Inside Ultra Disk Storage
Ultra Disk Storage is our next generation distributed block storage service that provides disk semantics for Azure IaaS VMs and containers. We designed Ultra Disk Storage with the goal of providing consistent performance at high IOPS without compromising our durability promise. Hence, every write operation replicates to the storage in three different racks (fault domains) before being acknowledged to the client. Compared to Azure Premium Storage, Ultra Disk Storage provides its extreme performance without relying on Azure Blob storage cache, our on-server SSD-based cache, and hence it only supports un-cached reads and writes. We also introduced a new simplified client on the compute host that we call virtual disk client (VDC). VDC has full knowledge of virtual disk metadata mappings to disks in the Ultra Disk Storage cluster backing them. That enables the client to talk directly to storage servers, bypassing load balancers and front-end servers used for initial disk connections. This simplified approach minimizes the layers that a read or write operation traverses, reducing latency and delivering performance comparable to enterprise flash disk arrays.
Below is a figure comparing the different layers an operation traverses when issued on an Ultra disk compared to a Premium SSD disk. The operation flows from the client to Hyper-V to the corresponding driver. For an operation done on a Premium SSD disk, the operation will flow from the Azure Blob storage cache driver to the load balancers, front end servers, partition servers then down to the stream layer servers as documented in this paper. For an operation done on an Ultra disk, the operation will flow directly from the virtual disk client to the corresponding storage servers.
Comparison between the IO flow for Ultra Disk Storage versus Premium SSD Storage
One key benefit of Ultra Disk Storage is that you can dynamically tune disk performance without detaching your disk or restarting your virtual machines. Thus, you can scale performance along with your workload. When you adjust either IOPS or throughput, the new performance settings take effect in less than an hour.
Azure implements two levels of throttles that can cap disk performance, a “leaky bucket” VM level throttle that is specific to each VM size, described in documentation. A key benefit of Ultra Disk Storage is a new time-based disk level throttle that is applied at the disk level. This new throttle system provides more realistic behavior of a disk for a given IOPS and throughput. Hitting a leaky bucket throttle can cause erratic performance, while the new time-based throttle provides consistent performance even at the throttle limit. To take advantage of this smoother performance, set your disk throttles slightly less than your VM throttle. We will publish another blog post in the future describing more details about our new throttle system.
Available regions
Currently, Ultra Disk Storage is available in the following regions:
- East US 2
- North Europe
- Southeast Asia
We will expand the service to more regions soon. Please refer to the FAQ for the latest on supported regions.
Virtual machine sizes
Ultra Disk Storage is supported on DSv3 and ESv3 virtual machine types. Additional virtual machine types will be supported soon. Refer to the FAQ for the latest on supported VM sizes.
