RHEL is now standardized on the ext4 file system, which has been a long time coming. Using ext4, the OS can support volumes as large as 16TB. RHEL 6 also supports XFS and GFS2, which scale up to 100TB.
We finally see support for NFSv4 and a bunch of updates in the high-availability space, including better log file management across clusters and a redesigned Web management interface. In addition, RHEL 6 can take advantage of RAS (reliability, availability, serviceability) features found in newer CPUs such as the Intel Nehalem-EX, which allow it to handle hot-swap CPU and RAM events and deal with bad memory pages marked by hardware.
On the IP storage networking front, booting from iSCSI LUNs is now supported and easily managed at install time. There's now embedded support for Fibre Channel over Ethernet.
Note there is no Itanium version of RHEL 6.
RHEL 6 software updates and upgrades
On the software side, you'll find a plethora of updates and new frameworks, such as the new System Security Services Daemon (SSSD) that offers a pluggable interface for authentication, supporting PAM (Pluggable Authentication Modules), NSS (Network Security Services), and offline credential caching.
Awaited updates abound in the LAMP space, with the inclusion of Apache 2.2, PHP 5.3.2, and Perl 5.10.1. Somewhat disappointing is that MySQL 5.5 has been passed over for MySQL 5.1.47, which is based on a release from more than two years ago. On the plus side, Memcached 1.4.4 and Tomcat 6 are present. The base compiler has been updated to GCC 4.4.
Quite a few of the new features revolve around virtualization, not the least of which are WHQL (Windows Hardware Quality Labs)-certified drivers that usher in official Microsoft support for Windows virtual servers. Previous to this certification, Microsoft would tell you to go pound sand if you admitted you were running Windows on an RHEL-driven virtualization platform.
The KVM hypervisor has also been bumped up a notch or two, with features like guest CPU affinity; CPU masking, in which all VMs see the same CPU type no matter what's actually in the box; and KSM (Kernel Samepage Merging), which reduces RAM usage by allowing virtual machines to share memory pages. The new limits for virtual servers are 64 virtual CPUs and 256GB of RAM.
On the virtualization security front, sVirt is included. Based on SELinux, sVirt uses mandatory access controls to isolate virtual machines and prevent them from interfering with each other.
RHEL 6 should also be more power efficient than the previous version. Red Hat claims that by leveraging a tickless kernel and other power-reducing tweaks, a RHEL 6 server can lower idle power consumption by 20 percent or more. I have not yet tested this in the lab.
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