[CentOS-virt] xen device mapping/translation

Tue Jan 15 18:13:17 UTC 2013
Andy Grimm <agrimm at gmail.com>

See https://bugzilla.redhat.com/show_bug.cgi?id=729586

On Tue, Jan 15, 2013 at 1:10 PM, Luis Fernando Alen
<luis.alen at izap.com.br>wrote:

> 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,
> z00dax).
> 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
> www.izap.com.br
> Ligue com tarifa local de todo o Brasil 4020.3000
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