虚拟磁盘的格式有很多,这里不一一列举,简单介绍下linux下的常用格式raw,qcow2 
 

 在实际使用过程中可能会碰到这样一种情况,因某种原因导致虚拟机无法启动,但又需要读取虚拟机内的文件,可以通过挂载虚拟磁盘来进行读取,并且可以直接写入。 
 

RAW 
 
 KVM官方默认的虚拟磁盘文件格式为RAW。 
 
 对于RAW格式可以通过losetup工具来模拟出一个新设备加以挂载。 
 
示例:挂载rhel7-mode.img 
 
A.标准分区 
 
 root@jun-live:images# 
 pwd 
 
 /var/lib/libvirt/images 
 
 root@jun-live:images# 
 qemu-img info rhel7-mode.img 
 
 image: rhel7-mode.img 
 
 file format: 
 raw 
 
 virtual size: 8.0G (8589934592 bytes) 
 
 disk size: 3.5G 
 
1.将rhel7-mode.img映射为loop10 
 
 root@jun-live:images# 
 losetup /dev/loop10 rhel7-mode.img 
 
 root@jun-live:images# 
 ls /dev/mapper/loop10* 
 
 ls: cannot access /dev/mapper/loop10*: 
 No such file or directory 
 
 此时,内核没有识别到该设备的分区表,和普通硬盘刚分完区后重新加载分区表类似,需要kpartx -a 
 
 2.加载设备分区表 
 
root@jun-live:images#kpartx -l
loop18p1 : 0 409600 /dev/loop18 2048
loop18p2 : 0 16365568 /dev/loop18 411648
 loop: device /dev/loop18 still in use, retrying delete
 loop deleted : /dev/loop18 
 
 通过kpartx -l我们其实就已经可以看到,rhel7-mode.img包含两个分区,并且自动映射在loop18上,但loop18已经被其它镜像所使用,所以报了个资源忙。 
 
 root@jun-live:images# 
 kpartx -a /dev/loop10 
 
 root@jun-live:images# 
 ls /dev/mapper/loop10* 
 
/dev/mapper/loop10p1@  /dev/mapper/loop10p2@ 
 
 root@jun-live:images# 
 fdisk -u -l /dev/loop10 
 

 Disk /dev/loop10: 8589 MB, 8589934592 bytes 
 
 255 heads, 63 sectors/track, 1044 cylinders, total 16777216 sectors 
 
 Units = sectors of 1 * 512 = 512 bytes 
 
 Sector size (logical/physical): 512 bytes / 512 bytes 
 
 I/O size (minimum/optimal): 512 bytes / 512 bytes 
 
 Disk identifier: 0x00067969 
 

        Device Boot      Start         End      Blocks   Id  System 
 
 /dev/loop10p1   *        2048      411647      204800   83  Linux 
 
 Partition 1 does not end on cylinder boundary. 
 
 /dev/loop10p2          
 411648    16777215     8182784   8e  Linux LVM 
 
 可以看到,有两个分区(实际上loop10p1为boot分区,loop10p2为LVM) 
 
 3.挂载标准分区 
 
 root@jun-live:images# 
 mount /dev/mapper/loop10p1 /mnt/iso/ 
 
 root@jun-live:images# 
 mount|grep /mnt/iso 
 
 /dev/mapper/loop10p1 on /mnt/iso type 
 xfs (rw) 
 
 root@jun-live:images# 
 ls /mnt/iso/ 
 
 config-3.10.0-123.el7.x86_64 
 
 grub2/ 
 
 initramfs-0-rescue-ddcdf1804942b72473cc9c4ae83b8ef1.img 
 
 initramfs-3.10.0-123.el7.x86_64.img 
 
 initramfs-3.10.0-123.el7.x86_64kdump.img 
 
 initrd-plymouth.img 
 
 symvers-3.10.0-123.el7.x86_64.gz 
 
 System.map-3.10.0-123.el7.x86_64 
 
 vmlinuz-0-rescue-ddcdf1804942b72473cc9c4ae83b8ef1* 
 
 vmlinuz-3.10.0-123.el7.x86_64* 
 
 4.测试写入 
 
 root@jun-live:images# 
 touch /mnt/iso/test.losetup 
 
 root@jun-live:images# 
 ls /mnt/iso/ 
 
 config-3.10.0-123.el7.x86_64 
 
 grub2/ 
 
 initramfs-0-rescue-ddcdf1804942b72473cc9c4ae83b8ef1.img 
 
 initramfs-3.10.0-123.el7.x86_64.img 
 
 initramfs-3.10.0-123.el7.x86_64kdump.img 
 
 initrd-plymouth.img 
 
 symvers-3.10.0-123.el7.x86_64.gz 
 
 System.map-3.10.0-123.el7.x86_64 
 
test.losetup 
 
 vmlinuz-0-rescue-ddcdf1804942b72473cc9c4ae83b8ef1* 
 
 vmlinuz-3.10.0-123.el7.x86_64* 
 
 可以直接读写,再来看看LVM 
 
B.RAW---lvm 
 
1.取消设备映射 
 
 root@jun-live:images# 
 umount /mnt/iso/ 
 
 root@jun-live:images# 
 mount /dev/mapper/loop10p2 /mnt/iso 
 / 
 
 mount: unknown filesystem type 'LVM2_member' 
 
