Every storage vendor on earth is claiming their version is “virtual storage”. But what does virtualized storage mean? There are many definitions and I will try to put some clarity to how this term is being used. For the sake of this blog, I will contrast the differences between four products in the marketplace today that are touted as virtual storage environments; Hitachi VSP (Virtual Storage Appliance), EMC VPLEX, Falconstor NSS (Network Storage Services) and IBM SVC.
These deserve the label virtualization tools because they are (or contain as part of them) create an abstraction layer between the host environment and the storage. I separate these products into two distinct groups; In-line Appliances or Storage Platforms.
The EMC VPLEX, Falconstor NSS and IBM SVC are all appliances that sit in the fabric between the hosts and storage. The all virtualize external storage and each has a set of storage vendors that are certified behind them.
The Falconstor NSS product has been out the longest of the three and is feature rich encorporating synchronous & asynchronous replication, snapshotting and thin (dynamic) provisioning. Like an active/active asynchronous SAN controller, a volume is owned by a particular NSS appliance. If a host image, such as a VM, lives in two places, only one can write at any one time. NSS is implemented in a clustered pair and there is a quorum to shift over ownership from one NSS appliance to another in the cluster. Asynchronous replication to a disaster recovery site is limited to Ethernet only.
The EMC VPLEX addressed a different concern – the ability to write from to the same volume from two different physical VPLEX engines and the VPLEX will manage through a separate 10G network and quorum the synchronization of the data that sits on EMC or non-EMC disk behind it. They have a fairly robust list of supported storage platforms on the matrix including the ability to virtualize other virtualization devices such as Hitachi’s VSP platform. This allows customers to achieve the ultimate in availability, data protection and mobility within and between datacenters. The EMC VPLEX is, however feature deficient today and EMC promises that this will change in the near future. All snapshotting and advanced provisioning are done at the platform being virtualized taking away some of the benefits that should be inherent in virtualization.
Although I have not encountered IBM’s SVC often and have not worked extensively with the product, it offers the ability to virtualize in the same manner as Falconstor and appears to have a reasonably complete set of provisioning and optimization features.
In-Line Storage Controller Virtualization
The Hitachi VSP is a unique breed of product expanding on the earlier USP-V technology. The VSP or Virtual Storage Platform is a storage subsystem and a virtualization engine in one. It provides all of the benefits of a Tier 0/1 storage platform with the ability to house internal disk with the ability to virtualize external storage. This is a double-edged sword as it provides the ability to provision, store data, tier within or externally and replicate within a single subsystem, however, the active/active unit is physically on chassis. Access, like with the Falconstor NSS and IBM SVC, is based on ownership. Only one VSP can have a host to LUN relationship at one time. VSPs can replicate synchronously or asynchronously over fabric.
Features and architecture greatly separate the storage virtualization platforms. When searching for a virtualization technology, first outline the goals your organizations requirements and put them in a priority order. Maybe I absolutely need complete mobility of applications and operating systems across synchronous distances. Maybe that is not as important as extending consistent advanced provisioning and snapshot functionality to several platforms that do not have the functionality today.