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Preparing an AMI through a Loopback File

From Amazon Elastic Compute Cloud: 2009-10-31

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Preparing an AMI through a Loopback File

This method involves doing a full operating system installation on a clean root file system, but avoids having to create a new root disk partition and file system on a physical disk. Once you have installed your operating system, the resulting image can be bundled as an AMI with the ec2-bundle-image utility. For more information on the AMI tools, go to the Amazon Elastic Compute Cloud Command Line Reference.

[Note] Note

This method only works with AMIs that use instance stores for their root devices. This method is not applicable for AMIs backed by Amazon EBS.

Before selecting an AMI, determine whether the instance types you plan to launch are 32-bit or 64-bit. For more information, see Instance Types

Make sure you are using GNU Tar 1.15 or later.

These examples use Fedora Core 4. Please make any adjustments for your distribution.

AMI Creation Process

1 How to Create a File to Host the AMI.
2 How to Create a Root File System inside the File.
3 How to Mount the File through Loopback.
4 How to Prepare for the Installation.
5 How to Install the Operating System.
6 How to Configure the Operating System.

How to Create a File to Host the AMI

The dd utility can create files of arbitrary sizes. Make sure to create a file large enough to host the operating system, tools, and applications that you will install. For example, a baseline Linux and UNIX installation requires about 700MB, so your file should be at least 1 GB.

To create a file to host the AMI

  • Enter the following command:

                  # 
              dd if=/dev/zero of=image_name bs=1M count=size

    The <image_name> is the name of the image file you are creating and <size> is the size of the file in megabytes.

Example

The following command creates a one gigabyte file (1024*1MB).

# dd if=/dev/zero of=my-image.fs bs=1M count=1024
1024+0 records in
1024+0 records out

How to Create a Root File System inside the File

There are several variations on the mkfs utility that can create a file system inside the image file you are creating. Typical Linux and UNIX installations default to ext2 or ext3 file systems.

To create an ext3 file system

  • Enter the following command:

                  # 
              
                mke2fs -F -j 
                <image_name>

    The <image_name> is the name of the image file.

Example

The following command creates an ext3 file system.

# mke2fs -F -j my-image.fs
mke2fs 1.38 (30-Jun-2005)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
131072 inodes, 262144 blocks
13107 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=268435456
8 block groups
32768 blocks per group, 32768 fragments per group
16384 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376

Writing inode tables: done
Creating journal (8192 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 24 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

How to Mount the File through Loopback

The loopback module allows you to use a normal file as if it were a raw device, which gives you a file system within a file. Mounting a file system image file through loopback presents it as part of the normal file system. You can then modify it using your favorite file management tools and utilities.

To mount the file through loopback

  1. Create a mount point in the file system where the image will be attached:

                  # 
              
                mkdir 
                <image_mountpoint>

    The <image_mountpoint> is the location where the image will be mounted.

  2. Mount the file system image:

                  # 
              mount -o loop  <image_name> <image_mountpoint>

    The <image_name> is the name of the image file and <image_mountpoint> is the mount location.

Example

The following commands create and mount the my-image.fs image file.

# mkdir /mnt/ec2-fs
# mount -o loop my-image.fs /mnt/ec2-fs

How to Prepare for the Installation

Before the operating system installation can proceed, you must create and prepare the newly created root file system.

To prepare for the installation

  1. Create a /dev directory and populate it with a minimal set of devices. You can ignore the errors in the output.

    # mkdir /mnt/ec2-fs/dev
# /sbin/MAKEDEV -d <image_mountpoint>/dev -x console
# /sbin/MAKEDEV -d <image_mountpoint>/dev -x null
# /sbin/MAKEDEV -d <image_mountpoint>/dev -x zero

    The <image_mountpoint> is the mount location.

  2. Create the fstab file within the /etc directory and add the following:

    /dev/sda1  /         ext3    defaults        1 1
none       /dev/pts  devpts  gid=5,mode=620  0 0
none       /dev/shm  tmpfs   defaults        0 0
none       /proc     proc    defaults        0 0
none       /sys      sysfs   defaults        0 0

  3. Create a temporary yum configuration file (e.g., yum-xen.conf) and add the following content.

    [main]
cachedir=/var/cache/yum
debuglevel=2
logfile=/var/log/yum.log
exclude=*-debuginfo
gpgcheck=0
obsoletes=1
reposdir=/dev/null

[base]
name=Fedora Core 4 - $basearch - Base
mirrorlist=http://fedora.redhat.com/download/mirrors/fedora-core-$releasever
enabled=1

[updates-released]
name=Fedora Core 4 - $basearch - Released Updates
mirrorlist=http://fedora.redhat.com/download/mirrors/updates-released-fc$releasever
enabled=1

    This ensures all the required basic packages and utilities are installed. This file can be located anywhere on your main file system (not on your loopback file system) and is only used during installation.

