Release Notes for Red Hat Enterprise Linux 6
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The Release Notes document the major features and enhancements implemented in the Red Hat Enterprise Linux 6 release.
Red Hat is pleased to announce the availability of Red Hat Enterprise
Linux 6. Red Hat Enterprise Linux 6 is the next generation of Red Hat's
comprehensive suite of operating systems, designed for mission-critical
enterprise computing and certified by top enterprise software and
This release is available as a single kit on the following architectures:
IBM Power (64-bit)
In this release, Red Hat brings together improvements across the
server, systems and the overall Red Hat open source experience.
The Red Hat Enterprise Linux installer (also known as
assists in the installation of Red Hat Enterprise Linux 6. This section
of the release notes provides an overview of the new features
implemented in the installer for Red Hat Enterprise Linux 6.
The Red Hat Enterprise Linux 6 Installation Guide
provides detailed documentation of the installer and the installation process.
2.1. Installation Methods
The installer provides three main interfaces to install Red Hat Enterprise Linux: kickstart, the graphical installer and the text-based installer.
2.1.1. Graphical Installer
The Red Hat Enterprise Linux graphical installer steps the user
through the major steps involved in preparing a system for installation.
The Red Hat Enterprise Linux 6 installation graphical installer
introduces major usability enhancements for disk partitioning and
The graphical installer now allows a user to choose basic storage devices or specialized storage devices. Basic Storage Devices
typically do not need any additional configuration settings before the
device is usable. A new interface has been implemented for configuring
specialized storage devices. Firmware RAID devices, Fibre Channel over
Ethernet (FCoE) devices, multipath devices, and other storage area
network (SAN) devices can now be easily configured using the new
Figure 1. Specialized Storage Devices Configuration
The interface for choosing partitioning layouts has been enhanced,
providing detailed descriptions and diagrams for each default
Figure 2. Partitioning Layout Choices
The Installer allows storage devices to be specified as either
install target devices or data storage devices prior to installation.
Figure 3. Specifying Storage Devices
Kickstart is an automated installation
method that system administrators use to install Red Hat Enterprise
Linux. Using kickstart, a single file is created, containing the answers
to all the questions that would normally be asked during a typical
Red Hat Enterprise Linux 6 introduces improvements to the validation
of kickstart files, allowing the installer to capture issues with
kickstart file syntax before an installation commences.
2.1.3. Text-based Installer
The text-based installer is provided primarily for systems with
limited resources. The text-based installer has been simplified,
permitting installation to the default disk layouts, and installation of
new and updated packages.
Figure 4. Text-based Installer
Some installations require advanced installation options that are
not present in the text-based installer. If the target system cannot run
the graphical installer locally, use the Virtual Network Computing
(VNC) display protocol to complete the installation.
2.2. Creating Backup Passphrases During Installation
Currently, creating backup passphrases for encrypted devices during
installation can only be achieved during a kickstart installation. More
information on this new feature, including how to utilize this feature
in a kickstart installation of Red Hat Enterprise Linux 6, refer to the Disk Encryption appendix in the Installation Guide.
2.3. DVD Media Boot Catalog Entries
The DVD media for Red Hat Enterprise Linux 6 include boot catalog
entries for both BIOS- and UEFI-based computers. This allows the media
to boot systems based on either firmware interface. (UEFI is the Unified
Extensible Firmware Interface, a standard software interface initially
developed by Intel and now managed by the Unified EFI Forum. It is
intended as a replacement for the older BIOS firmware.)
Some systems with very old BIOS implementations will not boot from
media which include more than one boot catalog entry. Such systems will
not boot from a Red Hat Enterprise Linux 6 DVD but may be bootable using
a USB drive or over a network using PXE.
UEFI and BIOS boot configurations differ significantly from each
other and are not interchangeable. An installed instance of Red Hat
Enterprise Linux 6 will not boot if the firmware it was configured for
is changed. You cannot, for example, install the operating system on a
BIOS-based system and then boot the installed instance on a UEFI-based
2.4. Installation Crash Reporting
Red Hat Enterprise Linux 6 features enhanced installation crash
reporting in the installer. If the installer encounters an error during
the installation process, details of the error are reported to the user
with the option to report the issue to Red Hat support.
Figure 5. Installation Error Reporting
To assist troubleshooting and debugging of installations, additional
details are now included in log files produced by the installer. Further
information on installation logs, and how to use them for
troubleshooting can be found in the following sections of the Installation Guide.
The Storage Administration Guide
provides further instructions on how to effectively manage file systems on Red Hat Enterprise Linux 6. Additionally, the Global File System 2
document details specific information on configuring and maintaining
Red Hat Global File System 2 for Red Hat Enterprise Linux 6.
3.1. Fourth Extended Filesystem (ext4) Support
The fourth extended filesystem (ext4) is based on the third extended
filesystem (ext3) and features a number of improvements. These include
support for larger file systems and larger files, faster and more
efficient allocation of disk space, no limit on the number of
subdirectories within a directory, faster file system checking, and more
robust journaling. The ext4 file system is selected by default and is
XFS is a highly scalable, high-performance file system which was
originally designed at Silicon Graphics, Inc. It was created to support
filesystems up to 16 exabytes (approximately 16 million terabytes),
files up to 8 exabytes (approximately 8 million terabytes) and directory
structures containing tens of millions of entries.
XFS supports metadata journaling, which facilitates quicker crash
recovery. The XFS file systems can also be defragmented and expanded
while mounted and active.
