How to mount a snapshot from boto? - amazon-web-services

Question

I'm using boto 2.5.1, Python 2.7, Ubuntu Precise. I want to mount a snapshot on an EC2 instance. I've gotten as far as creating a volume from the snapshot, but then I can't figure out how to attach it. If I do:
[setup stuff elided]
c = EC2Connection()
print volume
print instance
c.attach_volume(volume, instance, "/dev/snap")
I get the amazingly unhelpful exception:
vol-2df00677
i-1509d364
Traceback (most recent call last):
File "./mongo_pulldown.py", line 48, in <module>
main()
File "./mongo_pulldown.py", line 28, in main
c.attach_volume(volume, instance, "/dev/snap")
File "/home/roy/deploy/current/python/local/lib/python2.7/site-packages/boto/ec2/connection.py", line 1530, in attach_volume
return self.get_status('AttachVolume', params, verb='POST')
File "/home/roy/deploy/current/python/local/lib/python2.7/site-packages/boto/connection.py", line 985, in get_status
raise self.ResponseError(response.status, response.reason, body)
boto.exception.EC2ResponseError: EC2ResponseError: 400 Bad Request
with no clue of what I've done wrong. I'm assuming the device name is arbitrary, and the attach call will create the device as part of the process? Or does the device have the exist already?
How can I get some more useful diagnostic than just "Bad Request"?

Answer 1

The attach_volume method takes an instance_id and a volume_id but you are passing objects. Try this:
c.attach_volume(volume.id, instance.id, "/dev/sdh")
The device_name should be a reasonable device name for the OS you are using. You can find more about what that value should be here.
boto uses standard Python logging so you can configure it to log as much or as little as you want. This gist shows a shortcut approach to get full debug logging. However, boto can only log what it has access to and it's possible the response from EC2 just doesn't provide much information.

Answer 2

Well, it turns out that I was passing something bad for device. I was passing '/dev/snap'. When I changed that to 'xvdg', things worked. It appears that '/dev/xvdg' also works (and has the same effect; the '/dev/' part appears to be ignored).
I wrote a little function to find the next available unused device name:
def get_device_name():
for c in 'fghijklmnop':
name = "xvd%s" % c
path = "/dev/%s" % name
try:
os.stat(path)
except OSError:
return path
I was hoping that by using a fixed name outside of the set that's normally used, I could avoid having to worry about this silliness.