Wednesday, November 28, 2012

Understand Persistent VM Disk (2)

Since all persistent PVM disks are eventually stored in blob storage, there must be a way to manage these disks and the .vhd files in the blob storage. The tools and portal to manage these are simple and straight forward, but understanding below key concepts would be very helpful for you to use those tools.

There are three stages of a .vhd file or a PVM disk:
  1. raw .vhd file in blob storage (use any blob explore tool to manage this)
  2. .vhd files recognized by PVM as disks (use Azure portal to create/delete disk)
  3. PVM disk attached to a PVM instance (use Azure portal to attach/detach disk)

O/S disk vs. Data disk: this is decided when you recognize a raw blob file as a disk. O/S disk can be used to create a VM instance.

Image vs. Disk: Image is "sysprepared" O/S disk. You can use either Image or O/S Disk to create a VM instance. If you use O/S Disk, all the old user name, machine name etc. persist, while everything is as from scratch if you use Image.

Understand Persistant VM disks (1)

Persistent VM (PVM) is Azure's IaaS offering comparable to Amazon's EC2. Unlike EC2 which uses local disks for the majority of the time, PVM uses Azure blob storage to store disks. This design has multiple implications.

On the good side, your data in the disks are more reliable, as every Azure blob automatically has 3 local copies which are always synchronized and another 3 copies in a different data center 100+ miles away which is delay synchronized. On the bad side, the disk I/O performance might be impacted. For this reason, PVM do use both Azure blob backed disks and local only disks to optimize the disk I/O, so it's important to understand where each disk lives. Below is the explanation:

C: disk is the OS disk. Persisted in blob storage.

D: disk is local disk. Not persisted, but very fast.

F: disk is additional user data disk. Persisted in blob.