There are several things to look at when considering a scale-out NAS system.
1. Is a single namespace provided across all the nodes (also called controllers or heads) so that a file system can be spread across the nodes but the user does not need to take any special action for accessing a file? There are different ways that a single namespace can be implemented, and some may be better than others. Mounting or sharing a file system on a scale-out NAS system should require no more effort than if it was on a single-node system.
2. Does the management software manage across all nodes as an aggregate but still allow individual node communication to detect problems in the system?
3. Is there load balancing across nodes? The load balancing can be automatic when files are stored to distribute data across the different nodes. Will data automatically be redistributed across nodes (in the background) for capacity or load balancing?
4. Can it scale independently? In other words, can you scale nodes for more performance and the underlying storage for more capacity? This provides the greatest flexibility in usage. If the answer is yes, then how many nodes can the system scale to include? And how much capacity (including storage controllers) can it scale to?
5. Is there a back channel for communication between nodes? This requires another communication path between nodes rather than using the same path clients may be using to access data. Examples of this may be an InfiniBand connection between nodes or a 10-Gigabit Ethernet connection. Usually there would be a pair of back channels for availability.
6. Are there any features that are not included that would normally be part of standard NAS systems? A few to consider are: snapshots, remote replication, NDMP support, NFS and native CIFS support, security controls such as Active Directory, LDAP and file locking for shared access between CIFS and NFS, anti-virus software support and quotas.
7. Does the scale-out NAS support both small and large files? Some of the distributed file systems used for scale-out NAS come from the high-performance computing area where the optimization was around large files. It is important to understand whether the system supports small files and large files equally.