3.3. Block Discard — Enhanced Support for Thinly Provisioned LUNs and SSD Devices
Filesystems in Red Hat Enterprise Linux 6 use the new block discard
feature to allows a storage device to be informed when the filesystem
detects that portions of a device (also known as blocks) are no longer
in active use. While few storage devices feature block discard
capabilities, newer solid state drives (SSDs) utilize this feature to
optimize internal data layout and invoke proactive wear levelling.
Additionally, some high end SCSI devices use block discard information
to help implement thinly provisioned LUNs.
3.4. Network File System (NFS)
A Network File System (NFS) allows remote hosts to mount file systems
over a network and interact with those file systems as though they were
mounted locally. This enables system administrators to consolidate
resources onto centralized servers on the network. Red Hat Enterprise
Linux 6 supports NFSv2, NFSv3, and NFSv4 clients. Mounting a file system
via NFS now defaults to NFSv4.
Additional improvements have been made to the NFS in Red Hat
Enterprise Linux 6, providing enhanced support over Internet Protocol
version 6 (IPv6)
4.1. Storage Input/Output Alignment and Size
Recent enhancements to the SCSI and ATA standards allow storage
devices to indicate their preferred (and in some cases, required) I/O
alignment and I/O size. This information is particularly useful with
newer disk drives that increase the physical sector size from 512 byes
to 4K bytes. This information may also be beneficial for RAID devices,
where the chunk size and stripe size may impact performance.
The Red Hat Enterprise Linux 6 provides the ability to read and
utilize this information, and optimize how data is read and written from
4.2. Dynamic Load Balancing with DM-Multipath
Device Mapper Multipathing (DM-Multipath) creates a single conceptual
device from the multiple cables, switches and controllers that connect
servers to storage arrays. This enables centralized management of
connection devices (also known as paths) and makes it possible to
balance loads over all available paths.
DM-Multipath in Red Hat Enterprise Linux 6 introduces two new options
when dynamically balancing load over paths. Paths can now be
dynamically selected depending on either the queue size of each path or
previous I/O time data.
The DM Multipath
book provides information on using the Device-Mapper Multipath feature of Red Hat Enterprise Linux 6.
4.3. Logical Volume Manager (LVM)
Volume management creates a layer of abstraction over physical
storage by creating logical storage volumes. This provides greater
flexibility over just using physical storage directly. Red Hat
Enterprise Linux 6 manages logical volumes using the Logical Volume
system-config-lvm is a graphical user interface provided in Red Hat Enterprise Linux to manage logical volumes. The functionality provided by
system-config-lvm is in the process of transitioning to a more maintainable tool named
gnome-disk-utility (also referred to as
palimpsest). As a result, Red Hat will be very selective in updating
gnome-disk-utility reaches feature parity with
system-config-lvm, Red Hat reserves the right to remove
system-config-lvm during the life of Red Hat Enterprise Linux 6.
4.3.1. LVM Mirror Improvements
LVM supports mirrored volumes. By creating mirrored logical volumes,
LVM ensures that data written to an underlying physical volume is
mirrored onto a separate physical volume.
18.104.22.168. Merging Snapshots
Red Hat Enterprise Linux 6 introduces the ability to merge a
snapshot of a logical volume back into the origin logical volume. This
allows system administrators to revert any changes that have occurred on
a logical volume by merging back to the point preserved by a snapshot.
For more information about the new snapshot merge feature, consult the
22.214.171.124. Four-Volume Mirrors
LVM in Red Hat Enterprise Linux 6 supports creating a logical volume with up to four mirrors.
126.96.36.199. Mirroring Mirror Logs
LVM maintains a small log (on a separate device) which it uses to
keep track of which regions are in sync with the mirror or mirrors. Red
Hat Enterprise Linux 6 provides the ability to mirror this log device.
4.3.2. LVM Application Library
Red Hat Enterprise Linux 6 features the new LVM Application Library
(lvm2app), allowing the development of LVM based storage management
The Power Management Guide
provides information on effectively managing power consumption on Red Hat Enterprise Linux 6.
The introduction of the tickless kernel in Red Hat Enterprise Linux 6 (refer to Section 12.4.2, “Tickless Kernel”
) allows the CPU to enter the idle state more frequently, reducing power consumption and improving power management. The new powertop
tool provides the ability to identify specific components of kernel and
userspace applications that frequently wake up the CPU. powertop
was used in development to identify and tune many applications in this
release, reducing unnecessary CPU wake up by a factor of 10.
tuned is a system tuning daemon that monitors system components and dynamically tunes system settings. Utilizing ktune (the static mechanism for system tuning), tuned can monitor and tune devices (e.g. hard disk drives and ethernet devices). Red Hat Enterprise Linux 6 also introduces diskdevstat for monitoring disk operations and netdevstat for monitoring network operations.
6.1. Strong Package Checksums
RPM provides support for signed packages using strong hash algorithms
such as SHA-256 in order to ensure package integrity and increase
security. Red Hat Enterprise Linux 6 packages are transparently
compressed with the XZ lossless compression library, which implements
the LZMA2 compression algorithm for greater compression (thus reducing
package size) and faster unpacking (when installing RPMs). Further
information on the stronger package checksums is available in the Deployment Guide
All Red Hat Enterprise Linux 6 packages are digitally signed with a
new 4096-bit RSA hardware signing key and using a SHA-256 hash. The RPM package signing
document gives details of this new key.
6.3. The PackageKit Package Manager
Red Hat provides PackageKit for viewing, managing, updating,
installing and uninstalling packages and package groups. PackageKit
allows for quick repository enablement and disablement, a graphical and
searchable transaction log, and PolicyKit integration. Further
information on Package Kit is available in the Deployment Guide
Via its plugin architecture, Yum provides new or enhanced support for
various capabilities such as delta RPMs (using the presto plugin), RHN
communication (rhnplugin), and auditing and applying—using a calculated
least-invasive (minimal) number of updates—only relevant security fixes
to a system (security plugin).
