Xen实战之PV domU安装

声明

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引言

本文介绍了在Xen4.2平台上安装通过xm创建半虚拟化Ubuntu的domU客户机过程，并对domu配置文件项进行了阐述。本文使用Xen4.2的dom0运行在Centos6.5上。

安装准备

1. 创建domU使用的逻辑分区

首先需要创建domU使用的逻辑分区LV(Logical Volume)，这要求在进行dom0安装时进行逻辑卷组VG(Volume Group)的创建或在新的硬盘分区上进行创建。作者在centos dom0安装时预留了用于domU使用的逻辑卷组xen_domu，创建domU的逻辑卷都可以从此卷组上分配。

i. 查看系统目前的逻辑卷组的情况

[root@centos xen]# vgdisplay 
  --- Volume group ---
  VG Name               xen_domu
  System ID             
  Format                lvm2
  Metadata Areas        1
  Metadata Sequence No  6
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                2
  Open LV               1
  Max PV                0
  Cur PV                1
  Act PV                1
  VG Size               250.00 GiB
  PE Size               4.00 MiB
  Total PE              63999
  Alloc PE / Size       4096 / 16.00 GiB
  Free  PE / Size       59903 / 234.00 GiB
  VG UUID               EsdDpB-9fuy-EqF7-8URN-DEAg-MGub-fsndjw
  
  ......

ii. 使用lvcreate创建所需的逻辑分区

[root@centos xen]# lvcreate -L 8G -n ubuntu64 xen_domu
  Logical volume "ubuntu" created
[root@centos xen]# lvdisplay /dev/xen_domu/ubuntu64
  --- Logical volume ---
  LV Path                /dev/xen_domu/ubuntu64
  LV Name                ubuntu64
  VG Name                xen_domu
  LV UUID                3bEYBI-NWD2-8FH8-Qc09-rlnQ-9JhN-LHFlWO
  LV Write Access        read/write
  LV Creation host, time centos, 2014-02-21 17:12:13 -0500
  LV Status              available
  # open                 1
  LV Size                8.00 GiB
  Current LE             2048
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:3  

2. 准备配置文件

你会需要 domU 安装用的 initrd 映像及内核。你可以将它们放在一个合适的目录，并将它们改为恰当的名称。作者将相关文件放置在了/srv/ubuntu64目录下。

[root@centos ~]# cd /srv
[root@centos srv]# mkdir ubuntu64
[root@centos ubuntu64]# wget http://mirrors.sohu.com/ubuntu/dists/precise/main/installer-amd64/current/images/netboot/xen/initrd.gz
[root@centos ubuntu64]# wget http://mirrors.sohu.com/ubuntu/dists/precise/main/installer-amd64/current/images/netboot/xen/vmlinuz

Xen 在每个本地使用一个配置文件。这些本地设置在安装时会稍有分别，因为我们要提供所安装内核，甚至乎一些开机参数。Xen缺省将配置文件放置在/etc/xen目录下，这样在使用xm create创建domU客户机时就不需要指定路径参数。

/etc/xen/ubuntu.cfg配置文件的具体内容如下（具体每个参数含义可参考配置文件中的说明）：

#  -*- mode: python; -*-
#============================================================================
# Python configuration setup for 'xm create'.
# This script sets the parameters used when a domain is created using 'xm create'.
# You use a separate script for each domain you want to create, or 
# you can set the parameters for the domain on the xm command line.
#============================================================================

#----------------------------------------------------------------------------
# Kernel image file.
kernel = "/srv/ubuntu64/vmlinuz"

# Optional ramdisk.
ramdisk = "/srv/ubuntu64/initrd.gz"

# The extra option specifies a string that is appended, unchanged, to the
# domU kernel options.
extra = "debian-installer/exit/always_halt=true --console=hvc0"

# A name for your domain. All domains must have different names.
name = "ubuntu"

# Initial memory allocation (in megabytes) for the new domain.
#
# WARNING: Creating a domain with insufficient memory may cause out of
#          memory errors. The domain needs enough memory to boot kernel
#          and modules. Allocating less than 32MBs is not recommended.
memory = 1024

#----------------------------------------------------------------------------
# Define the disk devices you want the domain to have access to, and
# what you want them accessible as.
# Each disk entry is of the form phy:UNAME,DEV,MODE
# where UNAME is the device, DEV is the device name the domain will see,
# and MODE is r for read-only, w for read-write.
# /dev/xvd is the standard name for Xen virtual disks, by analogy with
# the hd and sd device nodes. The first virtual disk is /dev/xvda, which can
# be partitioned into /dev/xvda1, and so on. 
disk = ['phy:/dev/xen_domu/ubuntu64,xvda,w']

# The bootloader line specifies a program that will be run within dom0
# to load and initialize the domain kernel. For example, you can specify
# bootloader=pygrub to get a domain that, on startup, presents a GRUBlike
# boot menu.
# bootloader = "pygrub"

#----------------------------------------------------------------------------
# Define network interfaces.

