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.
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'
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”.
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.
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.