EBS FAQ and how-to's
By dkumar on Oct 13, 2009
Previously we posted a blog entry titled “OpenSolaris supports EBS - provides capability to create ZFS” that explains how to use the Amazon's Elastic Block Storage with OpenSolaris EC2 instances. This document combines the EBS with OpenSolaris ZFS technology. While we tried to cover the details needed but there a few questions have been asked several times. In this entry, I will try to explain those details and feel free to ask more questions so we can make it as clear as possible:
- When an EBS device is attached to OpenSolaris instance, how do I identify these drives from within the instance?
- Can I use some automated scripts to mount these EBS devices during the instance startup ? is this answered last (below)?
- Why do I sometimes detach/attach being not successful ?
I will answer these questions in following sections.
When a disk is attached to an OpenSolaris instance it can be viewed in number of ways and the simplest one is to use the format(1M) command. Following is the output of the format command on a default OpenSolaris EC2 instance (without any EBS device being attached) :
Searching for disks...done
AVAILABLE DISK SELECTIONS:
0. c7d0 <DEFAULT cyl 1274 alt 0 hd 255 sec 63>
1. c7d1 <DEFAULT cyl 19464 alt 0 hd 255 sec 63>
Specify disk (enter its number):
What this tells us is there are two default disks wwhere the controller is 7 and disk is 0(c7d0) and 1(c7d1). It is important to note that this is an OpenSolaris 2009.06 AMI and any AMI which is based on this should have the same controller number. For different OpenSolaris versions the controller number may change and the Getting Started Guide or the format command can be referred to get this information. Any further disk attachment through EBS commands (ec2-attach-volume) will have the same controller ID and a new disk id which will change based on the argument we give to this command. So far it can be easily assumed that -d (a unique number greater than 1) will result in an EBS device appearing as below with the format command within the EC2 instance:
c7d<decimal value of <<a unique number greater than 1> treated as hex number>
$ ec2-attach-volume vol-63d6250a -d 3 -i i-cf65b5a7
will result in:
c7d3 <DEFAULT cyl 2048 alt 0 hd 128 sec 32>
And so a command like this will result in the following:
$ ec2-attach-volume vol-63d6250a -d 10 -i i-cf65b5a7
will result in:
c7d16 <DEFAULT cyl 2048 alt 0 hd 128 sec 32>
Now if the disk number is already assigned and you try to do that again, it will result in following error:
$ ec2-detach-volume vol-63d6250a -d 2 -i i-cf65b5a7
Client.InvalidAttachment.NotFound: The volume 'vol-63d6250a' is not attached to instance 'i-cf65b5a7' as device '2'.
So it must be unique and unused number.
Also, for detaching a mounted ZFS or regular UFS? file system, we will have to do few things before we can do a clean detach:
For the EBS volumes that is part of ZFS we have to do the following:
all applications that are running on top of the ZFS pool.
Export the ZFS pools with:
$ zpool export pool_name
Detach the EBS volumes from the ECS instance.
Also clean up devices with:
$ devfsadm -C -v
For regular UFS? mounted file systems using the newfs(1M) and mount(1M) commands:
Unmount the mounted volume:
$ umount /ebs-vol
clean up devices with:
$ devfsadm -C -v