Red Hat Enterprise Linux 6 技术指南

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"Red Hat Enterprise Linux 6 Technical Notes en US" 本文档是Red Hat Enterprise Linux 6的技术笔记，属于技术发布文档，旨在为用户提供该版本操作系统的技术细节和相关信息。Red Hat Enterprise Linux（RHEL）是一款广泛使用的商业Linux发行版，特别针对企业级应用进行了优化，以确保稳定性和安全性。在RHEL 6中，包含了多项关键的技术特性： 1. **内核更新**：RHEL 6基于Linux内核的稳定版本，提供了对硬件的广泛支持和性能改进。这可能包括对新处理器架构的支持、内存管理优化以及网络和存储驱动的增强。 2. **XFS文件系统**：文档中提到了XFS，它是一个高性能、日志结构的文件系统，适用于大型数据集和高吞吐量的环境。RHEL 6支持XFS，允许用户利用其强大的扩展性和性能优势。 3. **安全增强**：RHEL 6集成了 SELinux（安全增强型Linux），这是一种强制访问控制机制，能够提供细粒度的安全策略，防止恶意软件和未经授权的访问。 4. **虚拟化**：RHEL 6引入了KVM（Kernel-based Virtual Machine）作为其主要的虚拟化平台，允许在同一台物理服务器上运行多个独立的操作系统实例，提高了硬件资源的利用率。 5. **JBoss企业应用平台**：作为Red Hat的中间件产品，JBoss在RHEL 6中被支持，为企业级应用提供了一个可扩展的、基于Java的平台。 6. **数据库支持**：RHEL 6提供了对MySQL数据库的支持，MySQL是一种流行的开源关系型数据库管理系统，适用于多种规模的应用。 7. **认证和兼容性**：RHEL 6遵循严格的兼容性和认证标准，以确保与广泛的企业级硬件和软件生态系统兼容。 8. **更新和维护**：Red Hat的订阅服务提供了持续的更新、安全补丁和故障排除支持，确保RHEL 6系统的长期稳定性和安全性。 9. **开发工具和库**：RHEL 6包含一套完整的开发工具和库，如GCC编译器和各种编程语言的运行时环境，支持软件开发和部署。 10. **网络和服务管理**：文档可能涵盖网络配置、DNS、DHCP、NTP等服务的管理和配置，这些是任何企业网络基础设施的关键组成部分。通过阅读这份技术笔记，用户可以深入了解RHEL 6的系统架构、安装流程、系统管理、故障排查方法以及如何利用RHEL 6的各种功能来满足企业的特定需求。同时，由于文档遵循CC-BY-SA许可协议，用户可以在遵循特定条款的情况下自由分享和修改文档内容。

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Technical Notes

• A change in the package that the sos tool uses to determine the installed version of Red Hat

Enterprise Linux will cause the tool to incorrectly identify the major release version. This adversely

impacts a small number of non-default sos plugins and may cause incomplete information to be

captured from the system when these plugins are enabled. The affected plugins are:

• general (only when using the non-default all_logs option)

• cluster (diagnostics may not be run)

Users affected by this problem should retrieve any missing data manually from systems.

3. Virtualization

Para Virtualization on Hardware Virtualized Machines (PV on HVM)

Red Hat Enterprise Linux 6 guests under Red Hat Enterprise Linux 5 Xen hosts can now utilize

the PV on HVM drivers to improve the performance of I/O on virtualized network devices (xen-vnif)

and virtualized block storage devices.

To enable Xen PV on HVM support in a Red Hat Enterprise Linux 6 HVM guest, add the following

to the kernel boot command line:

xen_pv_hvm=enable

Note, however, that due to conflicts with network configuration scripts, it is recommended that the

xen guest vif specification set 'type=netfront' if the emulated rtl8139 device is not desired as the

primary network interconnect.

virtio network device packet transmission algorithms

The virtio network device has two available algorithms for transmitting packets. The default is

to use an asynchronous bottom half transmitter which typically shows good performance for all

workloads. The alternate implementation uses a timer to delay transmit in an attempt to batch

multiple packets together. The timer approach typically results higher latency, but may improve

overall efficiency. To change from the default algorithm to the timer based approach, use the

following procedure to create a wrapper script around qemu-kvm and specify it as the emulator for

guests that require it.