Yum also ships with the yum-config-manager
utility, which shows exhaustive information about all set configuration
options and parameters for each individual repository. Further
information on updates to Yum is available in the Deployment Guide
Clusters are multiple computers (nodes) working in concert to increase
reliability, scalability, and availability to critical production
services. High Availability using Red Hat Enterprise Linux 6 can be
deployed in a variety of configurations to suit varying needs for
performance, high-availability, load balancing, and file sharing.
The Cluster Suite Overview
document provides an overview of Red Hat Cluster Suite for Red Hat Enterprise Linux 6. Additionally, the High Availability Administration
document describes the configuration and management of Red Hat cluster systems for Red Hat Enterprise Linux 6.
7.1. Corosync Cluster Engine
Red Hat Enterprise Linux 6 utilizes the Corosync Cluster Engine for core cluster functionality.
7.2. Unified Logging Configuration
The various daemons that High Availability employs now utilize a
shared unified logging configuration. This allows system administrators
to enable, capture and read cluster system logs via a single command in
the cluster configuration.
7.3. High Availability Administration
Conga is an integrated set of software components that provides
centralized configuration and management for Red Hat Enterprise Linux
High Availability. One of the primary components of Conga is luci, a
server that runs on one computer and communicates with multiple clusters
and computers. In Red Hat Enterprise Linux 6 the web interface that is
used to interact with luci has been redesigned.
7.4. General High Availability Improvements
In addition to the features and improvements detailed above, the
following features and enhancements to clustering have been implemented
for Red Hat Enterprise Linux 6.
Enhanced support for Internet Protocol version 6 (IPv6)
SCSI persistent reservation fencing support is improved.
Virtualized KVM guests can now be run as managed services.
The Security Guide
assists users and administrators in learning the processes and
practices of securing workstations and servers against local and remote
intrusion, exploitation and malicious activity.
8.1. System Security Services Daemon (SSSD)
The System Security Services Daemon (SSSD) is a new feature in Red
Hat Enterprise Linux 6 that implements a set of services for central
management of identity and authentication. Centralizing identity and
authentication services enables local caching of identities, allowing
users to still identify in cases where the connection to the server is
interrupted. SSSD supports many types of identity and authentication
services, including: Red Hat Directory Server, Active Directory,
OpenLDAP, 389, Kerberos and LDAP.
The Deployment Guide
contains a section that describes how to install and configure the
System Security Services Daemon (SSSD), and how to use the features that
8.2. Security-Enhanced Linux (SELinux)
Security-Enhanced Linux (SELinux) adds Mandatory Access Control (MAC)
to the Linux kernel, and is enabled by default in Red Hat Enterprise
Linux 6. A general purpose MAC architecture needs the ability to enforce
an administratively-set security policy over all processes and files in
the system, basing decisions on labels containing a variety of
Traditionally, SELinux is used to define and control how an
application interacts with the system. SELinux in Red Hat Enterprise
Linux 6 introduces a set of policies that allows system administrators
to control what particular users can access on a system.
SELinux in Red Hat Enterprise Linux 6 features the new security
sandbox feature. The security sandbox adds a set of SELinux policies
that enables a system administrator to run any application within a
tightly confined SELinux domain. Using the sandbox, system
administrators can test the processing of untrusted content without
damaging the system.
8.2.3. X Access Control Extension (XACE)
The X Window System (commonly referred to a "X") provides the base
framework for displaying the graphical user interface (GUI) on Red Hat
Enterprise Linux 6. This release features the new X Access Control
Extension (XACE), which permits SELinux to access decisions made within
X, specifically, controlling information flow between window objects.
8.3. Backup Passphrases for Encrypted Storage Devices
Red Hat Enterprise Linux provides the ability to encrypt the data on
storage devices, assisting in the prevention of unauthorized access of
the data. Encryption is achieved by transforming the data into a format
that can only be read using a specific encryption key. This key — which
is created during the installation process, and protected by a
passphrase — is the only way to decrypt the encrypted data.
Figure 6. Decrypting Data
However, if the passphrase is misplaced, the encryption key cannot be
used, and data on the encrypted storage device cannot be accessed.
Red Hat Enterprise Linux 6 provides the ability to save encryption
keys and create backup passphrases. This feature allows for the recovery
of an encrypted volume (including the root device) even when the
original passphrase is misplaced.
libvirt is a C language application programming interface (API) for
managing and interacting with the virtualization capabilities of Red Hat
Enterprise Linux 6. In this release, libvirt features the new sVirt
component. sVirt integrates with SELinux, providing security mechanisms
to prevent unauthorized access of guests and hosts in a virtualized
8.5. Enterprise Security Client
The Enterprise Security Client (ESC) is a simple GUI that allows Red
Hat Enterprise Linux to manage smart cards and tokens. New smart cards
can be formatted and enrolled, meaning that new keys are generated and
certificates requested for the smart card automatically. The smart card
lifecycle can be managed, as well, so that lost smart cards can have
their certificates revoked and expired certificates can be renewed. The
ESC works in conjunction with a larger public-key infrastructure
management product, either Red Hat Certificate System or Dogtag PKI.
9.1. Multiqueue Networking
Every data packet transferred over a network device represents
processing which must be completed by a CPU. The low-level network
implementation in Red Hat Enterprise Linux 6 allows network device
drivers to divide network packet processing across multiple queues.
Dividing these processes allows a system to better utilize the multiple
processors and CPU cores present on modern systems.
