Most traditional sort of persistence storage is based on block-based devices. Such devices, which include hard disk media or network-attached storage, are typically formatted and mounted as a file system available to the operating and, by extension, to applications.

While Amazon.com's Elastic Compute Cloud service had offered persistence storage options with its S2 and SimpleDB services, it did not offer the equivalent of hard disk storage to its dynamically provisioned elastic compute instances. Instead, users wishing to rely on persistence data between restarts of a virtual Amazon instance had to either store and fetch an entire virtual image to and from S3, or periodically back data up to S3.

With its "Elastic Block Store" (EBS) service, Amazon finally introduces a persistence storage platform that is, for all practical purposes, very similar to traditional disk-based storage—except that an Elastic block device is a virtual disk, and its data persistence is ensured by Amazon's highly redundant and available infrastructure.

With the new offering, developers can create elastic storage blocks of between 1GB and 1TB in size. Amazon CTO Werner Vogels describes EBS as a natural extension to Amazon's S3 and SimpleDB offerings:

Amazon EBS will be offered in the form of storage volumes which you can mount into your EC2 instance as a raw block storage device. It basically looks like an unformatted hard disk. Once you have the volume mounted for the first time you can format it with any file system you want or if you have advanced applications such as high-end database engines, you could use it directly.

Developers can create multiple volumes, in size ranging from 1 GB to 1TB. This volume will be created within a specified Availability Zone and will be accessible by your EC2 instances running in that Availability Zone. As to be expected with a volume abstraction only one instance can have the volume mounted at any given time. Volumes can migrate and be reattached to other instances if necessary for failure handling or application migration reasons.

The consistency of data written to this device is similar to that of other local and network-attached devices; it is under control of the developer when and how to force flush data to disk if you want to bypass the traditional lazy-writer functionality in the operating systems file-cache. Because of the session oriented model for access to the volume you do not need to worry about eventual consistency issues.

Vogels also points out that in contrast to traditional disk storage:

Amazon EBS isn't just a massive volume storage array within an Availability Zone, it provides a unique feature that allows for the creation of novel storage management scenarios: the ability to create snapshots and store those snapshots into Amazon S3. These snapshots can then be used as the starting point for creating new volumes within any availability zone.

Because EBS volumes appear as regular block storage devices, developers can even create their own striped volumes or RAID on top of EBS. As with other Amazon Web services, EBS is priced on a pay-per-use basis, with $0.10 per GB per month of storage.

What do you think of Amazon's latest Web service offering?