Can you attach Amazon EBS to multiple instances?

Question

UPDATE (April 2015): For this use-case, you should start looking at the new Amazon Elastic File System (EFS), which is designed to be multiply attached in exactly the way you are wanting. The key difference between EFS and EBS is that they provide different abstractions: EFS exposes the NFSv4 protocol, whereas EBS provides raw block IO access.

Below you'll find my original explanation as to why it's not possible to safely mount a raw block device on multiple machines.

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ORIGINAL POST (2011):

Even if you were able to get an EBS volume attached to more than one instance, it would be a _REALLY_BAD_IDEA_. To quote Kekoa, "this is like using a hard drive in two computers at once"

Why is this a bad idea? ... The reason you can't attach a volume to more than one instance is that EBS provides a "block storage" abstraction upon which customers run a filesystem like ext2/ext3/etc. Most of these filesystems (eg, ext2/3, FAT, NTFS, etc) are written assuming they have exclusive access to the block device. Two instances accessing the same filesystem would almost certainly end in tears and data corruption.

In other words, double mounting an EBS volume would only work if you were running a cluster filesystem that is designed to share a block device between multiple machines. Furthermore, even this wouldn't be enough. EBS would need to be tested for this scenario and to ensure that it provides the same consistency guarantees as other shared block device solutions ... ie, that blocks aren't cached at intermediate non-shared levels like the Dom0 kernel, Xen layer, and DomU kernel. And then there's the performance considerations of synchronizing blocks between multiple clients - most of the clustered filesystems are designed to work on high speed dedicated SANs, not a best-effort commodity ethernet. It sounds so simple, but what you are asking for is a very nontrivial thing.

Alternatively, see if your data sharing scenario can be NFS, SMB/CIFS, SimpleDB, or S3. These solutions all use higher layer protocols that are intended to share files without having a shared block device subsystem. Many times such a solution is actually more efficient.

In your case, you can still have a single MySql instance / fileserver that is accessed by multiple web front-ends. That fileserver could then store it's data on an EBS volume, allowing you to take nightly snapshot backups. If the instance running the fileserver is lost, you can detach the EBS volume and reattach it to a new fileserver instance and be back up and running in minutes.

"Is there anything like S3 as a filesystem?" - yes and no. Yes, there are 3rd party solutions like s3fs that work "ok", but under the hood they still have to make relatively expensive web service calls for each read / write. For a shared tools dir, works great. For the kind of clustered FS usage you see in the HPC world, not a chance. To do better, you'd need a new service that provides a binary connection-oriented protocol, like NFS. Offering such a multi-mounted filesystem with reasonable performance and behavior would be a GREAT feature add-on for EC2. I've long been an advocate for Amazon to build something like that.

Update (2020) It is now possible!

This is possible now with the newest instance types running in AWS Nitro within the same Availability Zone. There are some caveats but this is great for certain use cases that need the speed of EBS and where EFS isn't feasible.

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-volumes-multi.html

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Original Post (2009)

No, this is like using a hard drive in two computers.

If you want shared data, you can setup a server that all your instances can access. If you are wanting a simple storage area for all your instances, you can use Amazon's S3 storage service to store data that is distributed and scalable.

Moving to the cloud, you can have the exact same setup, but you can possibly replace the fileserver with S3, or have all your instances connect to your fileserver.

You have a lot of options, but sharing a hard drive between instances is probably not the best option.

No, according to the EBS docs: "A volume can only be attached to one instance at a time".

How are you using the shared storage currently? If it's just for serving files from the fileserver, have you considered setting up a system so that you could proxy certain requests to a process on the fileserver rather than having the webservers serve those files?

Multiple Web Servers accessing MySQL Server & File Server is normal in AWS. Some of the best practices to be followed for the above mentioned architecture are:

Point 1) MySQL on EC2 can be setup as Master-Slave in async/semi sync mode in AWS. EBS-OPT+PIOPS in RAID 0 is recommended for High performance DB

Point 2) Alternatively you can use Amazon RDS + Multi-AZ mode. For read scaling Multiple RDS Read Replica's can be attached to MySQL RDS.

Point 3) EBS Volume cannot be attached to Multiple EC2's simultaneously. You can create File server based on GlusterFS on Amazon EC2 using EBS. Multiple Web Servers can talk to single GlusterFS simultaneously on AWS infra.

Point 4)In case your application can be integrated with S3 as file store, then it is preferred because of the stability it brings in to the architecture. You can also access S3 using tools like S3fuse as well from your application.

EBS just announced this is possible: https://aws.amazon.com/about-aws/whats-new/2020/02/ebs-multi-attach-available-provisioned-iops-ssd-volumes/

I'm fairly sure you can't, but you can clone an EBS and attach it to another instance.

This is useful for fixed datasets, or for testing on 'real' data but doesn't allow more than 1 instance to operate on a single block store