Performing NAS virtualisation via a virtualised file system gives a storage administratour greater flexibility in responding to the needs of the enterprise. Think of a virtualised file system as a DNS server for files. For instance, most people do not know the IP address of searchstorage.com, but most of them realise that if they type searchstorage.com into their browser, they'll get to the SearchStorage web site.
Similarly, with virtualised file systems, users don't need to know which NAS appliance is hosting the file they want to access. When they access the file, the virtualised file system knows its location and points the user to the file.
Virtualised file systems come in three forms: They can be software-only solutions built into the NAS head; software solutions embedded in the operating system; or an appliance combining hardware and software.
When the software solution is built into the NAS head, this allows for seamless integration between the file system and the NAS environment. OnStor's family of NAS appliances is an example of this type of implementation.
However, the software can also be built into the operating system of the clients that connect into the NAS environment. An example of this approach is Microsoft DFS (Distributed File System). Such OS-specific implementations may have a hard time with mixed platforms (Unix and Windows on the same NAS), the granularity at which file decisions can be made and the performance impact of tracking billions of files.
When a virtualised file system comes in the form of an appliance that sits in-band to the rest of the network, it has the advantage of being NAS-neutral but, like the OS-embedded implementation, this type of system also tends to struggle with mixed environments. As a result, many virtualised file system appliances have specific market segment focuses. Attune Systems offers an appliance and focuses solely on the Windows market. Acopia also chooses the appliance route and focuses on NFS.
A global file system has several advantages over a traditional file system. First there can be one mount point for the entire enterprise. Even if you have four different NAS solutions from four different manufacturers, you can give your users a single point of access to the environment. This solves the common problem of too many drive letters and not enough letters in the alphabet as well as users of not knowing which drive contains which data.
With this type of infrastructure in place, data can be moved from one file server instance to another without having to update user configurations. While these moves usually need to be performed at a folder level, the ability to move data transparently between NAS appliances is a useful capability. For example, if one NAS appliance were configured for high capacity but modest performance, you could move older stale data from a high-performance NAS to the high-capacity NAS. Simply identify data that has not been accessed in a period of time and move it to the higher-capacity NAS. This is all seamless to the user. The global file system tracks all the metadata in a consolidated database, so there are no stub files to be left behind and account for as is the case with traditional data movement products.
A virtualised file system can also aid in data migration. If you are decommissioning your current NAS device and upgrading to a new one, a virtualised file system allows you to move the data to the new NAS in real-time, during working hours, without user disruption. As files are moved, they are checked for activity: If they are active files they are skipped until they become released by the user, if they are inactive they are copied and the metadata information about that file is updated. The moment the user accesses that file again, they are redirected to the new NAS. Even if the intent is to keep the older NAS, a policy can be set up to only direct new or recently modified data to the new NAS, allowing a gradual fade-in of the new appliance.
While virtualised file systems provide better management of back-end storage, NAS virtualisation via virtual file servers focuses on network I/O and NAS head performance issues. Similar to a standard server virtualisation solution, virtual file servers are file server instances that run as virtual machines beneath a virtual file server host. This provides similar capabilities to standard virtual server solutions. A grid of physical NAS appliances can be networked together, all with common access to storage. Within each NAS appliance, virtual file server instances can be created. They look and act like standalone file servers: They can be assigned a unique IP address, serve unique file systems and can be made unseen to the other virtual servers in the NAS Appliance. This provides security to make sure that secure data that is on virtual server "A" can not be seen by any of the other virtual servers' clients.
The true value in virtual servers is their ability to move from one physical NAS appliance to another. If a physical NAS needs routine maintenance or if it becomes oversubscribed and file servers need to be re-distributed for performance reasons, virtual servers can be redistributed in real time. While this could be done with a combination of a virtualised operating system like VMware or Virtual Iron and a file server, companies like OnStor have built this functionality into their NAS operating system, making the connections seamless and removing the LUN management issues that cause challenges for virtualised operating systems.
Complete NAS virtualisation is the combination of virtualised file systems and virtual file server capabilities. This allows for seamless transport of data and server instances between physical NAS hardware and physical storage. This ability is not only critical for managing storage and storage-related costs on file server data. Because of VMware's new capability to boot from NFS-mounted storage, more NAS appliances are being deployed in those environments.
A NAS now has the responsibility of not only serving files but also serving files that happen to be boot images for a virtual infrastructure. As a result, the task of accurately projecting resource utilisation across both the virtual infrastructure and a file server infrastructure is nearly impossible. Having the flexibility to move both file systems and virtual file servers brings great flexibility to respond to unexpected impacts on data center performance caused by broader virtualisation.
About the authour: George Crump is founder of Storage Switzerland, an analyst firm focused on the virtualisation and storage marketplaces. It provides strategic consulting and analysis to storage users, suppliers, and integratours. An industry veteran of more than 25 years, Crump has held engineering and executive management positions at various IT industry manufacturers and integratours. Prior to Storage Switzerland, he was CTO at one of the nation's largest integratours.
This was first published in July 2008