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当我们需要扩展模板镜像的虚拟磁盘大小时，比如原来的虚拟磁盘大小为20G，现在我们想将其扩展到30G，那么我们可以根据如下步骤来操作。

整个流程可以分为三个阶段：

1、扩展KVM镜像磁盘文件大小到30G。

2、扩展磁盘分区大小为30G。

3、扩展文件系统大小为30G。

假若当前有一个名为 test_extend.img 的模板镜像，其格式为 qcow2， virtual_size为 20G

1、首先可以使用qemu-img来查看该模板镜像的元信息：

root@cason:~/image# qemu-img info test_extend.img 
image: test_extend.img
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 309M
cluster_size: 65536
Format specific information:compat: 1.1lazy refcounts: false
root@cason:~/image# 

2、由于需要使用到qemu-nbd，因此我们需要先确认nbd模板已经被load：

root@cason:~/image# lsmod | grep nbd
root@cason:~/image#

     执行如上命令发现没有任何输出，则表示当前系统并没有加载nbd模块

3、加载nbd模板，并再次确认（若没有nbd模块，则可以参考这里进行编译安装）：

root@cason:~/image# modprobe nbd max_part=8
root@cason:~/image# lsmod | grep nbd
nbd                    17603  0 
root@cason:~/image# 

     此时nbd模板已经正常加载了

4、通过qemu-img命令来扩展虚拟磁盘的virtual_size为 30G：

root@cason:~/image# qemu-img resize test_extend.img 32212254720
Image resized.
root@cason:~/image# qemu-img info test_extend.img 
image: test_extend.img
file format: qcow2
virtual size: 30G (32212254720 bytes)
disk size: 309M
cluster_size: 65536
Format specific information:compat: 1.1lazy refcounts: false
root@cason:~/image# 

5、下面通过qemu-nbd挂载test_extend.img到/dev/nbd0设备上：

root@cason:~/image# qemu-nbd -c /dev/nbd0 ./test_extend.img 
root@cason:~/image# ll /dev/nbd* | grep nbd0
brw-rw---- 1 root disk 43,   0 11月 20 18:09 /dev/nbd0
brw-rw---- 1 root disk 43,   1 11月 20 18:09 /dev/nbd0p1
root@cason:~/image# 

     挂载成功后，在/dev/下会看到如上信息，其中/dev/nbd0p1表示该虚拟磁盘仅有一个分区

6、下面开始通过fdisk来扩展虚拟磁盘分区大小：

root@cason:~/image# fdisk /dev/nbd0Command (m for help): pDisk /dev/nbd0: 32.2 GB, 32212254720 bytes
255 heads, 63 sectors/track, 3916 cylinders, total 62914560 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: 0x000e8e8dDevice Boot      Start         End      Blocks   Id  System
/dev/nbd0p1   *        2048    41943039    20970496   83  LinuxCommand (m for help): d
Selected partition 1Command (m for help): pDisk /dev/nbd0: 32.2 GB, 32212254720 bytes
255 heads, 63 sectors/track, 3916 cylinders, total 62914560 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: 0x000e8e8dDevice Boot      Start         End      Blocks   Id  SystemCommand (m for help): n
Partition type:p   primary (0 primary, 0 extended, 4 free)e   extended
Select (default p): p
Partition number (1-4, default 1): 
Using default value 1
First sector (2048-62914559, default 2048): 
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-62914559, default 62914559): 
Using default value 62914559Command (m for help): pDisk /dev/nbd0: 32.2 GB, 32212254720 bytes
255 heads, 63 sectors/track, 3916 cylinders, total 62914560 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: 0x000e8e8dDevice Boot      Start         End      Blocks   Id  System
/dev/nbd0p1            2048    62914559    31456256   83  LinuxCommand (m for help): a
Partition number (1-4): 1Command (m for help): pDisk /dev/nbd0: 32.2 GB, 32212254720 bytes
255 heads, 63 sectors/track, 3916 cylinders, total 62914560 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: 0x000e8e8dDevice Boot      Start         End      Blocks   Id  System
/dev/nbd0p1   *        2048    62914559    31456256   83  LinuxCommand (m for help): w
The partition table has been altered!Calling ioctl() to re-read partition table.
Syncing disks.
root@cason:~/image# 

     如此以来，我们已经将该虚拟磁盘的分区大小扩展到了30G

7、至此，我们还需要将文件系统扩展到30G：

root@cason:~/image# e2fsck -fp /dev/nbd0p1 
/dev/nbd0p1: Deleted inode 131076 has zero dtime.  FIXED.
/dev/nbd0p1: 18489/1310720 files (0.2% non-contiguous), 281286/5242624 blocksroot@cason:~/image# resize2fs /dev/nbd0p1 
resize2fs 1.42.9 (4-Feb-2014)
Resizing the filesystem on /dev/nbd0p1 to 7864064 (4k) blocks.
The filesystem on /dev/nbd0p1 is now 7864064 blocks long.root@cason:~/image# qemu-nbd -d /dev/nbd0
/dev/nbd0 disconnected

至此，我们就完成了扩展虚拟磁盘大小的操作。