1. create the wrapper script

$ cat > /usr/libexec/qemu-kvm.txtimer << EOF

#!/bin/sh

exec /usr/libexec/qemu-kvm \`echo "\$@" | sed

's|virtio-net-pci|virtio-net-pci,tx=timer|g'\`

EOF

2. Make script executable

$ chmod 755 /usr/libexec/qemu-kvm.txtimer

3. Set selinux permissions

$ restorecon /usr/libexec/qemu-kvm.txtimer

4. Create selinux module

Technical Notes

To enable the old, insecure setting, edit /etc/libvirt/qemu.conf, set "relaxed_acs_check = 1", and

restart libvirtd. Note that this action will re-open possible security issues.

• Users upgrading from pre-release versions of Red Hat Enterprise Linux 6 (i.e. the virt-v2v

versions less than virt-v2v-0.6.2-2.el6) may be required to update the default virt-v2v configuration

file. Specifically, the 'viostor' app for Windows guests is replaced by the 'virtio' app, which now points

to the directory containing the complete driver. Refer to the updated default configuration file for

further details.

• I/O Advanced Programmable Interrupt Controller (I/O APIC) timer interrupts are not emulated as

non-maskable interrupts (NMIs) to virtualized guests. Consequently, if a virtualized guest uses the

kernel parameter nmi_watchdog=1, the guest kernel will panic on boot.

• The balloon service on Windows 7 guests can only be started by the "Administrator" user.

• Direct Asynchronous IO (AIO) that is not issued on filesystem block boundaries, and falls into a hole

in a sparse file on ext4 or xfs filesystems, may corrupt file data if multiple I/O operations modify the

same filesystem block. Specifically, if qemu-kvm is used with the aio=native IO mode over a sparse

device image hosted on the ext4 or xfs filesystem, guest filesystem corruption will occur if partitions

are not aligned with the host filesystem block size. Generally, do not use aio=native option along

with cache=none for QEMU. This issue can be avoided by using one of the following techniques:

1. Align AIOs on filesystem block boundaries, or do not write to sparse files using AIO on xfs or

ext4 filesystems.

2. KVM: Use a non-sparse system image file or allocate the space by zeroing out the entire file.

3. KVM: Create the image using an ext3 host filesystem instead of ext4.

4. KVM: Invoke qemu-kvm with aio=threads (this is the default).

5. KVM: Align all partitions within the guest image to the host's filesystem block boundary (default

4k).

• On Red Hat Enterprise Linux 6 KVM virtual guests, unmounting a filesystem on an mdraid

volume does not immediately free the underlying device for the mdadm --stop operation.

Consequently, during installation on a system with pre-existing mdraid volumes the following error

can appear while anaconda is looking for storage devices:

MDRaidError: mddeactivate failed for /dev/md1: 08:26:59,485 ERROR : Perhaps a

running process, mounted filesystem or active volume group?

To work around this issue, erase all data on the volume before installation by clearing the first

several sectors of the volume with zeros.

• Libvirt uses transient iptables rules for managing NAT or bridging to virtual machine guests. Any

external command that reloads iptables state ( such as running system-config-firewall) will overwrite

the entries needed by libvirt. Consequently, after running any command or tool that changes the

state of iptables, guests may lose access the network. To work around this issue, use the command

'service libvirt reload' to restore libvirt's additional iptables rules.

• Adding an rtl8139 NIC to an active Windows 2008 guest may result in the qemu-kvm process

exiting. To work around this issue, shutdown the guest before adding additional rtl8139 NICs.

Alternatively, install the virtio-net drivers and add a virtio NIC.

• KVM users with a mix of virtio and ata disks should verify the boot device that anaconda chooses

during installation. To verify the boot device, locate the "Install Target Devices" list in the disk

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