Amazon EC2 EBS Instances and Ephemeral Storage

Here’s a couple tidbits I’ve gleaned that are useful.

When  you start an “instance-store” Amazon EC2 instance, you get a certain amount of ephemeral storage allocated and mounted automatically.  The amount of space varies by instance size and is defined here.  The storage location and format also varies by instance size and is defined here.

The upshot is that if you start an “instance-store” small Linux EC2 instance, it automagically has a free 150 GB /mnt disk and a 1 GB swap partition up and runnin’ for ya.  (mount points vary by image, but that’s where they are in the Amazon Fedora starter.)

[root@domU-12-31-39-00-B2-01 ~]# df -k

Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1             10321208   1636668   8160252  17% /
/dev/sda2            153899044    192072 145889348   1% /mnt
none                    873828         0    873828   0% /dev/shm

[root@domU-12-31-39-00-B2-01 ~]# free
total       used       free     shared    buffers     cached
Mem:       1747660      84560    1663100          0       4552      37356
-/+ buffers/cache:      42652    1705008
Swap:       917496          0     917496

But, you say, I am not old or insane!  I use EBS-backed images, just as God intended.  Well, that’s a good point.  But when you pull up an EBS image, these ephemeral disk areas are not available to you.  The good news is, that’s just by default.

The ephemeral storage is still available and can be used (for free!) by an EBS-backed image.  You just have to set the block devices up either explicitly when you run the instance or bake them into the image.

Runtime:

You refer to the ephemeral chunks as “ephemeral0”, “ephemeral1”, etc. – they don’t tell you explicitly which is which but basically you just count up based on your instance type (review the doc).  For a small image, it has an ephemeral0 (ext3, 15 GB) and an ephemeral1 (swap, 1 GB).  To add them to an EBS instance and mount them in the “normal” places, you do:

ec2-run-instances <ami id> -k <your key> --block-device-mapping '/dev/sda2=ephemeral0'
--block-device-mapping '/dev/sda3=ephemeral1'

On the instance you have to mount them – add these to /etc/fstab and mount -a or do whatever else it is you like to do:

/dev/sda3                 swap                    swap    defaults 0 0
/dev/sda2                 /mnt                    ext3    defaults 0 0

And if you want to turn the swap on immediately, “swapon /dev/sda3”.

Image:

You can also bake them into an image.  Add a fstab like the one above and when you create the image, do it like this, using the exact same –block-device-mapping flag:

ec2-register -n <ami id> -d "AMI Description" --block-device-mapping  /dev/sda2=ephemeral0
--block-device-mapping '/dev/sda3=ephemeral1' --snapshot your-snapname --architecture i386
--kernel<aki id>  --ramdisk <ari id>

Ta da. Free storage that doesn’t persist.  Very useful as /tmp space.  Opinion is split among the Linuxerati about whether you want swap space nowadays or not; some people say some mix of  “if you’re using more than 1.8 GB of RAM you’re doing it wrong” and “swapping is horrid, just let bad procs die due to lack of memory and fix them.”  YMMV.

Ephemeral EBS?

As another helpful tip, let’s say you’re adding an EBS to an image that you don’t want to be persistent when the instance dies.  By default, all EBSes are persistent and stick around muddying up your account till you clean them up.   If you don’t want certain EBS-backed drives to persist, what you do is of the form:

ec2-modify-instance-attribute --block-device-mapping "/dev/sdb=vol-f64c8e9f:true" i-e2a0b08a

Where ‘true’ means “yes, please, delete me when I’m done.”  This command throws a stack trace to the tune of

Unexpected error: java.lang.ClassCastException: com.amazon.aes.webservices.client.InstanceBlockDeviceMappingDescription
cannot be cast to com.amazon.aes.webservices.client.InstanceBlockDeviceMappingResponseDescription

But it works, that’s just a lame API tools bug.