9.2. Internet Protocol version 6 (IPv6)
The next-generation Internet Protocol version 6 (IPv6) specification
is designed as the successor to Internet Protocol version 4 (IPv4). IPv6
specifies a wide range of improvements over IPv4, including: expanded
addressing capabilities, flow labeling and simplified header formats.
9.2.1. Optimistic Duplicate Address Detection
Duplicate Address Detection (DAD) is a feature of the Neighbor
Discovery Protocol portion of IPv6. Specifically, DAD is tasked with
checking if an IPv6 address is already being used. Red Hat Enterprise
Linux features Optimistic Duplicate Address Detection, a speed
optimization of DAD.
9.2.2. Intra-Site Automatic Tunnel Addressing Protocol
Red Hat Enterprise Linux 6 features support for the Intra-Site
Automatic Tunnel Addressing Protocol (ISATAP). ISATAP is a protocol
designed to assist in the transition from IPv4 to IPv6, by providing a
mechanism to connect IPv6 routers and hosts over IPv4 network
Netlabel is a new kernel-level feature in Red Hat Enterprise Linux 6
that provides network packet labeling services for Linux Security
Modules (LSMs). Labeling data packets using netlabel allows an LSM to
better enforce security requirements on incoming network packets.
9.4. Generic Receive Offload
The low-level network implementation in Red Hat Enterprise Linux 6
features Generic Receive Offload (GRO) support. The GRO system increases
the performance of inbound network connections by reducing the amount
of processing done by the CPU. GRO implements the same technique as the
Large Receive Offload (LRO) system, but can be applied to a wider range
of transport layer protocols.
Red Hat Enterprise Linux 6 contains enhanced support for wireless
networking and devices. Support for the wireless local area networking
using the IEEE 802.11 set of standards has been improved, with added
support for 802.11n based wireless networking.
Red Hat Enterprise Linux 6 introduces a new, seamless graphical boot
sequence that commences immediately after the hardware has initialized.
The new graphical boot sequence provides the user with simple visual
feedback on the progress of the system boot, and seamlessly switches to
the login screen. The Red Hat Enterprise Linux 6 graphical boot sequence
is enabled by the Kernel Modesetting feature and is available on ATI,
Intel and NVIDIA graphics hardware.
System Administrators are still able to view detailed progress of
the boot sequence by pressing the F11 key at any time during the
Suspend and resume is a current feature in Red Hat Enterprise Linux
that allows a machine to be placed into and removed from a low power
state. The new kernel modesetting feature enables enhanced support for
the suspend and resume feature. Previously, graphics hardware was
suspended and resumed via userspace applications. In Red Hat Enterprise
Linux 6, this functionality has moved into the kernel, providing a more
reliable mechanism for enabling low power mode.
10.3. Multiple Display Support
Red Hat Enterprise Linux 6 features enhanced support for workstations
with multiple displays. When an additional display is attached to a
machine, the graphics driver detects it and automatically adds it to the
desktop. Conversely, when a display is unplugged, the graphics driver
automatically removes it from the desktop.
By default, the additional display is added in a spanning layout to the left of the current display.
The automatic detection of additional displays is useful in
situations where displays are added and removed frequently (e.g. setting
up a laptop with an external projector)
10.3.1. Display Preferences
The new Display Preferences dialog provides the ability to further customize multiple display layouts.
Figure 7. Display Preferences Dialog
The new dialog provides the ability to instantly change the
positioning, resolution, refresh rate and rotation settings for each
individual display that is currently attached to a machine.
10.4. nouveau Driver for NVIDIA Graphics Devices
Red Hat Enterprise Linux 6 features the new nouveau driver as default
for NVIDIA graphics devices up to and including the NVIDIA GeForce 200
series. nouveau supports 2D and software video acceleration and kernel
The previous default driver for NVIDIA hardware (nv) is still available in Red Hat Enterprise Linux 6.
Red Hat Enterprise Linux 6 introduces the Intelligent Input Bus
(IBus) as the default input method framework for Asian languages.
10.5.2. Choosing and Configuring Input Methods
Red Hat Enterprise Linux 6 includes
im-chooser, a graphical user interface to enable and configure input methods. im-chooser (located under
System > Preferences > Input Method in the main menu) allows the user to easily enable and configure the input methods available on the system.
10.5.3. Indic Onscreen Keyboard
The new Indic Onscreen Keyboard (iok) is a screen based virtual
keyboard for Indic languages, enabling input using Inscript keymap
layouts and other 1:1 key mappings.
10.5.4. Indic Collation Support
Red Hat Enterprise Linux 6 includes improved sorting for Indic
languages. The order of menus and other interface elements are now
correctly sorted in Indic languages.
Font support in Red Hat Enterprise Linux 6 has been improved, with
updates to fonts for Chinese, Japanese, Korean, Indic and Thai
The majority of applications on the Red Hat Enterprise Linux 6
desktop have been updated. The following section documents the most
Red Hat Enterprise Linux 6 introduces version 3.5 of the Mozilla Firefox web browser.
Red Hat Enterprise Linux 6 includes version 3 of the Mozilla
Thunderbird email client, providing tabbed messaging, smart folders, and
a message archive. For further details on new features in Thunderbird
3, refer to the Thunderbird Release Notes
10.6.3. OpenOffice.org 3.1
Red Hat Enterprise Linux 6 features OpenOffice.org 3.1, adding
support for reading a wider range of file formats, including Microsoft
Office OOXML format. Additionally, OpenOffice.org has improved file
locking support and has the ability to render graphics using
Figure 8. OpenOffice.org 3.1
Full details on all the features in this version of OpenOffice.org are available in the OpenOffice.org Release Notes
NetworkManager is the desktop tool that is used to set up, configure and manage a wide range of network connection types.
