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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 changedmtime
— The last time the file (or directory) data was modifiedatime
— 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
A standard Linux mount option,
noatime
, can be specified when the file system is mounted, which disables atime
updates on that file system.
Usage
mount BlockDevice MountPoint
-o noatime
BlockDevice
- Specifies the block device where the GFS file system resides.
MountPoint
- Specifies the directory where the GFS file system should be mounted.
Example
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
4.11.2. Tune GFS atime
Quantum
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
Changing the
atime_quantum
Parameter Value
gfs_tool settuneMountPoint
atime_quantumSeconds
MountPoint
- Specifies the directory where the GFS file system is mounted.
Seconds
- Specifies the update period in seconds.
Example
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