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4.11. Configuring atime Updates

Each file inode and directory inode has three time stamps associated with it:
  • ctime — The last time the inode status was changed
  • mtime — The last time the file (or directory) data was modified
  • atime — The last time the file (or directory) data was accessed
If atime updates are enabled as they are by default on GFS and other Linux file systems then every time a file is read, its inode needs to be updated.
Because few applications use the information provided by atime, those updates can require a significant amount of unnecessary write traffic and file-locking traffic. That traffic can degrade performance; therefore, it may be preferable to turn off atime updates.
Two methods of reducing the effects of atime updating are available:
  • Mount with noatime
  • Tune GFS atime quantum

4.11.1. Mount with noatime
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A standard Linux mount option, noatime, can be specified when the file system is mounted, which disables atime updates on that file system.

Usage
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mount BlockDevice MountPoint -o noatime
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BlockDevice
Specifies the block device where the GFS file system resides.
MountPoint
Specifies the directory where the GFS file system should be mounted.

Example
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In this example, the GFS file system resides on the /dev/vg01/lvol0 and is mounted on directory /gfs with atime updates turned off. 
mount /dev/vg01/lvol0 /gfs -o noatime
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4.11.2. Tune GFS atime Quantum
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When atime updates are enabled, GFS (by default) only updates them once an hour. The time quantum is a tunable parameter that can be adjusted using the gfs_tool command.
Each GFS node updates the access time based on the difference between its system time and the time recorded in the inode. It is required that system clocks of all GFS nodes in a cluster be synchronized. If a node's system time is out of synchronization by a significant fraction of the tunable parameter, atime_quantum, then atime updates are written more frequently. Increasing the frequency of atime updates may cause performance degradation in clusters with heavy work loads.
To see the current values of the GFS tunable parameters, including atime_quantum, you can use the gfs_tool gettune, as described in Section 4.5, “Displaying GFS Tunable Parameters”. The default value for atime_quantum is 3600 seconds.
The gfs_tool settune command is used to change the atime_quantum parameter value. It must be set on each node and each time the file system is mounted. The setting is not persistent across unmounts.

Usage
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Changing the atime_quantum Parameter Value 
gfs_tool settune MountPoint atime_quantum Seconds
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MountPoint
Specifies the directory where the GFS file system is mounted.
Seconds
Specifies the update period in seconds.

Example
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In this example, the atime update period is set to once a day (86,400 seconds) for the GFS file system on mount point /gfs. 
gfs_tool settune /gfs atime_quantum 86400
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