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I'm actually creating a customized script based off of Alestic.com's alestic-git project to create my own custom ami image for Ubuntu Precise 12.04 for an EBS backed AMI instance.

The interesting thing I'm encountering happens after registering the AMI and starting off an instance from this new AMI file. Everything starts out fine but somehow the ephemeral device (/dev/sdb) is not showing or being detected at all. EC2 metadata shows the ephemeral0 as available in /dev/sdb, but the instance itself does not detect or have /dev/xvdb device exist.

I've tried myriad ways to figure out why, but still scratching my head.

I forgot to mention that I'm trying to create an 64-bit image and tested it on m1.medium and micro instances.

Anyone happen to know what's going on?

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I've realized what the issue is. On micro instances, there are no instance storage (duh!). I've been testing the images using micros, and just now I've been launching mediums and they appear without problems. Consider this resolved. –  Chris May 11 '12 at 22:13
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2 Answers 2

As you already realized, Amazon EC2 instance type t1.micro does not come with ephemeral storage at all, whereas the m1.small and c1.medium instance types have it readily available and in use for /mnt and /swap.

Please be aware of a related caveat though: While all larger instance types come with ephemeral storage in principle, it is not necessarily attached/formatted/mounted for most Linux and UNIX instance types by default (it is for Windows instances though), rather this is an exception for the m1.small and c1.medium instance types only (the logic behind this escapes me, guess it simply must be accepted as a historical fact for the time being).

Consequently, you need to attach/mount/format the ephemeral storage devices yourself on most larger instance types in case you want to facilitate this plentiful and free storage (which should only be used for strictly temporary data you can afford to lose or rebuild easily of course).

The documentation for Amazon EC2 Instance Storage provides more details, e.g. lists of Instance Stores Available On Instance Types and Instance Store Device Names.

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I believe you have to do the ephemeral device block association during the creation of the instance, not during the AMI-creation process. That's done using the -b parameter for the ec2-run-instances command.

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That makes sense, but how come the original images from Ubuntu's official AMIs come with ephemerals attached directly and can be launched from AWS Console without any issue? These are the same images they use to publish the AMIs. –  Chris May 11 '12 at 20:15
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