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RHEL6 isn't revolutionary. But it does a nice job of advancing ideas that first appeared in other releases. For example, Security Enhanced Linux (SELinux), a security-focused subset of Linux, offers partitioning of resources so that user processes can't hijack kernel root-privileged processes.
RHEL6 takes SELinux and adds sandboxing policies that allow sysadmins or processes to further isolate sessions or applications. Policy controls also allow admins to confine session or resource access as well.
We were heartened by these extensions, as they're needed tools to isolate both users and processes from destabilizing busy servers.
And while Novell's SUSE Linux 11 first championed a production release of the Linux tickless kernel in a corporate distribution of Linux, Red Hat goes further toward kernel-based power management.
A tickless kernel doesn't interrupt the processor every thousandth of a second, waking it up from power saving states. This feature has been available in Linux for a while, but not often implemented because there are some applications that are built with that need a System Tick timer clock.
The powertop application in RHEL6 is used to actively command and monitor power usage in great detail. Applications can be tuned to spoof needless tick-based interruptions to the CPU without reducing functionality of the application. These noisy applications become quieter, and the CPU sleep states can become longer with tuning. When the CPU sleeps, it uses far less power.
Control groups, first seen in SLES 11, are also implemented in RHEL6. The cgroups allow tasks to be grouped together as an object, in terms of their accessibility to system resources. Tasks and cgroups can be confined in terms of CPU strokes (and which CPU), memory allocation, network I/O, storage, or access to the system scheduler.
Red Hat also added Aggressive Link Power Management that works (for now) only on SATA host bus adapters/controllers to jump to a low power state when there's no pending disk I/O. Coupled with aggressive use of powertop, an administrator has the ability to assert more active control over server/instance power consumption.
In an ideal future world, applications would set their use based on configuration information, but there are no real standards for this today, so administrators are left to tune application instances for power consumption.
Installation has become more sophisticated. We installed RHEL6 onto VMware ESXi, which had a configuration wrapper available to deal with RHEL6 specifics before RHEL6 was released.
The installation GUI also has detailed specs to install storage devices. If you want your server to use iSCSI or Fibre Channel over Ethernet, you get device and method-specific help and the same is provided for detected storage-area network (SAN) devices or firmware-based RAID drives.