Explore Windows Azure drives, disks, and mirrors

Using Windows Http://www.aliyun.com/zixun/aggregation/13357.html ">azure virtual machine Preview, we have two new special types of blobs stored in Windows Azure Storage: Windows Azure Virtual machine disk and Windows Azure virtual machine mirroring. Of course, we also have the Windows Azure Drive preview version. In the remainder of this article, we will refer to these storage, disks, mirrors, and drives. This article explores what drives, disks, and mirrors are and how to interact with the storage.

Virtual hard disk (VHDs)

drives, disks, and mirrors are virtual hard disks stored in your storage account as page BLOBs (BLOB (Binary SCM object), binary large objects. There are actually several slightly different VHD formats: fixed, dynamic, and differential comparisons. Currently, Windows Azure supports only the format named fixed. The logical disk in this format is arranged in a linear file format so that the disk offset x is stored at the BLOB offset x. One end of the blob, with a small footer describing the VHD property. All of these are stored in a page blob in the standard format of the VHD, so that you can use this VHD and mount on a non-cloud server if you choose to use it. In general, a fixed format wastes space because most disks have a large number of unused ranges. However, we store our "fixed" VHDs as page blobs, which is a sparse format, so we can get the benefits of "fixed" and "inflated" disks at the same time.

Uploading virtual hard disks to Windows Azure storage

You can upload your VHD to your storage account and use it as a PaaS (platform or service) or IaaS (infrastructure as a service). When you want to upload your VHD to storage, use a tool that understands that the page blob is sparse and uploads only some of the actual data VHD. In addition, if you have dynamic VHDs, to use a tool, when uploading, you will convert your dynamic VHD to a fixed VHD. Csupload will perform both operations for you, and it is part of the Windows Azure SDK.

Durability and durability

Because drives, disks, and mirrors are stored in storage, your data is persisted even if your virtual machine is moved to another physical computer. This means that your data takes advantage of the durability provided by the Windows Azure storage architecture, and that your data that is not buffered and written to your hard disk is replicated 3 times and stored before it is successfully returned to your application, so that it is permanently returned.

Drive (PaaS (platform as a service))

The drive is used by the PaaS role (worker role, Web role, and VM role) to mount the VHD and assign a drive letter. There are many details about how you can use these drives here. The drive that implements kernel mode is running within your VM, so your disk IO to the virtual machine drive will cause network IO in Windows Azure Storage and a blob from the VM to your page. The following figure shows that the driver runs on the VM and communicates with the storage through the virtual machine's virtual network adapter.

PaaS Each role allows mounting to 16 drives.

Disk (IaaS (infrastructure as a service))

When you create a Windows Azure virtual machine, the platform connects at least one disk to the VM for your operating system. This disk is also stored as a page blob in the memory VHD. When you write to the VM disk, changes to the disk will change the internal storage page blob. You can also attach additional disks to your VM as data disks, which will also be stored in storage as page blobs.

Unlike drives, the code that stores and communicates on your disk is not in your VM, so doing disk IO does not cause network activity on the VM, although it will cause network activity on that physical node. The following diagram shows how the driver operates in the host operating system and the VM communicates with the driver through the disk interface and then through the host network adapter to the memory.

Some of the virtual machines can be loaded, from 16 data disks to a large virtual machine, to a data disk, an extra small number of virtual machine disk limits.