虚拟机分配磁盘40GB，实际系统分区只用了20GB，需要手工扩展到40GB，操作方法如下：
查看磁盘信息（确认主分区只有17GB）：
[root@test-web1 ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/centos-root 17G 3.2G 14G 19% /
devtmpfs 1.9G 0 1.9G 0% /dev
tmpfs 1.9G 0 1.9G 0% /dev/shm
tmpfs 1.9G 8.8M 1.9G 1% /run
tmpfs 1.9G 0 1.9G 0% /sys/fs/cgroup
/dev/vda1 1014M 179M 836M 18% /boot
tmpfs 380M 12K 380M 1% /run/user/42
tmpfs 380M 0 380M 0% /run/user/0
查看磁盘总大小为42.9GB，实际只分配了20GB左右，还有20GB可以扩展：
[root@test-web1 ~]# fdisk -l
Disk /dev/vda: 42.9 GB, 42949672960 bytes, 83886080 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 label type: dos
Disk identifier: 0x000bb34f
Device Boot Start End Blocks Id System
/dev/vda1 * 2048 2099199 1048576 83 Linux
/dev/vda2 2099200 41943039 19921920 8e Linux LVM
Disk /dev/mapper/centos-root: 18.2 GB, 18249416704 bytes, 35643392 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 /dev/mapper/centos-swap: 2147 MB, 2147483648 bytes, 4194304 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
新增磁盘主分区，依次输入n p 回车 回车 回车 w：
[root@test-web1 ~]# fdisk /dev/vda
Welcome to fdisk (util-linux 2.23.2).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.
Command (m for help): n
Partition type:
p primary (2 primary, 0 extended, 2 free)
e extended
Select (default p): p
Partition number (3,4, default 3): 回车
First sector (41943040-83886079, default 41943040): 回车
Using default value 41943040
Last sector, +sectors or +size{K,M,G} (41943040-83886079, default 83886079): 回车
Using default value 83886079
Partition 3 of type Linux and of size 20 GiB is set
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
WARNING: Re-reading the partition table failed with error 16: Device or resource busy.
The kernel still uses the old table. The new table will be used at
the next reboot or after you run partprobe(8) or kpartx(8)
Syncing disks.
查看磁盘信息，增加了一个/dev/vda3：
[root@test-web1 ~]# fdisk -l
Disk /dev/vda: 42.9 GB, 42949672960 bytes, 83886080 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 label type: dos
Disk identifier: 0x000bb34f
Device Boot Start End Blocks Id System
/dev/vda1 * 2048 2099199 1048576 83 Linux
/dev/vda2 2099200 41943039 19921920 8e Linux LVM
/dev/vda3 41943040 83886079 20971520 83 Linux
Disk /dev/mapper/centos-root: 18.2 GB, 18249416704 bytes, 35643392 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 /dev/mapper/centos-swap: 2147 MB, 2147483648 bytes, 4194304 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
挂载分区：
[root@test-web1 ~]# partprobe
Warning: Unable to open /dev/sr0 read-write (Read-only file system). /dev/sr0 has been opened read-only.
[root@test-web1 ~]# pvcreate /dev/vda3
Physical volume "/dev/vda3" successfully created.
[root@test-web1 ~]# vgscan
Reading volume groups from cache.
Found volume group "centos" using metadata type lvm2
[root@test-web1 ~]# vgextend centos /dev/vda3
Volume group "centos" successfully extended
[root@test-web1 ~]# lvextend -L +19G /dev/mapper/centos-root
Size of logical volume centos/root changed from <17.00 GiB (4351 extents) to <36.00 GiB (9215 extents).
Logical volume centos/root successfully resized.
[root@test-web1 ~]# xfs_growfs /dev/mapper/centos-root
meta-data=/dev/mapper/centos-root isize=512 agcount=4, agsize=1113856 blks
= sectsz=512 attr=2, projid32bit=1
= crc=1 finobt=0 spinodes=0
data = bsize=4096 blocks=4455424, imaxpct=25
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0 ftype=1
log =internal bsize=4096 blocks=2560, version=2
= sectsz=512 sunit=0 blks, lazy-count=1
realtime =none extsz=4096 blocks=0, rtextents=0
data blocks changed from 4455424 to 9436160
确认分区大小是否变了：
[root@test-web1 ~]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/centos-root 36G 3.2G 33G 9% /
devtmpfs 1.9G 0 1.9G 0% /dev
tmpfs 1.9G 0 1.9G 0% /dev/shm
tmpfs 1.9G 8.8M 1.9G 1% /run
tmpfs 1.9G 0 1.9G 0% /sys/fs/cgroup
/dev/vda1 1014M 179M 836M 18% /boot
tmpfs 380M 12K 380M 1% /run/user/42
tmpfs 380M 0 380M 0% /run/user/0