I’ve seen this question posed quite a few times both in the VMware developer and VMTN forum asking what exactly is the vStorage API?

So what is it?

If your answer was VMware marketing term, then you win 50 Schrute Bucks!

The VMware vStorage API is actually a blanket umbrella term that encompasses 4 separate individual APIs, all with different functionalities:

1. vStorage API for Data Protection (VADP)
2. vStorage API for Site Recovery Manager (VASRM)
3. vStorage API for Multi-pathing (VAMP)
4. vStorage API for Array Integration (VAAI)

I’m actually going to quote one of Chad Sakac’s reply on a blog about these 4 APIs as he has done a great job of explaining the differences:

1) vStorage API for Data Protection – a set of APIs focused on local backup/recovery use cases.

2) vStorage APIs for Site Recovery Manager – a set of APIs focused on array vendor remote replication such that they can be orchestrated by Site Recovery Manager.

3) vStorage APIs for Multipathing (otherwise known as the Pluggable Storage Architecture). A set of APIs for 3rd parties to extend vSphere’s core multipathing architectures.

4) vStorage APIs for Array Integration (VAAI). Technically not in vSphere 4, but have been discussed in VMworld events in the past. Will be available in future vSphere-generation releases. This allow array vendors to “offload” various tasks from the ESX host’s vmkernel stack – things like writing blocks that make up VMs, copying/snapshotting blocks, doing thin-provisioning out of space handling, and also a much more advanced global locking mechanism than VMFS uses today. These each will make common actions 5x-10x faster (clone, deploy from template, create a FT VM), and improve VMFS scaling by an order of magnitude. More on that here, for folks that are interested (note that when I wrote this, vStorage API for Data Protection was called the VCB Backup Framework)

Generally when the vStorage APIs are brought up, most people think about backups and the new Change Block Tracking feature in vSphere. That is because VMware and other 3rd party backup vendors has done a good job of marketing this "must have" feature. Leveraging Change Block Tracking helps decrease the duration of a backup by only copying the blocks that have changed and some users have seen up to 5-10x increase in speed.

This is great! But now you might ask the question, how do I use vStorage API for Data Protection and how does it tie into VMware's VCB product? Both the vStorage API for Data Protection and VCB are backup API frameworks, they are similar in functionality but different in features (for a break down of the two, take a look at this VMware blog post). VADP will be pretty much hidden from the end user's perspective, you just need to ensure that backup vendors are implementing it and utilizing Change Block Tracking to efficiently backup your VMs. VADP is only available in vSphere and not in VI 3.5.

If you want to write your own backup application, then you will need to know how to hook into VADP. As far as I understand today, and correct me if I'm wrong, the vStorage API for Data Protection is actually the combination of the vSphere 4.0 API + VMware Virtual Disk Development Kit (VDDK) which are both available to users to develop against. There's actually a guide within the VDDK page on Designing Backup Solutions for VMware vSphere which goes into great detail on creating a backup solution using the Change Block Tracking feature.

The three other APIs (VASRM, VAMP and VAAI) are targeted at third party hardware/software vendors and storage array providers to hook in their special sauces with the various VMware solutions. This includes hooking into Site Recovery Manager, PSA (Pluggable Storage Architecture) plugins and offloading VMware operations such as VM cloning and storage vMotions, etc. onto the actual storage array. These APIs are only exposed to partners who provide solutions to one of these features and are not available to the public for general use.

I personally think vStorage API is going to be a game changer and Change Block Tracking is just one of the many cool features to come!

Hopefully this all made sense and if you're interested to learn more about the vStorage API and some of the upcoming features, take a look at these additional resources:

http://www.ntpro.nl/blog/archives/1461-Storage-Protocol-Choices-Storage-Best-Practices-for-vSphere.html
http://virtualgeek.typepad.com/virtual_geek/2008/09/so-what-does-vs.html
http://www.vmware.com/products/vstorage-apis-for-data-protection/
http://www.yellow-bricks.com/2009/03/19/pluggable-storage-architecture-exploring-the-next-version-of-esxvcenter/