Hu Yoshida

VSAN for Direct Attach Storage and VVOL for Enterprise Storage

Blog Post created by Hu Yoshida Employee on Nov 11, 2014

Oct 9, 2014


In June of last year 2013, VMware announced VSAN. VMware Virtual SAN is embedded in the VMware vSphere kernel and enables clusters of servers with direct attached hard disk drives and solid state drives (HDD and SSD) to create a shared datastore. The management architecture of VSAN enables administrators to specify storage attributes, such as capacity, performance, and availability, in the form of policies on a per VM basis. VSAN brings the benefits of software-defined storage to direct attached storage. Since it eliminates the need for a storage controller, it requires SSDs for read and write caching and the support of the hypervisor for other storage controller functions like provisioning, snap shots, clones, data replication, and data protection. Does this mean that storage arrays will go away to be replaced by commodity HDDs and SSDs?


At VMworld in San Francisco, Pat Gelsinger acknowledged that this is not the case with the announcement of virtual volumes (VVOL). VVOL promises to be the next big thing in the storage ecosystem due to its ability to immensely simplify storage management and drive up IT efficiency by leveraging the unique capabilities of different storage systems, whether they are block or file. VVOL provides VM-level granularity to IT administrators by providing an abstraction layer in the form of a storage API between control plane and storage systems. . I covered the benefits of VVOL in a previous post.

Using vSphere APIs for Storage Awareness (VASA), companies like Hitachi Data Systems can create providers for their storage and converged solutions products to present their unique capabilities to vCenter for consumption. A VVOL is a storage container, provisioned using the information provided through VASA and aligned to VM boundaries. VVOLs can use these storage providers to set policies and push information down to the external storage to make the storage VM-aware and able to negotiate with individual VMs rather than the traditional way of dealing with a set of VMs in a LUN or file share. It contains the data store, a set of data services expected from the storage system and a set of policies requested for the VM. This is the way to implement software-defined storage by leveraging the services of the storage array and offloading the servers so that they can support more VMs.

While VSAN may be easy to manage it  lacks the enterprise performance and capabilities that an enterprise storage array can provide. For instance, the reason VSAN requires direct attached SSDs is to provide read and write caching. While SSD flash has a 25 microsecond read and 200 microsecond write with 1.5 MS format time, it is slow compared to the nano second speeds of DRAM in a storage controller cache. VMware also does not support a VSAN cluster over distance. You cannot build a vSphere Metro Storage Cluster for mission critical applications, which require zero downtime on a VSAN cluster.  This requires an enterprise storage array. As a result VMware is encouraging vSAN for tier two workloads such as VDI, ROBO etc.

The key benefit of VVOL for Hitachi customers is that VVOL enables per-VM data services, such as replication, snapshot, caching, etc., to leverage Hitachi storage features like thin provisioning and active/active devices that are made available through Hitachi’s VVOL provider. These services run more efficiently on the storage array due to the co-processing benefit of storage controllers. VVOL will enable enterprise class software defined storage. VSAN will not, since it uses direct attached disk and SSD, and is limited to the capabilities provided by the host server.

Gelsinger positioned VSAN and VVOL in his key note speech at VMworld. “We’re in VSAN 2.0 beta as well. Great, amazing, adoption and momentum with that. We are also as part of vSphere 6.0, releasing VVOL. Before this, when we announced VSAN, I apologized to you, our industry partners, particularly in the storage area. Because one of our thesis’s of disruptive innovation is always enabling the ecosystem to come along. And VSAN did not enable you to do that. VVOL does and we’re committed to delivering this to participate in the software automation and policy management of that platform – as we continue to innovate on VSAN and the integrated technology that comes as part of vSphere 6.”