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Amazon EC2: Creating EBS-backed Instances with Ephemeral Storage and Automatically Deleting the EBS Storage upon Termination

March 12, 2012

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I use Amazon EC2 extensively. One of the things I noticed over the past couple of years is a move from instance-store to EBS-backed instances. I’ve read the literature on EBS-backed instances and, quite honestly, I don’t care about the benefits. If an instance dies, I can re-start is and have it up and running in no time, since virtually everything is scripted. That said, I’m not going to fight the trend.

One thing I do miss, though, is that instance-store instances have a large chunk of ephemeral storage available for use for free. With EBS-backed instances, the ephemeral storage is usually not available. It is, though, if you go through the motion of creating your own AMI or find one configured as outlined here.

To take the easiest route, launch an EBS-backed instance of the AMI you’d like to use with ephemeral storage. Make whatever changes you wish to it once you have it running. You might want to add this to the /etc/fstab, adjusting the device name and filesystem as required for your version of Linux:

/dev/xvda2 /mnt ext4 defaults 1 2

Now, stop the instance and take a snapshot of it. The snapshot will be our new AMI when done, so it will persist as long as you want to keep the AMI around.

Then execute the following command:

ec2-register -n AMI_Name -d AMI_Description -a PLATFORM —kernel KERNEL —ramdisk RAMDISK —root-device-name /dev/sda1 -b /dev/sda1=SNAPSHOT_NAME:10:true -b /dev/sda2=ephemeral0

Each of the variables above are defined here:

  • AMI_Name: A friendly name you assign to the AMI
  • AMI_Description: A longer description you assign to your AMI
  • PLATFORM: The platform, either "i386" or "x86_64"
  • KERNEL: The kernel ID of the kernel to use, which can be found using ec2-describe-images or observing the kernel used while the original instance is running
  • RAMDISK: The ramdisk ID to use, which should also match that specified in ec2-describe-images or observing the one used while the original instance is running (this is often not specified)
  • SNAPSHOT_NAME: The name of the snapshot you created above. Note the '10' following indicates the size of the EBS volume to create for the root filesystem and 'true' means that the volume should be deleted when the instance is terminated (you may prefer to set this to false)

One of the other things I really do not like about EBS-based instances is that when you terminate them, the EBS storage is left behind and you have to clean that up separately. Using “true” as a part of the -b parameter means that the EBS storage will be deleted automatically when the instance is terminated.

Note that the ec2-register command will return the name of your new AMI.

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Paul E. Jones

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