In Red Hat Enterprise Linux 6, NetworkManager provides enhanced
support for Mobile Broadband devices, IPv6 and added support for
connecting to Bluetooth Personal Area Network (PAN) devices.
Red Hat Enterprise Linux 6 provides KDE 4.3 as an alternative desktop environment.
KDE 4.3 features an entirely new user experience, featuring:
The new Plasma Desktop Workspace, including Plasma Widgets for a more customizable desktop.
Oxygen, with enhanced icon and sound themes.
Enhancements to the KDE Window Manager (kwin)
dolphin file browser has replaced
konqueror as the KDE default.
Documentation for Red Hat Enterprise Linux 6 is comprised of 18
separate documents. Each of these documents belongs to one or more of
the following subject areas:
11.1. Release Documentation
The Release Notes
document the major new features in Red Hat Enterprise Linux 6.
The Red Hat Enterprise Linux Technical Notes
contains detailed information specific to this release, including:
Technology Previews, package change details and known issues.
Migration Planning Guide
The Red Hat Enterprise Linux Migration Planning Guide
documents migration from Red Hat Enterprise Linux 5 to Red Hat Enterprise Linux 6.
11.2. Installation and Deployment
The Installation Guide
documents relevant information regarding the installation of Red Hat Enterprise Linux 6
The Deployment Guide
documents relevant information regarding the deployment, configuration and administration of Red Hat Enterprise Linux 6.
Storage Administration Guide
The Storage Administration Guide
provides instructions on how to effectively manage storage devices and
file systems on Red Hat Enterprise Linux 6. It is intended for use by
system administrators with intermediate experience in either Red Hat
Enterprise Linux or Fedora distributions of Linux.
Global File System 2
The Global File System 2
book provides information about configuring and maintaining Red Hat GFS2 (Global File System 2) for Red Hat Enterprise Linux 6.
Logical Volume Manager Administration
The Logical Volume Manager Administration
book describes the LVM logical volume manager, including information on running LVM in a clustered environment.
The Security Guide
is designed to assist users and administrators in learning the
processes and practices of securing workstations and servers against
local and remote intrusion, exploitation and malicious activity.
SELinux User Guide
The SELinux User Guide
covers the management and use of Security-Enhanced Linux for those with
minimal or no experience with the framework. It serves as an
introduction to SELinux and explains the terms and concepts in use.
Managing Confined Services
The Managing Confined Services
guide is designed to assist advanced users and administrators when
using and configuring Security-Enhanced Linux (SELinux). It is focused
on Red Hat Enterprise Linux and describes the components of SELinux as
they pertain to services an advanced user or administrator might need to
configure. Also included are real-world examples of configuring these
services and demonstrations of how SELinux complements their operation.
11.4. Tools & Performance
Resource Management Guide
The Resource Management Guide
documents tools and techniques for managing system resources on Red Hat Enterprise Linux 6.
Power Management Guide
The Power Management Guide
explains how to manage power consumption on Red Hat Enterprise Linux 6
systems effectively. This document discusses different techniques that
lower power consumption (for both server and laptop), and how each
technique affects the overall performance of a system.
The Developer Guide
describes the different features and utilities that make Red Hat
Enterprise Linux 6 an ideal enterprise platform for application
SystemTap Beginners Guide
The SystemTap Beginners Guide
provides basic instructions on how to use SystemTap to monitor
different subsystems of Red Hat Enterprise Linux in finer detail.
SystemTap Tapset Reference
The SystemTap Tapset Reference
guide describes the most common tapset definitions users can apply to SystemTap scripts.
Cluster Suite Overview
The Cluster Suite Overview
document provides an overview of High Availability for Red Hat Enterprise Linux 6.
The Cluster Administration
document describes the configuration and management of Red Hat High Availability systems for Red Hat Enterprise Linux 6.
Virtual Server Administration
The Virtual Server Administration
book discusses the configuration of high-performance systems and
services with Red Hat Enterprise Linux 6 and the Linux Virtual Server
The DM Multipath
book provides information on using the Device-Mapper Multipath feature of Red Hat Enterprise Linux 6.
The Virtualization Guide
details the process to install, configure and manage the virtualization technologies in Red Hat Enterprise Linux 6.
Control groups are a new feature of the Linux kernel in Red Hat
Enterprise Linux 6. Each control group is a set of tasks on a system
that have been grouped together to better manage their interaction with
system hardware. Control groups can be tracked to monitor the system
resources that they use. Additionally, system administrators can use
control group infrastructure to allow or to deny specific control groups
access to system resources such as memory, CPUs (or groups of CPUs),
networking, I/O, or the scheduler. Management of control groups in
userspace is provided by
enabling system administrators to create new control groups, start new
processes in a specific control group and set control group parameters.
Control Groups and other resource management features are discussed in detail in the Red Hat Enterprise Linux 6 Resource Management Guide
12.2.1. Completely Fair Scheduler (CFS)
A process (or task) scheduler is a specific kernel subsystem that is
responsible for assigning the order in which processes are sent to the
CPU. The kernel (version 2.6.32) shipped in Red Hat Enterprise Linux 6
O(1) scheduler with the new
Completely Fair Scheduler (CFS). The CFS implements the fair queuing scheduling algorithm.
