Storage High Availability

Virtual machines are backed up by storage accounts by storing their VHD files on them. While availability sets provide High Availability to compute instance, it does not ensure High Availability of VHD files for virtual machines stored in storage accounts. The VHD files for all VMs might be placed on the same storage cluster and any cluster failure can render all virtual machines non-available or less available than required. In short, it is not only computed services that needs to be highly available, but even storage accounts storing VHD files should be placed on separate clusters such that in an event of failure, at least one or some virtual machines continue to be available both from a computer and storage perspective.

Azure provides managed disks as a concept that provides disk management facilities. Managed disks provide better reliability for availability sets by ensuring that the disks of VMs in an availability set are sufficiently isolated from each other to avoid single points of failure. It does this by automatically placing the disks in different storage clusters. If a storage cluster fails due to hardware or software failure, only the VM instances with disks on those stamps fail. Each virtual machine VHD in an availability set should be placed in a separate storage account. Although, virtual machines from different availability sets can be placed in a storage account.