However, NAS appliances do pose some disadvantages. First, they are file-based and not appropriate for every application...
-- some applications simply need the SAN . Second, their LAN connectivity can present a potential bottleneck for network users trying to access storage. Consequently, the choice of NAS appliance requires careful evaluation. Now that you've reviewed the essential issues involved in any NAS product, this guide focuses on specific considerations for dedicated NAS appliance products. You'll also find a series of specifications to help make on-the-spot product comparisons between vendors, like Agami Systems, Dell Inc., EMC Corp., IBM, ONStor Inc., Sun Microsystems Inc. and more.
@29474 Watch for additional or hidden fees. Not all of the features listed for a NAS appliance are standard. Some features and functionality may carry additional costs for upgrades or software licensing, adversely impacting the TCO. When comparing product costs, be sure to compare costs with all necessary features enabled, and factor in any upgrade or licensing costs involved in future scalability.
Consider the capacity and connectivity. Select a NAS appliance that will offer adequate storage capacity in the near term and suitable expandability into the future. An undersized NAS appliance will typically force users to purchase additional appliances -- resulting in additional capital expense and management overhead. Also, ensure that the NAS appliance provides suitable Fibre Channel (FC) or Ethernet connectivity with enough ports to support the expected storage traffic. For example, the Snap Server 18000 from Adaptec Inc. incorporates two Gigabit Ethernet (GigE) ports for connectivity. By comparison, IBM's N7000 Modular Disk Storage System supports up to 12 GigE ports or up to 16 2 Gbit FC ports.
Consider any necessary infrastructure changes. Heavy data transfers can easily overwhelm a LAN. Understand the implications of traffic changes on the intended network segment, and plan to accommodate upgrades or infrastructure changes that might be needed to achieve best performance. For example, adding a standalone NAS to a lightly used network segment may require a move from 100 Mbps Ethernet to GigE or the utilization of multiple GigE ports in busier environments. Organizations that need the performance of SAN storage may opt for a NAS gateway, rather than a NAS appliance. The NAS vendor might be able to help with network planning.
Consider the platform implications. The choice of platform can have a profound impact on the scalability, performance and manageability of NAS devices. Industry experts note that appliances based on Microsoft Windows (e.g., Windows Storage Server 2003 Release 2) are typically simpler to implement and manage, while proprietary appliances, such as EMC Corp.'s Celerra or Network Appliance Inc.'s (NetApp) products, often support greater scalability and performance. Proprietary appliances sometimes integrate more functionality, such as snapshots, which can add significant value to an enterprise.
Evaluate the support for RAID. Most NAS appliances offer data protection through internal RAID, so consider the RAID levels that will be most beneficial for your appliance. Support for RAID 0 (striping), RAID 1 (mirroring) and RAID 5 (parity) is common. RAID 6 (double parity) is appearing in appliances that rely on high-density SATA disk -- though support for RAID 6 is not yet universal. For example, Adaptec Inc.'s Snap Server 18000 and EMC's Celerra support RAID 1 and RAID 5, but the StoreVault S500 includes RAID-DP; a variation of RAID 6 double parity. Remember that your choice of RAID will impact the total usable storage capacity of the appliance. For example, RAID 1 mirrors disk contents, effectively cutting the total storage capacity in half. RAID 5 requires a disk in every RAID group dedicated to parity data. RAID 6 demands two additional disks in every RAID group for parity data.
Consider other forms of NAS data protection. Beyond local RAID features, a NAS appliance may offer support for snapshots, replication and backup. Be sure to identify the suitable snapshot or replication targets; a NAS appliance that can replicate to any heterogeneous storage platform may be preferable to an appliance that can only replicate to a duplicate heterogeneous appliance. Industry experts emphasize the importance of backup and recovery compatibility -- the NAS appliance should be compatible with your existing backup/recovery software and should not impose any special requirements on backup or recovery processes. As an example, the Adaptec Snap Server 18000 integrates BakBone Software Inc.'s NetVault software and offers backup agent support for Veritas, CA Inc., NetWorker and NDMP tools.
Consider internal support for tiered storage. Some NAS appliances can provide internal support for multiple disk types, and this can be a notable feature for organizations that practice tiered storage; for example, the Pantera 2200 Clustered NAS from ONStor supports serial attached SCSI (SAS) disks for performance and SATA disks for high-volume storage. This allows NAS appliances to be included within storage tiers. In many cases, both drive types can be supported simultaneously for internal tiering.
The NAS appliance product specifications page in this chapter covers the following products: