EC2 Root partition does not get resized to the available space

Issue

• When creating an instance in Amazon Web Services from an AMI, the user has the ability to select a volume size for the root device. The expected behavior is that the root filesystem of the instance will match the size of the EBS volume created on launch.

• Instances created using HVM AMIs for RHEL from Cloud Access do not resize the filesystem to match the device.

• If we select a HVM AMI for RHEL 6.5 (RHEL-6.5_HVM_GA-x86_64-4-HVM-Access2 ami-3225be02) and launch a new instance with a 10GB EBS volume, the resulting instance will have a 6GB root filesystem and a 10GB device. This is not the expected or desired outcome. 6GB is the default size of the device for a new instance based on the registered block device in the AMI, /dev/sda1=snap-80726aba:6:true:standard.

[root@host ~]# df -Th
Filesystem     Type   Size  Used Avail Use% Mounted on
/dev/xvda1     ext4   5.7G  1.9G  3.7G  35% /
none           tmpfs  1.9G     0  1.9G   0% /dev/shm
[root@ip-10-233-146-5 ~]# parted -l
Model: Xen Virtual Block Device (xvd)
Disk /dev/xvda: 10.7GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number  Start  End     Size    Type     File system  Flags
 1      983kB  6145MB  6144MB  primary  ext4

• If we select a PVM AMI for RHEL 6.5 (RHEL-6.5_GA-x86_64-4-Access2 ami-28f89018) and launch a new instance with a 10GB EBS volume, the resulting instance will have a 10GB root filesystem and device as expected.

[root@host ~]# df -Th
Filesystem     Type   Size  Used Avail Use% Mounted on
/dev/xvda1     ext4   9.9G  2.0G  7.9G  20% /
none           tmpfs  1.9G     0  1.9G   0% /dev/shm
[root@ip-10-236-141-4 ~]# parted -l
Model: Unknown (unknown)
Disk /dev/xvda1: 10.7GB
Sector size (logical/physical): 512B/512B
Partition Table: loop

Number  Start  End     Size    File system  Flags
 1      0.00B  10.7GB  10.7GB  ext4

• An Amazon Linux HVM AMI (amzn-ami-hvm-2014.03.1.x86_64-ebsami-383a5008) works as expected with the root filesystem matching the device size.

[root@host ~]# df -Th
Filesystem     Type      Size  Used Avail Use% Mounted on
/dev/xvda1     ext4      9.8G  1.1G  8.6G  11% /
devtmpfs       devtmpfs  1.9G   56K  1.9G   1% /dev
tmpfs          tmpfs     1.9G     0  1.9G   0% /dev/shm
[root@ip-10-233-151-2 ~]# parted -l
Model: Xen Virtual Block Device (xvd)
Disk /dev/xvda: 10.7GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt

Number  Start   End     Size    File system  Name                 Flags
128     1049kB  2097kB  1049kB               BIOS Boot Partition  bios_grub
 1      2097kB  10.7GB  10.7GB  ext4         Linux

• We also tested with the public HVM AMIs for RHEL 6.5 and 7 and saw the same behaviors. This points to an AMI creation issue.