  4. Enter the following:

    # mkdir <image_mountpoint>/proc
# mount -t proc none <image_mountpoint>/proc

    The <image_mountpoint> is the mount location. A groupadd utility bug in the shadow-utils package (versions prior to 4.0.7-7) requires you to mount the new proc file system manually with the preceding command.

Example

These commands create the /dev directory and populate it with a minimal set of devices:

# mkdir /mnt/ec2-fs/dev
# /sbin/MAKEDEV -d /mnt/ec2-fs/dev -x console
MAKEDEV: mkdir: File exists
MAKEDEV: mkdir: File exists
MAKEDEV: mkdir: File exists
# /sbin/MAKEDEV -d /mnt/ec2-fs/dev -x null
MAKEDEV: mkdir: File exists
MAKEDEV: mkdir: File exists
MAKEDEV: mkdir: File exists
# /sbin/MAKEDEV -d /mnt/ec2-fs/dev -x zero
MAKEDEV: mkdir: File exists
MAKEDEV: mkdir: File exists
MAKEDEV: mkdir: File exists

This example creates and mounts the /mnt/ec2-fs/proc directory. 

# mkdir /mnt/ec2-fs/proc
# mount -t proc none /mnt/ec2-fs/proc

How to Install the Operating System

At this stage, the basic directories and files are created and you are ready to install the operating system. Depending on the speed of the host and network link to the repository, this process might take a while.

To install the operating system

  • Enter the following command:

                  # 
              yum -c <yum_configuration_file> --installroot=<image_mountpoint> -y groupinstall Base

    The <yum_configuration_file> is the name of the yum configuration file and <image_mountpoint> is the mount location.

    You now have a base installation, which you can configure for operation inside Amazon EC2 and customize for your use.

Example

This example installs the operating system at the /mnt/ec2-fs mount point using the yum-xen.conf yum configuration file.

# yum -c yum-xen.conf --installroot=/mnt/ec2-fs -y groupinstall Base
Setting up Group Process
Setting up repositories
base                      100% |=========================| 1.1 kB    00:00
updates-released          100% |=========================| 1.1 kB    00:00
comps.xml                 100% |=========================| 693 kB    00:00
comps.xml                 100% |=========================| 693 kB    00:00
Setting up repositories
Reading repository metadata in from local files
primary.xml.gz            100% |=========================| 824 kB    00:00
base      : ################################################## 2772/2772
Added 2772 new packages, deleted 0 old in 15.32 seconds
primary.xml.gz            100% |=========================| 824 kB    00:00
updates-re: ################################################## 2772/2772
Added 2772 new packages, deleted 0 old in 10.74 seconds
...
Complete!

How to Configure the Operating System

After successfully installing the base operating system, you must configure the networking and hard drives to work in the Amazon EC2 environment.

To configure the operating system

  1. Edit (or create) /mnt/ec2-fs/etc/sysconfig/network-scripts/ifcfg-eth0 and make sure it contains at least the following information:

    DEVICE=eth0
BOOTPROTO=dhcp
ONBOOT=yes
TYPE=Ethernet
USERCTL=yes
PEERDNS=yes
IPV6INIT=no
    [Note] Note

    The Amazon EC2 DHCP server ignores hostname requests. If you set DHCP_HOSTNAME, the local hostname will be set on the instance but not externally. Additionally, the local hostname will be the same for all instances of the AMI, which might be confusing.

  2. Ensure that networking starts by making sure the following line appears in the /mnt/ec2-fs/etc/sysconfig/network file: 

    NETWORKING=yes

  3. Ensure that local disk storage on /dev/sda2 and swap space on /dev/sda3 are mounted at system startup by adding the following lines to /mnt/ec2-fs/etc/fstab:

    /dev/sda2  /mnt      ext3    defaults        0 0
/dev/sda3  swap      swap    defaults        0 0
    [Note] Note

    The /dev/sda2 and /dev/sda3 storage locations only apply to small instances. For more information on instance storage, see Instance Storage .

  4. Make sure all of your required services start at system startup by allocating them appropriate system run levels. For example, to enable the service my-service on multi-user and networked run levels, enter the following commands: 

    # chroot /mnt/ec2-fs /bin/sh
# chkconfig --level 345 my-service on
# exit

    Your new installation is successfully installed and configured to operate in the Amazon EC2 environment.

  5. Umount the image by entering the following commands: 

    # umount <image_mountpoint>/proc
# umount -d <image_mountpoint>

    The <image_mountpoint> is the mount location.

Example

The following example unmounts the installation from the /mnt/ec2-fs mount point.

# umount /mnt/ec2-fs/proc
# umount -d /mnt/ec2-fs

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