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So I logged into a new Amazon EC2 instance created with one of their new "standard" AMI's. Instead of seeing /dev/sda1 I see /dev/xvda1 . This relates to Xen somehow.

Here's what I can find that mentions this:

http://www.ioncannon.net/system-administration/1290/how-to-build-compile-a-custom-linux-kernel-for-ec2/

It says: "Apply the following patch to disable XSAVE". (someone must have done that before building the standard AMI)

Then it says: "Note that the root device here is /dev/xvda1 instead of /dev/sda1. This is caused by the XSAVE patch."

And finally it says: "Note that the devices here are /dev/sda1 and not /dev/xvda1. That is a little confusing but the AWS system doesn't see the devices in the same way your AMI will once it is booted."

Yes, it is kind of confusing and I would like to understand it.

Googling XSAVE and XVDA1 are not helpful at all.

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1 Answer 1

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XSAVE and /dev/xvda1 have nothing to do with each other. That link is incorrect.

XSAVE is a CPU feature that Xen should not present to a kernel running in a VM. See https://partner-bugzilla.redhat.com/show_bug.cgi?id=524719

When configuring Xen you present a block device to the guest VM (domU, "AMI"). By convention (was a requirement in the past) they are prefixed with "xvd" (Xen virtual disk). In the host OS (dom0, "AWS system") this block device will be called something different. In the EC2 case it is a SCSI-like disc hence the "sd" prefix.

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Why is it that my older EC2 instances (from other AMIs and of other sizes) have /dev/sda1 etc. drives. But my new ones have /dev/xvda1 ? –  user55882 Oct 1 '10 at 23:19
    
@paul, I'm a little confused myself but I think what happened is xvda1 etc has always been the device node inside Xen guests (xen virtual device a), however when Amazon started using it they patched the foo out of it so that devices on instances running any of the Amazon AKI's showed up as the more familiar sda. Now that we have the ability to run our own kernels and vendors are making their own, we're back to the standard set of Xen devices because our kernels are closer to stock. –  Caleb Oct 2 '10 at 19:53

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