EC2のUbuntuであれば、比較的かんたんにディスク拡張が行なえます。
AWS EBSのルートディスク拡張した際のサイズ拡張のやりかた
RedHat7では同じ手順で行えなかったのでここにやり方を記載しておきます。
$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 7.0 (Maipo)

# 必要なものをインストール
$ yum install cloud-utils-growpart
$ yum install gdisk

$ df -hT
Filesystem     Type      Size  Used Avail Use% Mounted on
/dev/xvda2     xfs       6.0G  1.4G  4.7G  24% /
devtmpfs       devtmpfs  7.8G     0  7.8G   0% /dev
tmpfs          tmpfs     7.7G     0  7.7G   0% /dev/shm
tmpfs          tmpfs     7.7G   17M  7.7G   1% /run
tmpfs          tmpfs     7.7G     0  7.7G   0% /sys/fs/cgroup

# 520GBのはずが６GBしか認識してない
$ fdisk -l /dev/xvda
WARNING: fdisk GPT support is currently new, and therefore in an experimental phase. Use at your own discretion.

Disk /dev/xvda: 558.3 GB, 558345748480 bytes, 1090519040 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: gpt


#         Start          End    Size  Type            Name
 1         2048         4095      1M  BIOS boot parti
 2         4096     12587007      6G  Microsoft basic

# こっちでも確認
$ lsblk
NAME    MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
xvda    202:0    0  520G  0 disk
├─xvda1 202:1    0    1M  0 part
└─xvda2 202:2    0    6G  0 part /

# fdisk 実行
$ fdisk /dev/xvda
WARNING: fdisk GPT support is currently new, and therefore in an experimental phase. Use at your own discretion.
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): p

Disk /dev/xvda: 558.3 GB, 558345748480 bytes, 1090519040 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: gpt


#         Start          End    Size  Type            Name
 1         2048         4095      1M  BIOS boot parti
 2         4096     12587007      6G  Microsoft basic

Command (m for help): d
Partition number (1,2, default 2): 2
Partition 2 is deleted

Command (m for help): n
Partition number (2-128, default 2):
First sector (34-1090519006, default 4096):
Last sector, +sectors or +size{K,M,G,T,P} (4096-1090519006, default 1090519006):
Created partition 2


Command (m for help): p

Disk /dev/xvda: 558.3 GB, 558345748480 bytes, 1090519040 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: gpt


#         Start          End    Size  Type            Name
 1         2048         4095      1M  BIOS boot parti
 2         4096   1090519006    520G  Linux filesyste

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.

# 最後に再起動
$ shutdown -r now

# 再起動後に確認
$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda2      520G  1.4G  519G   1% /
devtmpfs        7.8G     0  7.8G   0% /dev
tmpfs           7.7G     0  7.7G   0% /dev/shm
tmpfs           7.7G   17M  7.7G   1% /run
tmpfs           7.7G     0  7.7G   0% /sys/fs/cgroup