[ubuntu-cloud] Ephemeral Mounts?

Eric Hammond ehammond at thinksome.com
Thu Nov 17 02:09:53 UTC 2011

On 11/16/2011 04:06 PM, Thomas Oatess wrote:
> Can someone please explain an ephemeral mount to me?  How is it used?

Short answer for folks new to EC2:

  Run EBS boot instances and avoid using ephemeral storage like /mnt

This advice will help you avoid a number of common and serious problems
that can occur when using instance-store AMIs and ephemeral storage.

Longer answer:

Once you're using EC2 comfortably and want to start exploring more
advanced topics, you can learn about ephemeral storage and think about
if there might be use cases where you could gain a slight advantage in
cost by using it.

Ephemeral storage (including the root disk of instance-store instances)
is free but the contents are completely lost forever whenever an
instance is stopped, terminated, or fails.  There is no way to recover.

EBS storage (including the root disk of EBS boot instances) has a minor
cost for the storage and for IO transactions, but is persistent through
an instance being stopped or failing.

By default, the root disk of an EBS boot instance is deleted when the
instance is terminated, but you can modify the instance attributes to
change this.  Here's an article that talks about how to do it:

  Protect EC2 Instances from Accidental Termination and Loss of Data

There are a number of other important reasons to use EBS boot instances
instead of instance-store, but when you're just getting started, it's
simplest and safest just to stick with EBS boot.

Even as an expert, I always run EBS boot.  I sometimes use /mnt as a big
/tmp but that's about it.  Everything else goes on EBS.

Eric Hammond

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