12.2.2. Virtual Memory Pageout Scalability
Implemented by the kernel, virtual memory presents applications with
a single, contiguous block of memory addresses. The reality underlying
this presentation is complex, with actual physical addresses commonly
fragmented and even paged out to much slower devices such as fixed
disks. The virtual memory addresses are organized by the kernel into
standard units called pages. The kernel in Red Hat Enterprise Linux 6
features enhanced management of virtual memory pages, reducing the
processing load required on systems with large amounts of physical
12.3.1. Advanced Error Reporting (AER)
The kernel in Red Hat Enterprise Linux 6 features Advanced Error
Reporting (AER). AER is a new kernel feature that provides enhanced
error reporting for PCI-Express devices.
12.3.2. Kdump Auto Enablement
Kdump is now enabled by default on systems with large amounts of memory. Specifically, kdump is enabled by default on:
systems with more than 4GB of memory on architectures with a 4KB page size (i.e. x86 or x86_64), or
systems with more than 8GB of memory on architectures with larger than a 4KB page size (i.e PPC64).
On systems with less than the above memory configurations, kdump is not auto enabled. Refer to
/usr/share/doc/kexec-tools-2.0.0/kexec-kdump-howto.txt for instructions on enabling kdump on these systems.
12.4.1. Aggressive Link Power Management (ALPM)
The kernel in Red Hat Enterprise Linux 6 features support for
Aggressive Link Power Management (ALPM). ALPM is a power-saving
technique that helps the disk save power by setting a SATA link to the
disk to a low-power setting during idle time (i.e. when there is no
I/O). ALPM automatically sets the SATA link back to an active power
state once I/O requests are queued to that link.
Previously the kernel implemented a timer that periodically queried
the system to check if there were any outstanding tasks to process.
Consequently, the CPU would remain in an active state, consuming
unnecessary power. The kernel in Red Hat Enterprise Linux 6 enables the
new tickless kernel feature, replacing the periodic timer interrupts
with on-demand interrupts. The tickless kernel allows a CPU to enter
longer sleep states when idle, and wake only when a task is queued for
12.5. Analyzing Kernel Performance
12.5.1. Performance Counter for Linux (PCL)
The Linux Performance Counter infrastructure provides an abstraction
of performance counter hardware capabilities, such as instructions
executed, cache misses, and branches mis-predicted. PCL provides
per-task and per-CPU counters, and adds event capabilities on top of
these counters. Performance counter information can be used to profile
kernel functions and events, and assist in the analysis of kernel
Two new tools are available in Red Hat Enterprise Linux 6 to assist
in analyzing kernel performance. Ftrace provides call graph style
tracing for the kernel. The new perf tool monitors, logs and analyzes
system hardware events.
12.6. General Kernel Updates
12.6.1. Physical Address Extension (PAE)
The Physical Address Extension (PAE) is a feature implemented in
modern x86 processors. PAE extends memory addressing capabilities,
allowing more than 4 gigabytes (GB) of random access memory (RAM) to be
used. The default kernel shipped with the x86 architecture version of
Red Hat Enterprise Linux 6 is PAE enabled. A PAE enabled processor is a
minimum requirement for the x86 variant of Red Hat Enterprise Linux 6.
12.6.2. Loadable Firmware Files
Firmware files for which there is no appropriately licensed source
code have been removed from the Red Hat Enterprise Linux 6 kernel.
Modules that require loadable firmware now use a kernel interface to
request firmware from userspace.
SystemTap is a tracing and probing tool that allows users to study
and monitor the activities of the operating system (particularly, the
kernel) in fine detail. It provides information similar to the output of
tools like netstat, ps, top, and iostat; however, SystemTap is designed
to provide more filtering and analysis options for collected
Red Hat Enterprise Linux 6 features SystemTap version 1.1, which introduces many new features and enhancements, including:
Improved support for user-space probing.
Support for probing C++ programs with native C++ syntax.
A more secure script-compile server.
The new unprivileged mode, allowing non-root users to use SystemTap.
Unprivileged mode is new and experimental. The stap-server facility
on which it relies is undergoing work for security improvements and
should be deployed with care on a trustworthy network.
OProfile is a system-wide profiler for Linux systems. The profiling
runs transparently in the background and profile data can be collected
at any time.
Red Hat Enterprise Linux 6 features version 0.9.5 of OProfile, adding support for new Intel and AMD processors.
13.3. GNU Compiler Collection (GCC)
The GNU Compiler Collection (GCC) includes, among others, C, C++, and
Java GNU compilers and related support libraries. Red Hat Enterprise
Linux 6 features version 4.4 of GCC, which includes the following
features and enhancements:
Conformance to version 3.0 of the Open Multi-Processing (OpenMP) application programming interface (API).
Additional C++ libraries to utilize OpenMP threads
Further implementations of the next ISO C++ standard draft (C++0x)
Introduction of variable tracking assignments to improve debugging using the GNU Project Debugger (GDB) and SystemTap.
More information about the improvements implemented in GCC 4.4 is available from the GCC website.
13.4. GNU C Library (glibc)
The GNU C Library (glibc) packages contain the standard C libraries
used by multiple programs on Red Hat Enterprise Linux. These packages
contain the standard C and the standard math libraries. Without these
two libraries, the Linux system cannot function properly.
Red Hat Enterprise Linux 6 features version 2.11 of glibc, providing many features and enhancements, including:
An enhanced dynamic memory allocation (malloc) behaviour enabling
higher scalability across many sockets and cores. This is achieved by
assigning threads their own memory pools and by avoiding locking in some
situations. The amount of additional memory used for the memory pools
(if any) can be controlled using the environment variables
MALLOC_ARENA_TEST specifies that a test for the number of cores is performed once the number of memory pools reaches this value.
MALLOC_ARENA_MAX sets the maximum number of memory pools used, regardless of the number of cores.
