CentOS（Community ENTerprise Operating System）是Linux发行版之一，它是来自于Red Hat Enterprise Linux依照开放源代码规定开放的源代码所编译而成。由于出自同样的源代码，因此有些要求高度稳定性的服务器以CentOS替代商业版的Red Hat Enterprise Linux使用。两者的不同，在于CentOS并不包含封闭源代码软件.今天，centos 开发团队对外发布了4.x 分支的最新版本4.9,以应对RHEL的4.9版本。主要更新如下：
* The kernel-utils package provides microcode_ctl utility code and the
microcode data itself — supplied by Intel — to assist the kernel in
updating the CPU microcode at system boot time. This microcode supports
all current Intel x86- and Intel 64-based CPU models and takes advantage
of the mechanism built-in to Linux that allows microcode to be updated
after system boot. When loaded, the updated microcode corrects the
behavior of various Intel processors, as described in processor
specification updates issued by Intel for those processors. In the
CentOS-4.9 release the kernel-utils package has been updated with the
2010-09-14 version of Intel's microcode.

*  OProfile in CentOS 4.9 now supports Intel Xeon (formerly codenamed
Nehalem) processor performance events.

*  The KornShell (ksh) package in CentOS 4.9 has been upgraded to the
upstream release version ksh93t+.

*  CentOS-4.9 provides an updated coreutils package, containing checksum
utilities for the whole SHA-2 family, allowing a user to easily count or
verify SHA-224, SHA-256, SHA-384, and SHA-512 checksums.

* CentOS-4.9 features version 3.6.13 of the Mozilla Firefox open source
web browser.

*  SystemTap in CentOS-4.9 has been updated to version 1.3. Most
notably, this update provides support for the newer kprobe capabilities.

*  OpenOffice 2.0 is now included in this release as a Technology
Preview. This suite features several improvements, including ODF and PDF
functionalities, support for digital signatures and greater
compatibility with open suites in terms of format and interface. In
addition to this, the OpenOffice 2.0 spreadsheet has enhanced pivot
table support, and can now handle up to 65,000 rows.

系统升级：yum upgrade
下载：http://wiki.centos.org/Download
查看ReleaseNote:http://wiki.centos.org/Manuals/ReleaseNotes/CentOS4.9