fix the issue that disk space is not the size that aws ec2 have.

在申请aws ec2时，按照向导，在选择存储的时候默认硬盘大小是 8 G，这时候可以根据自己的需要输入一个合适的数字，例如100。完成向导并启动ec2 instance 后登陆机器。使用命令：

df -hT

发现硬盘的大小不是自己的设定的值，而还是 8 G，使用fdisk、mkfs来分区和格式化后，还是无法增大其空间。反复折腾多次，包括重启机器，问题依旧，后来发现其实很简单，只需要使用一条命令resize2fs就可以搞定。

resize2fs /dev/xvde

注意：“/dev/xvde” 根据自己的实际情况可能会不一样。使用fdisk或df命令都可以获知具体的设备号。 如果执行上述命令收到 The filesystem is already 2096896 blocks long. Nothing to do! 的错误，那么需要先做如下操作：

<<1>> Look at the filesystem, it is 6G
<<2>> Look at the disk and the partition, the disk is 21.5 GB but the partition is 6 GB (6291456 blocks)
<<3>> Start fdisk for that disk (xvda, so not the partition xvda1)
<<4>> Switch to sector display.
<<5>> Print the partition(s), and remember the start sector (2048 in the example).
<<6>> Delete the partition.
<<7>> Create a new partition.
<<8>> Make it primary.
<<9>> First partition.
<<10>> Enter the old start sector, do NOT make any typo here!!! (2048 in the example) 
<<11>> Hit enter to accept the default (this is the remainder of the disk)
<<12>> Print the changes and make sure the start sector is ok, if not restart at <<6>>
<<13>> Make the partition bootable. do NOT forget this!!!
<<14>> Enter your partition number (1 in the example)
<<15>> Write the partition info back, this will end the fdisk session.
<<16>> Reboot the server, and wait for it to come up (this may take longer than usual).
<<17>> Verify the filesystem size.
<<18>> If the filesystem is not around 20Gb as expected, you can use this command.

# df -h  <<1>>

Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1      6.0G  2.0G  3.7G  35% / 
tmpfs            15G     0   15G   0% /dev/shm

# fdisk -l  <<2>>

Disk /dev/xvda: 21.5 GB, 21474836480 bytes
97 heads, 17 sectors/track, 25435 cylinders
Units = cylinders of 1649 * 512 = 844288 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0003b587

    Device Boot      Start         End      Blocks   Id  System
/dev/xvda1   *           2        7632     6291456   83  Linux

# fdisk /dev/xvda  <<3>>

WARNING: DOS-compatible mode is deprecated. It's strongly recommended to
         switch off the mode (command 'c') and change display units to
         sectors (command 'u').

Command (m for help): u  <<4>>
Changing display/entry units to sectors

Command (m for help): p  <<5>>

Disk /dev/xvda: 21.5 GB, 21474836480 bytes
97 heads, 17 sectors/track, 25435 cylinders, total 41943040 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: 0x0003b587

    Device Boot      Start         End      Blocks   Id  System
/dev/xvda1   *        2048    12584959     6291456   83  Linux

Command (m for help): d  <<6>>
Selected partition 1

Command (m for help): n  <<7>>
Command action
   e   extended
   p   primary partition (1-4)
p  <<8>>
Partition number (1-4): 1  <<9>>
First sector (17-41943039, default 17): 2048  <<10>>
Last sector, +sectors or +size{K,M,G} (2048-41943039, default 41943039): <<11>>
Using default value 41943039

Command (m for help): p <<12>>

Disk /dev/xvda: 21.5 GB, 21474836480 bytes
97 heads, 17 sectors/track, 25435 cylinders, total 41943040 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: 0x0003b587

    Device Boot      Start         End      Blocks   Id  System
/dev/xvda1            2048    41943039    20970496   83  Linux

Command (m for help): a  <<13>>
Partition number (1-4): 1  <<14>>


Command (m for help): w  <<15>>
The partition table has been altered!

Calling ioctl() to re-read partition table.

WARNING: Re-reading the partition table failed with error 16: ...
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.

# reboot  <<16>>



# df -h  <<17>>
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       20G  2.0G   17G  11% / 
tmpfs            15G     0   15G   0% /dev/shm

# resize2fs /dev/xvda1  <<18>>
resize2fs 1.41.12 (17-May-2010)
Filesystem at /dev/xvda1 is mounted on /; on-line resizing required
old desc_blocks = 1, new_desc_blocks = 2
Performing an on-line resize of /dev/xvda1 to 5242624 (4k) blocks.
The filesystem on /dev/xvda1 is now 5242624 blocks long.

root@vs120 [~]#  df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       20G  7.8G   11G  42% /
tmpfs           498M     0  498M   0% /dev/shm
/usr/tmpDSK     399M   11M  368M   3% /tmp