Improved efficiency when using condition variables (condvars) with
priority inheritance (PI) mutual exclusion (mutex) operations by
utilizing support in the kernel for PI fast userspace mutexes.
Optimized string operations on the x86_64 architecture.
getaddrinfo() function now has support for the Datagram Congestion Control Protocol (DCCP) and the UDP-Lite protocol. Additionally,
getaddrinfo() now has the ability to look up IPv4 and IPv6 addresses simultaneously.
13.5. GNU Project Debugger (GDB)
The GNU Project Debugger (normally referred to as GDB) debugs
programs written in C, C++, and other languages by executing them in a
controlled fashion, and then printing out their data. Red Hat Enterprise
Linux 6 features version 7.0 of GDB.
This updated version of GDB introduces the new Python API, allowing
GDB to be automated using scripts written in the Python Programming
One notable feature of the Python API is the ability to format GDB
output (normally referred to as pretty-printing) using Python scripts.
Previously, pretty-printing in GDB was configured using a standard set
of print settings. The ability to create custom pretty-printer scripts
gives the user control of the way GDB displays information for specific
applications. Red Hat Enterprise Linux features a complete suite of
pretty-printer scripts for the GNU Standard C++ Library (libstdc++).
Enhanced C++ Support
Support for the C++ programming language in GDB has been improved. Notable improvements include:
Better handling of type names.
Many improvements to expression parsing.
The need for extraneous quoting has nearly been eliminated
"next" and other stepping commands work properly even when the inferior throws an exception.
GDB has a new "catch syscall" command. This can be used to stop the inferior whenever it makes a system call.
Independent Thread Debugging
Thread execution now permits debugging threads individually and
independently of each other; enabled by new settings "set target-async"
and "set non-stop".
Samba is a suite of programs which use NetBIOS over TCP/IP (NetBT) to
enable the sharing of files, printers and other information. This
package provides a Server Message Block or SMB server (also known as a
Common Internet File System or CIFS server) which can provide network
services to SMB/CIFS clients.
Red Hat Enterprise Linux 6 provides the following significant enhancements to Samba:
Internet Protocol version 6 support (IPv6)
Support for Windows 2008 (R2) trust relationships.
Support for Windows 7 domain members.
Support for Active Directory LDAP signing/sealing policy.
Improvements for libsmbclient
Better support for Windows management tools (mmc and User Manager)
Automatic machine password changes as domain member
New registry based configuration layer
Encrypted SMB transport between Samba client and server
Full support for Windows cross-forest, transitive trusts and one-way domain trusts
New NetApi remote management and winbind client C libraries
A new graphical user interface for joining Windows Domains
Refer to the Deployment Guide
for further information on Samba configuration on Red Hat Enterprise Linux 6.
15.1. Kernel-based Virtual Machine
Red Hat Enterprise Linux 6 includes full support for the Kernel-based Virtual Machine (KVM)
hypervisor on the AMD64 and Intel 64 architectures. KVM is integrated
into the Linux kernel, providing a virtualization platform that takes
advantage of the stability, features, and hardware support inherent in
Red Hat Enterprise Linux.
15.1.1. Memory enhancements
Transparent Hugepages increase the memory page size from 4
kilobytes to 2 megabytes. Transparent Hugepages provide significant
performance advantages on systems with highly contended resources and
large memory workloads. Additionally, Red Hat Enterprise Linux 6
provides support for utilizing Transparent Hugepages with KSM.
Extended Page Table age bits enables a host to make smarter
choices for swapping memory under memory pressure and allows swapping of
Transparent Hugepages by breaking the extended pages into smaller
15.1.2. Virtualized CPU features
Red Hat Enterprise Linux 6 supports up to 64 virtualized CPUs for a single virtualized guest.
CPU extensions present on the host processor can now be utilized
by virtualized guests. Support for these instruction sets allow
virtualized guests to take advantage of modern processor instruction
sets and hardware features.
x2apic virtual Advanced Programmable Interrupt Controller (APIC) improves virtualized
x86_64 guest performance by allowing direct guest APIC access and removing the overhead of emulated access.
New user space notifiers allow the caching of CPU registers,
avoiding the computationally expensive actions of preserving register
states of unused components during context switches.
Read copy update (RCU) kernel locking is now used for enhanced
symmetric multiprocessing support. RCU kernel locking provides greater
performance for networking functions and multi-processing systems.
Indirect ring entries (spin locks) for the para-virtualized (virtio) driver improve block I/O performance and allows more concurrent I/O operations.
Virtualized storage devices can now be added and removed (hot plugged) from guests during runtime.
Support for block alignment storage topology awareness. Underlying
storage hardware features and physical storage sector sizes (for
example, 4KB sectors) are presented to guests. This feature requires
compatible storage device information and commands. Guest topology
awareness allows virtualized guests to optimize file system layouts and
improve performance of applications using I/O optimizations.
Performance enhancements for the qcow2 virtualized image format.
MSI-X support which increases the number of interrupts available
to network devices. MSI-X support increases the performance of
Virtualized network devices can now be hot plugged and hot removed
from running guests. Network boot using gpxe for more advanced PXE
15.1.5. Kernel SamePage Merging
The KVM hypervisor in Red Hat Enterprise Linux 6 features Kernel SamePage Merging (KSM),
allowing KVM guests to share identical memory pages. Page sharing
reduces memory duplication, allowing a host with similar guest operating
systems to run more efficiently.
15.1.6. Device Assignment
Assignment devices can now be hot plugged and hot removed from running guests.
