0'''" to disable.

The example below uses the "'''sysctl'''" command to <i>disable</i> the "'''relatime'''" method for "'''atime'''" updates for the "'''/'''" ('''root''') file system. One needs to also use the "'''mount'''" command to "'''remount'''" the file system for the change to take effect.

<div class="screen">
 <div class="userInput"><span class="prompt">[root@probe ~]# </span>/sbin/sysctl -w fs.default_relatime=0
 </div>
 <pre class="computerOutput">
fs.default_relatime = 0</pre>
 <div class="userInput"><span class="prompt">[root@probe ~]# </span>/bin/mount -v -o remount /dev/sda1
 </div>
 <pre class="computerOutput">
/dev/sda1 on / type ext3 (rw)
[root@probe ~]#</pre>
</div>


='''relatime''' Method: Disable At Boot Time=

Use the following boot option to <i>disable</i> the "'''relatime'''" method for "'''atime'''" updates: "'''default_relatime=0'''".