# By default, no network interfaces are configured.  You may have one created
# with sensible defaults using an empty vif clause:
#
# vif = [ '' ]
#
# or optionally override backend, bridge, ip, mac, script, type, or vifname:
#
# vif = [ 'mac=00:16:3e:00:00:11, bridge=xenbr0' ]
#
# or more than one interface may be configured:
#
# vif = [ '', 'bridge=xenbr1' ]
vif = ['bridge=xenbr0']

#----------------------------------------------------------------------------
# Configure the behaviour when a domain exits.  There are three 'reasons'
# for a domain to stop: poweroff, reboot, and crash.  For each of these you
# may specify:
#
#   "destroy",        meaning that the domain is cleaned up as normal;
#   "restart",        meaning that a new domain is started in place of the old
#                     one;
#   "preserve",       meaning that no clean-up is done until the domain is
#                     manually destroyed (using xm destroy, for example); or
#   "rename-restart", meaning that the old domain is not cleaned up, but is
#                     renamed and a new domain started in its place.
#
# In the event a domain stops due to a crash, you have the additional options:
#
#   "coredump-destroy", meaning dump the crashed domain's core and then destroy;
#   "coredump-restart', meaning dump the crashed domain's core and the restart.
#
# The default is
#
#   on_poweroff = 'destroy'
#   on_reboot   = 'restart'
#   on_crash    = 'restart'
#
# For backwards compatibility we also support the deprecated option restart
#
# restart = 'onreboot' means on_poweroff = 'destroy'
#                            on_reboot   = 'restart'
#                            on_crash    = 'destroy'
#
# restart = 'always'   means on_poweroff = 'restart'
#                            on_reboot   = 'restart'
#                            on_crash    = 'restart'
#
# restart = 'never'    means on_poweroff = 'destroy'
#                            on_reboot   = 'destroy'
#                            on_crash    = 'destroy'
on_shutdown = 'destroy'
on_poweroff = 'destroy'

开始安装

当你设置好安装的配置以后，便可以引导domU的实例：

[root@centos ~]# xm create ubuntu.cfg
[root@centos ~]# xm list
Name                                        ID   Mem VCPUs      State   Time(s)
Domain-0                                     0  1024    16     r-----   7152.9
ubuntu                                      15  1024     1     ---s--      4.1

如果你正确地设置了这台 domU，安装桯序将会顺利地开始。假若你选择了手动式安装，你可以将一个主控台连接到这台domU：

[root@centos ~]# xm console ubuntu

安装完成后，这台domU将会重新引导并被销毁（因为这是重新引导的缺省动作，我们稍后会更改它）。

安装后的配置修改

之前的配置文件是用于安装domU，安装完成以后需要修改配置文件。

#kernel = "/srv/ubuntu/vmlinuz"
#ramdisk = "/srv/ubuntu/initrd.gz"
#extra = "debian-installer/exit/always_halt=true --console=hvc0"
name = "ubuntu"
memory = 1024
disk = ['phy:/dev/xen_domu/ubuntu64,xvda,w']
bootloader = "pygrub"
vif = ['bridge=xenbr0']
on_shutdown = 'preserve'
on_poweroff = 'preserve'

如你所见，这个新的配置文件不会再使用内核或initrd映像。取而代之，它会使用pygrub为开机程序。pygrub会尝试在虚拟映像档的分区内寻找一个藏有GRUB配置文件的文件系统。假若能找到GRUB的配置文件，它便会被用来引导内核。

同时，你也会注意到开关机选项参数由destroy更改为perserve，这意味着domU关机操作后，你仍然可以通过“xm list”查看到此客户机，直到你使用了"xm destroy“删除。

自动引导domU

如果你想在系统（dom0）开机时自动引导一个本地domU客户机，请将这个本地的配置文件链接到到/etc/xen/auto目录内。例如：

[root@centos ~]# cd /etc/xen/auto
[root@centos auto]# ln -s ubuntu.cfg /etc/xen/ubuntu.cfg

这样做亦会确保这个本地在系统关机时会被正确地关闭。

总结

本文简单介绍了Xen4.2下安装PV Ubuntu的主要步骤，希望能够指导感兴趣的阅读者迅速创建PV的客户机。

你可能在安装的过程中遇到各种问题，需要做的就是平静下来，了解每个步骤或参数的含义，逐步排除各种可能的问题；解决问题的过程远比顺利创建出来学到的知识和技能要多得多。