The para-virtualized serial device (virtio-serial) provides a simple
communication interface between the host's user space and the guest's
user space. virtio-serial can be used for communication where networking
is not be available or unusable.
sVirt is a new feature included with Red Hat Enterprise Linux 6.0
that integrates SELinux and virtualization. sVirt applies Mandatory
Access Control (MAC) to improve security when using virtualized guests.
sVirt improves security and hardens the system against bugs in the
hypervisor that might be used as an attack vector for the host or to
another virtualized guest.
Guest ABI stability provides enhanced migration support. Guests
PCI device numbers are preserved during migration and identical PCI
device positions are presented after migrating the guest.
Migration now accounts for CPU models. CPU models allow guests to
take advantage of new processor instruction sets. Guests can be migrated
to hosts with a compatible CPU model.
Enhancements to the migration protocol.
15.1.10. Guest Device ABI Stability
As part of the new qdev device model, the guest ABI is now stable
and will be kept consistent for newer releases. The devices and device
arrangements on guests will remain consistent in future updates. This
feature resolves issues with some operating system activation processes.
Red Hat Enterprise Linux 6 includes components providing functionality for the Simple Protocol for Independent Computing Environments (SPICE)
remote display protocol. These components are only supported for use in
conjunction with Red Hat Enterprise Virtualization products and are not
guaranteed to have a stable ABI. The components will be updated to
synchronize with functional requirements of Red Hat Enterprise
Virtualization products. Migration to future releases may require manual
operations on a per-system basis.
Red Hat Enterprise Linux 6 is supported as a Xen guest for the x86
and the AMD 64 and Intel 64 architectures. The para-virtualized
operations (pv-ops) are included in the Red Hat Enterprise Linux 6
kernel. The default Red Hat Enterprise Linux 6 kernel can be used as a
Xen para-virtualized guest and as a Xen fully virtualized guest on Red
Hat Enterprise Linux 5 hosts. Red Hat Enterprise Linux 6 includes the
para-virtualized drivers for fully virtualized Xen guest installations.
Red Hat Enterprise Linux 6 is not supported as a Xen host.
The Virtualization Guide
details the process to install, configure and manage the virtualization technologies in Red Hat Enterprise Linux 6.
Red Hat Enterprise Linux 6 features the new
tool, enabling system administrators to convert and import virtual
machines created on other systems such as Xen and VMware ESX to KVM.
virt-v2v provides a migration path for Xen guests running on a Red Hat Enterprise Linux 5 hypervisor.
16. Supportability and Maintenance
16.1. firstaidkit System Recovery Tool
Red Hat Enterprise Linux 6 includes the new
firstaidkit system recovery tool. By automating common recovery processes,
provides an interactive environment to assist in the troubleshooting
and recovery of a system that boots incorrectly. Additionally, system
administrators are able to create custom automated recovery processes
firstaidkit plugin infrastructure.
firstaidkit is considered a Technology Preview in Red Hat Enterprise Linux 6.
16.2.1. Installation Crash Reporting
16.3. Automated Bug Reporting Tool
Red Hat Enterprise Linux 6 features the new Automated Bug Reporting
Tool (ABRT). ABRT logs details of software crashes on a local system,
and provides interfaces (both graphical and command line based) to
report issues to Red Hat support.
Figure 10. Automated Bug Reporting Tool
17. Web Servers and Services
17.1. Apache HTTP Web Server
The Apache HTTP Server is a robust, commercial-grade open source Web
server. Red Hat Enterprise Linux 6 includes the Apache HTTP Server
2.2.15 as well as a number of server modules designed to enhance its
Apache in Red Hat Enterprise Linux 6 features support for the Server
Name Indication (SNI) protocol, which enables name-based virtual hosting
over Secure Sockets Layer (SSL) connections. Additionally, support for
the Web Server Gateway Interface (WSGI) has been added to Apache for
this release, enabling the use of python web application frameworks that
implement the WSGI standard.
17.2. PHP: Hypertext Preprocessor (PHP)
PHP is an HTML-embedded scripting language commonly used with the
Apache HTTP Web server. In Red Hat Enterprise Linux, PHP now supports
the Alternative PHP Cache (APC).
memcached is a high-performance distributed object caching server
that is designed to increase the performance of dynamic web applications
by reducing database load. memcached is a new feature in this release,
and provides bindings for C, PHP, Perl and Python programming languages.
PostgreSQL is an advanced Object-Relational database management
system (DBMS). The postgresql packages include the client programs and
libraries needed to access a PostgreSQL DBMS server.
Red Hat Enterprise Linux 6 features version 8.4 of PostgreSQL
MySQL is a multi-user, multi-threaded SQL database server. It
consists of the MySQL server daemon (mysqld) and many client programs
This release features version 5.1 of MySQL. For a list of all enhancements that this version provides, refer to the MySQL Release Notes
19. Architecture Specific Notes
Red Hat Enterprise Linux 6 is architecturally complete, and all supported architectures are now available.
Red Hat Enterprise Linux 6 will not provide support for the Intel®
Itanium® architecture. All Itanium-related development will be
incorporated into Red Hat Enterprise Linux 5 exclusively. Up to and
through March 2014, Red Hat Enterprise Linux 5 will provide support,
deliver new features, and enable new Itanium hardware in accordance with
the published Red Hat Enterprise Linux product life-cycle. In addition,
extended support for Red Hat Enterprise Linux 5 for Itanium is
available up to March 2017 from selected OEMs.
On the POWER architecture, Red Hat Enterprise Linux 6 requires a
POWER6 or higher CPU. POWER5 processors are not supported on Red Hat
Enterprise Linux 6.
A. Revision History
|Revision 1-0||Wed Nov 10 2010|
|Initial Release of the Red Hat Enterprise Linux 6 Release Notes|