Re: Next step.... more guidance requested

  • From: Andrew Stuart <andrew.stuart@xxxxxxxxxxxxxxxxxx>
  • To: pooka@xxxxxx
  • Date: Tue, 16 Jun 2015 14:55:39 +1000

Andrew: how do you specify the block storage to Amazon?

1: when you create an AMI (Amazon Machine Image, which is like a configuration
blueprint for creating VM's)
OR
2: when you launch an AMI from the command line you can override the device
mappings for that AMI

It is possible to override the device mappings when launching a VM using one of
the various command line tools such as ec2-run-instances. This link
http://docs.aws.amazon.com/AWSEC2/latest/CommandLineReference/ApiReference-cmd-RunInstances.html
explains the --block-device-mapping parameter.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/block-device-mapping-concepts.html
The above link says:
"A block device mapping defines the block devices (instance store volumes and
EBS volumes) to attach to an instance. You can specify a block device mapping
as part of creating an AMI so that the mapping is used by all instances
launched from the AMI. Alternatively, you can specify a block device mapping
when you launch an instance, so this mapping overrides the one specified in the
AMI from which you launched the instance.”
also:
"Each instance that you launch has an associated root device volume, either an
Amazon EBS volume or an instance store volume. You can use block device mapping
to specify additional EBS volumes or instance store volumes to attach to an
instance when it's launched. You can also attach additional EBS volumes to a
running instance;”

*****Device naming

Amazon uses block device names of the form /dev/sdX
the root device is /dev/sda1

However, in many cases the kernel renames the device names from /dev/sdX to the
form /dev/xvdX (XVD being Xen Block Device). This may well be the case for what
we are doing but I haven’t found the hard evidence yet.


Further reading:
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/RootDeviceStorage.html
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/device_naming.html




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