Edit: I am taking the unusual step of adding this after the original post was finished. As of the moment, it may be best to regard the conclusions in the post as tentative only. The post has generated a flurry of questions and controversy, with several people saying that I am wrong. Unfortunately, nobody has been able to provide a piece of definitive technical documentation (a white paper, or a support document, or relevant piece of text from an administration guide) that clearly describes the issue. Let me be clear: I am OK with being wrong. I kind of hope that I am--it would be just fantastic if we could get image level application consistent backups. But it is not OK that the answer to such an important issue is so obscure. So at the end of the day, if I am wrong but manage to get a clear and well documented answer, that is good enough for me! S.W.
Continuing the posts here and here, I wanted to have a very quick discussion of application consistent backup. This particular issue came up as a question to the other posts--and simultaneously in a discussion with W. Curtis Preston. And the issue is this: can you get .vmdk level application consistent backups of Microsoft guests under VMware ESX 4.0 with vSphere?
So, long question! Why are we asking this? Remember I concluded that you could not get an image level, application consistent backup, that is guaranteed to be consistent--even with vSphere 4.0.
So what is going on here? Was I dead wrong? Does VMware actually permit application consistent image level backups via VMware Tools and support for the MS VSS provider?
It turns out the answer is simple: no they don't. VMware's support for VSS is for the basic file system level VSS provider. Not for the extended application VSS provider used by Exchange and SQL. So, even when you take an image level (.vmdk) backup of a guest OS, and even when you have VMware Tools installed, all you can quiesce is the file system, not the application. As a result, you may get an application consistent backup, or you may not. It is certainly not guaranteed--because the application itself is not quiesced.
Therefore, final answer: if you need application consistent backups, you must do a guest level backup. An image level backup is simply not good enough. Even with a MS Windows 2003 or higher guest, even though VMware supports VSS. Yes you may do image level backups too, but they will only complement, no replace, guest level backups.
And, as I concluded before, if you need to do guest level backups, there is no better choice than Avamar. The combination of its dramatic reduction in network I/O for backup by as much as 99.5% over a standard full backup, the reduction in backup duration by as much as 90%, and the very significant reduction in net CPU utilization demonstrated by Avamar are simply an unbeatable combination for the backup of consolidated systems like large VMware infrastructures.