 LVM不能直接挂载,需要作特殊处理。 
 
 root@jun-live:images# 
 kpartx -d /dev/loop10 
 
 root@jun-live:images# 
 losetup -d /dev/loop10 
 

2.通过偏移量重新映射 
 
 fdisk -u 参数指明使用 sector 为单位记录偏移量(offset)而不是 cylinder 为单位 
 
 root@jun-live:images# 
 bc 
 
 bc 1.06.95 
 Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
 This is free software with ABSOLUTELY NO WARRANTY.
 For details type `warranty'.
411648*512
210763776
 quit
 root@jun-live:images#losetup /dev/loop10 rhel7-mode.img -o 
 3.lvm扫描 
 
 确认卷组名是否重复,如果有重复需要重命名 
 
 root@jun-live:images# 
 pvscan 
 
   
 WARNING: Duplicate VG name vg0: Existing 2gwi76-SA4G-sf4y-B3qd-yIbZ-P7e8-J2U7fP (created here) takes precedence over rJGSsG-3ebw-8c2c-88bM-Am6M-Xycu-WNcQsh 
 
PV /dev/loop10   VG vg0   lvm2 [7.80 GiB / 3.11 GiB free]
   PV /dev/sda8     VG vg1   lvm2 [29.80 GiB / 19.80 GiB free]
  PV /dev/sda5     VG vg0   lvm2 [100.00 GiB / 12.44 GiB free]
   Total: 3 [137.61 GiB] / in use: 3 [137.61 GiB] / in no VG: 0 [0   ]可以看到,该RAW设备里的卷组名也叫vg0,和我本地物理机的卷组重名了,这个时候需要根据卷组的UUID来重命名。
root@jun-live:images#vgs -v
     Finding all volume groups
... ...
   WARNING: Duplicate VG name vg0: 2gwi76-SA4G-sf4y-B3qd-yIbZ-P7e8-J2U7fP (created here) takes precedence over rJGSsG-3ebw-8c2c-88bM-Am6M-Xycu-WNcQsh
     Archiving volume group "vg0" metadata (seqno 4).
     Archiving volume group "vg0" metadata (seqno 113).
     Creating volume group backup "/etc/lvm/backup/vg0" (seqno 113).
   VG   Attr   Ext   #PV #LV #SN VSize   VFree  VG UUID                                VProfile
  vg0  wz--n- 4.00m   1   3   0   7.80g  3.11g rJGSsG-3ebw-8c2c-88bM-Am6M-Xycu-WNcQsh        
  vg0  wz--n- 4.00m   1   9   1 100.00g 12.44g 2gwi76-SA4G-sf4y-B3qd-yIbZ-P7e8-J2U7fP        
   vg1  wz--n- 4.00m   1   1   0  29.80g 19.80g suc7X2-gTB4-2QZd-YS76-hUEs-9wcK-zLQPbz        
 root@jun-live:images#vgrename rJGSsG-3ebw-8c2c-88bM-Am6M-Xycu-WNcQsh virt_vg0
   WARNING: Duplicate VG name vg0: Existing 2gwi76-SA4G-sf4y-B3qd-yIbZ-P7e8-J2U7fP (created here) takes precedence over rJGSsG-3ebw-8c2c-88bM-Am6M-Xycu-WNcQsh
 ... ...
  Volume group "vg0" successfully renamed to "virt_vg0"4.激活lvm                                            
 root@jun-live:images#lvscan
  inactive          '/dev/virt_vg0/swap' [512.00 MiB] inherit
   inactive          '/dev/virt_vg0/root' [4.00 GiB] inherit
   inactive          '/dev/virt_vg0/home' [200.00 MiB] inherit
   ACTIVE            '/dev/vg1/ubuntu_root' [10.00 GiB] inherit
   ACTIVE            '/dev/vg0/swap' [1.00 GiB] inherit
   ACTIVE            '/dev/vg0/ct6_home' [500.00 MiB] inherit
   ACTIVE            '/dev/vg0/ct6_root' [8.00 GiB] inherit
   ACTIVE   Original '/dev/vg0/win2008r2' [40.00 GiB] inherit
   ACTIVE   Snapshot '/dev/vg0/win2008r2-snap' [12.00 GiB] inherit
   ACTIVE            '/dev/vg0/virt' [25.00 GiB] inherit
   ACTIVE            '/dev/vg0/test' [500.00 MiB] inherit
   ACTIVE            '/dev/vg0/xfs' [500.00 MiB] inherit
   ACTIVE            '/dev/vg0/xfs_journal' [100.00 MiB] inheritroot@jun-live:images#vgchange -ay virt_vg0
   3 logical volume(s) in volume group "virt_vg0" now activeroot@jun-live:images#lvscan|grep virt_vg0
   ACTIVE            '/dev/virt_vg0/swap' [512.00 MiB] inherit
   ACTIVE            '/dev/virt_vg0/root' [4.00 GiB] inherit
   ACTIVE            '/dev/virt_vg0/home' [200.00 MiB] inherit 
 5.挂载lvm 
 
