If you can still ping the servers without any problem, check if your system is behind a proxy.
If you are running Loaded plugins: langpacks, presto, refresh-packagekit Existing lock /var/run/yum.pid: another copy is running as pid 1880.
Loaded plugins: fastestmirror, presto Loading mirror speeds from cached hostfile Could not retrieve mirrorlist
 Update all plugins, uninstall any plugins you no longer need or disable all plugins, to see if that resolves the problem.
See Issues related to plugins for plugin locations and additional troubleshooting information, including how to disable plugins if your browser will not start.
Depending on your browser version and OS, Firefox or Sea Monkey may hang, crash, or simply fail to start because of a problematic plugin.
If possible, visit Mozilla's Plugin Check page with another browser that Plugin Check supports (e.g., Safari or Opera) to help identify installed plugins.
to manage software raid arrays; it assumes you have a good understanding of software RAID and Linux in general, and it will just explain the commands line usage of mdadm.