[CentOS-virt] xen device mapping/translation

Tue Jan 15 18:10:58 UTC 2013
Luis Fernando Alen <luis.alen at izap.com.br>

Hello, list.

Yesterday I was pleased to see that Centos has released official images at
the aws marketplace. Nice job.

Today I started playing with the Centos 6.3 image (
https://aws.amazon.com/marketplace/pp/B00A6L6F9I, on which I plan to deploy
a gluster cluster in production soon) and noticed a weird thing.

EBS Volumes attached to sd<X> are translated to xvd<Y> at the OS level.
However, after a few research and IRC chat, I figured out that it's not
weird, it's actually a normal and expected behavior (thanks for your help,

sdX is actually mapped to xvdX+4. There is a consistent offset of 4.
Suppose you attach an ebs volume to /dev/sdf. It'll be translated to xvdj
at the OS level. sdg to xvdk, sdh to xvdl and so on.

Allright. After having figured the mystery out, it became easy to work on
automations that deal with ebs volumes and file systems, such as volumes
created, attached and mounted on the fly, snapshots that freeze file
systems and so on...

However, I really do think to myself: Wouldn't it be cleaner if the image
use simple translation (sdX to xvdX)? If I'm not wrong, Rightscale uses
this on their Centos images and it's much simpler. There's no extra work
needed to deal with that 4 offset when you want to automate things.

Is there a reasonable reason for the 4 offset which makes it unchangeable?

It's just a thought. I think it's worth considering it..

Luis Alen
Ligue com tarifa local de todo o Brasil 4020.3000
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