 root@jun-live:images# 
 mount /dev/virt_vg0/root /mnt/iso/ 
 
 root@jun-live:images# 
 ls /mnt/iso/ 
 
 bin@   dev/  home/  lib64@  mnt/  proc/  run/   srv/  tmp/  var/ 
 
 boot/  etc/  lib@   media/  opt/  root/  sbin@  sys/  usr/ 
 
 root@jun-live:images# 
 mount|grep /mnt/iso 
 
 /dev/mapper/ 
 virt_vg0-root on /mnt/iso type xfs (rw) 
 

6.读写完毕后还原 
 
 root@jun-live:images# 
 umount /mnt/iso 
 
 root@jun-live:images# 
 lvchange -an virt_vg0 
 
 root@jun-live:images# 
 kpartx -d /dev/loop10 
 
 root@jun-live:images# 
 losetup -d /dev/loop10 
 

补充: 
 
 对于还没有分区的RAW虚拟磁盘可以直接以loop格式挂载: 
 
mount -o loop  foo.raw /mnt 
 



qcow2 
 

 [root@test ~]# 
 lsb_release -a 
 
 LSB Version:  
 

      :core-4.1-amd64:core-4.1-noarch 
  
 Distributor ID:    CentOS 
  
 Description:    CentOS Linux release 7.0.1406 (Core) 
  
 Release:    
  7.0.1406 
  
 Codename:    Core 
  
 [root@test ~]# 
  uname -a 
  
 Linux test 
  3.10.0-123.el7.x86_64 #1 SMP Mon Jun 30 12:09:22 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux 
  
 先确认内核是否支持CONFIG_BLK_DEV_NBD模块 
 
 [root@test boot]# grep NBD /boot/config-3.10.0-123.el7.x86_64 
 
 # CONFIG_BLK_DEV_NBD is not set 
 
 发现CentOS 7默认不支持,怎么办,重新编译内核模块 
 
 [root@test boot]# yum -y install gcc gcc-c++ make kernel-devel ncurses-devel elfutils-libelf-devel 
 
 [root@test boot]# cd /usr/src/kernels/3.10.0-123.el7.x86_64/ 
 
 [root@test 3.10.0-123.el7.x86_64]# cp -f /boot/config-3.10.0-123.el7.x86_64 .config 
 
 [root@test 3.10.0-123.el7.x86_64]# vim .config 
 
CONFIG_BLK_DEV_NBD=m
更详细的内核及模块编译方法,请参看: 
 Ubuntu14.04升级内核3.14.25 
 
 Device Drivers  ---> Block devices  ---> Network block device support 
 


  



  



  
 编译安装完成后,再次尝试,映射原理同上面的RAW 
 
 [root@test boot]# 
 grep NBD /boot/config-3.10.0-123.el7.x86_64
CONFIG_BLK_DEV_NBD=m 
 


 或直接编译内核模块 
 
福利:CentOS 其他版本的source rpm这里都可以找到,如kpatch, php等
 以CentOS7.2 
  3.10.0-327为例
 
  mv /usr/src/kernels/$(uname -r) /usr/src/kernels/$(uname -r)-old
 rpm -ivh http://vault.centos.org/7.2.1511/os/Source/SPackages/kernel-3.10.0-327.el7.src.rpm tar -xvf ~/rpmbuild/SOURCES/linux-3.10.0-327.el7.tar.xz
 mv linux-3.10.0-327.el7 /usr/src/kernels/$(uname -r)

 cd /usr/src/kernels/$(uname -r)
 make mrproper
 cp -f /usr/src/kernels/$(uname -r)-old/Module.symvers /usr/src/kernels/$(uname -r)
 cp -f /boot/config-$(uname -r) /usr/src/kernels/$(uname -r)/.config
 make oldconfig
 make prepare
 make scripts
make CONFIG_BLK_DEV_NBD=m M=drivers/block
 cp -f /usr/src/kernels/$(uname -r)/drivers/block/nbd.ko /lib/modules/$(uname -r)/kernel/drivers/block/
depmod -a

 modprobe nbd
 modinfo nbd 
   
 


 
 
qemu-nbd -b 0.0.0.0 -p 10809 -e 10 -t centos7-s1.qcow2
 
 
/usr/libexec/qemu-kvm -enable-kvm -name demo -m 1024 -hda nbd:192.168.130.254:10809  -vnc :19 -snapshot
 
 
 
  
 
qemu-nbd -k /tmp/nbd.sock -e 10 -t centos7-s1.qcow2
 
 
/usr/libexec/qemu-kvm -enable-kvm -name demo -m 1024 -hda nbd:unix:/tmp/nbd.sock -vnc :19 -snapshot