思路
修改 qcow2 镜像可以分为两类：扩大和缩小。
扩大的操作思路是：先扩大镜像（磁盘），后修改分区表扩大分区信息。整个过程如下图所示，第一步先把镜像整个容量扩大，第二步把 sda2 扩大，把余下的空间填满。
缩小的操作思路和扩大相反：先修改分区表缩小分区信息，后缩小镜像（硬盘）。
操作过程
扩大
现在把一个 120G 的镜像扩大到 200G，其中多出来的 80G 全部划到 sda3 中。
首先查看镜像的大小和分区表：
$ qemu-img info image1.qcow2
image: image1.qcow2
file format: qcow2
virtual size: 120G (128849018880 bytes)
disk size: 2.0G
cluster_size: 65536
$ virt-df image1.qcow2
Filesystem 1K-blocks Used Available Use%
image1.qcow2:/dev/sda1 194241 25578 158423 14%
image1.qcow2:/dev/sda3 119391468 1824776 111495244 2%
第一步：创建空镜像
- 先创建出一个 200G 的空镜像：
$ qemu-img create -f qcow2 image2.qcow2 200G
Formatting 'image2.qcow2', fmt=qcow2 size=214748364800 encryption=off cluster_size=65536
第二步：virt-resize
- 将老镜像扩大到 200G，并把数据复制到新镜像中：
$ virt-resize --expand /dev/sda3 image1.qcow2 image2.qcow2
Examining image1.qcow2 ...
**********
Summary of changes:
/dev/sda1: This partition will be left alone.
/dev/sda2: This partition will be left alone.
/dev/sda3: This partition will be resized from 115.8G to 195.8G. The
filesystem ext4 on /dev/sda3 will be expanded using the 'resize2fs'
method.
**********
Setting up initial partition table on image2.qcow2 ...
Copying /dev/sda1 ...
Copying /dev/sda2 ...
100%
Copying /dev/sda3 ...
100%
Expanding /dev/sda3 using the 'resize2fs' method ...
Resize operation completed with no errors. Before deleting the old
disk, carefully check that the resized disk boots and works correctly.
如果镜像很大用时会很长，要有心里准备。。
完成后效果：
$ qemu-img info image2.qcow2
image: image2.qcow2
file format: qcow2
virtual size: 200G (214748364800 bytes)
disk size: 121G
cluster_size: 65536
$ virt-df image2.qcow2
Filesystem 1K-blocks Used Available Use%
image2.qcow2:/dev/sda1 194241 25578 158423 14%
image2.qcow2:/dev/sda3 201960128 1824476 189869968 1%
缩小
现在把一个 120G 的镜像缩小到 100G。
基本信息：
$ virt-df Centos65v10.qcow2
Filesystem 1K-blocks Used Available Use%
Centos65v10.qcow2:/dev/sda1 194241 29758 154243 16%
Centos65v10.qcow2:/dev/sda3 119391468 2103976 111216044 2%
$ qemu-img info Centos65v10.qcow2
image: Centos65v10.qcow2
file format: qcow2
virtual size: 120G (128849018880 bytes)
disk size: 2.2G
cluster_size: 65536
第一步：缩小分区表
注意：这步会修改镜像内容，请事先备份镜像。
guestfish -a Centos65v10.qcow2
><fs> launch
><fs> list-partitions
/dev/sda1
/dev/sda2
/dev/sda3
><fs> resize2fs-size /dev/sda3 100G
><fs> sync
><fs> exit
此时分区表已经被修改：
$ virt-df Centos65v10.qcow2
Filesystem 1K-blocks Used Available Use%
Centos65v10.qcow2:/dev/sda1 194241 29758 154243 16%
Centos65v10.qcow2:/dev/sda3 103081248 2104036 95734336 3%
第二步：缩小镜像
为了安全起见多留了一个空间，所以镜像总大小是 105G。
$ qemu-img create -f qcow2 Centos65_small.qcow2 105G
$ virt-resize --shrink /dev/sda3 Centos65v10.qcow2 Centos65_small.qcow2
Examining Centos65v10.qcow2 ...
**********
Summary of changes:
/dev/sda1: This partition will be left alone.
/dev/sda2: This partition will be left alone.
/dev/sda3: This partition will be resized from 115.8G to 98.8G. The
filesystem ext4 on /dev/sda3 will be expanded using the 'resize2fs'
method.
**********
Setting up initial partition table on Centos65_small.qcow2 ...
Copying /dev/sda1 ...
Copying /dev/sda2 ...
……
继续是等待……
参考文档
- https://rwmj.wordpress.com/2010/09/27/virt-resize-shrink-now-works/
- http://libguestfs.org/virt-resize.1.html
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