I follow the Microsoft Azure article How to Attach a Data Disk to a Windows Virtual Machine and I am told not to use D:\ as a drive label because it already exists. Why is this?
Virtual machines created in Windows Azure are created with temporary storage assigned automatically.
Depending on the OS this can appear as:
- Windows Virtual Machine “D:\”
- Linux Virtual Machine “/dev/sdb1/”
Essentially, the temporary storage is used for the paging file of the running VM. Using local storage on the physical host helps to increase IOPS and lower latency when compared to standard Azure storage.
It is obviously possible to store data on this drive BUT do not use it to store data that you are not willing to lose! The reason for this is, the temporary storage is created on the physical machine that is hosting your virtual machine. Should your virtual machine move to a different host due to hardware hardware failure or local host updating, the OS disk will be recreated from your storage account. However the temporary storage will be reallocated on the new physical host and any data will not be migrated from the original host. Other causes for the temporary storage to be recreated include when you resize your VM or when your VM is shutdown and restarted.
The size of the temporary storage changes between virtual machines but an up to date size can be found on this Microsoft Azure article Virtual Machine and Cloud Service Sizes for Azure
This image is from the Microsoft Azure Support Team Blog, but shows this process.