CentOS : showing error message No space left on device

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http://serverfault.com – I am using CentOS-5.5 its not stasting. while booting its showing "GDM could not write a new authorization entry to disk.Possible out of diskspace.Error: No space left on device" if press then showing one more message "Could not start X server (your graphical environment) due to some internal error. please contact your system administrator or check your syslog to diagnose. In meantime this display will be disabled.Please restart GDM when the problem is corrected. and if check filesystem uses by "df -h" its showing Filesystem Size used Avail Use% Mounted on /dev/mapper/VolGrou (HowTos)