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Over the last several years, storage hardware has become increasingly sophisticated. With so many new features and capabilities being introduced each year, data storage administrators may have a tough time determining which storage offering will best fit their organization's needs: NAS, DAS or SAN.
This article explains what NAS is, how it differs from competing technologies and factors to consider before you commit to a network-attached storage device.
What is NAS?
NAS is a mid-level storage offering. At its simplest, a network-attached storage device is a storage appliance that is directly connected to the network. Like other devices on the network, a NAS appliance is assigned an IP address and communication between servers and network-attached storage is usually based on the Transmission Control Protocol/Internet Protocol. NAS uses a traditional file system and the storage is accessible at the file level (usually through a file share).
Those who are new to storage sometimes confuse SAN and NAS. The acronyms are very similar and they both refer to types of networked storage. The main difference between SAN and NAS is that a SAN is a higher end storage offering. A SAN refers to a network dedicated solely to providing block-level access to storage hardware. In contrast, a NAS device is network connected, but a dedicated network is not required.
SAN networks are based around the use of Fibre Channel (FC), but there are exceptions. FC switches are arranged in such a way as to provide redundant access to storage hardware. In addition, server hardware in a SAN environment uses the SCSI protocol to communicate with storage hardware, whereas NAS devices are IP-based.
DAS refers to storage that is local to a server. The term DAS can also refer to a server's own internal disks. In addition, DAS can refer to an external storage array that is directly attached to a server via a storage controller or USB connection (as opposed to being attached through a network connection).
Do I need NAS?
Before implementing NAS, you need to consider how it will be used. You should think of NAS as general-purpose storage: a great choice for some use cases, but not the best option in other instances.
NAS is generally accessible at the file level, which makes it a good choice for storing unstructured data. If you are looking for a storage appliance to replace an aging file server, NAS is a good option. But not all NAS appliances support the use of NTFS permissions.
Depending on the device, NAS may not be a good option for storing structured data. While there might not be anything physically stopping you from storing a database on NAS, a NAS appliance might not be able to deliver the performance your database requires to operate efficiently. Even if the appliance is equipped with fast disks, the network connection will almost always be a limiting factor. Because of performance limitations (and other factors), many application vendors do not support NAS with their database-driven applications.
Technologies such as NAS and SAN can also be used for shared storage, which attaches multiple servers to a common storage device. For example, failover clusters commonly use shared storage so that all the cluster nodes can access the same data.
When deciding whether a NAS appliance for shared storage is a viable option, you need to look at the software vendor's requirements and the capabilities of the NAS device. Some NAS devices support iSCSI connectivity. If your network-attached storage device supports iSCSI and the software product you are using does not specifically prohibit the use of NAS, then network-attached storage can be used as a shared storage medium. Keep in mind that NAS does not provide the same level of performance as a SAN.
Some organizations consider NAS an option for storing virtual machines (VMs). On the surface, this seems like a good idea because NAS devices are generally high capacity/low cost (at least comparatively speaking) and can be configured to provide fault tolerance. However, two factors must be considered before placing VMs on NAS:
- Performance. Network connectivity is often a performance bottleneck for NAS appliances and may cause VMs to perform poorly.
- Vendor support. Microsoft, for example, allows you to store a Hyper-V virtual machine on NAS, but only if the NAS device supports SMB 3.0.
A network-attached storage device can also be used as a backup target, as long as the backup application does not require raw, block-level storage.
Another consideration when deciding to implement NAS is the overall health of your network. If your network is nearing the saturation point, it may not make sense to add NAS until existing issues have been addressed. In addition, some NAS devices do not support IPv6, which could be a problem for organizations transitioning (or planning to transition) from IPv4.
Scalability is another concern. While SAN environments can be pricy, they can be scaled to ridiculous levels. NAS devices have a finite storage capacity that is limited by the number of drive bays and, in some cases, by firmware. Although products exist to turn collections of NAS appliances into scale-out file servers, it can be a complicated process and not every NAS appliance supports scale-out architectures.
You also need to consider your options for protecting against NAS failure. Presumably, you will be backing up the device's contents (unless the NAS device is the backup), but what happens if a storage controller fails? Unless the appliance has built-in redundancy or you have replicated the device's contents to a secondary appliance, such a failure could cause a massive data loss event. In this type of situation, restoring the backup might not be an option because the controller failure affects the appliance as a whole. This isn't to say that you should avoid purchasing a NAS device, but you should have a contingency plan in case the appliance fails.
What class of NAS device should I buy?
Network-attached storage devices vary widely in terms of cost and capability. For those who might be trying to decide between a network-attached storage device geared toward the SMB market and an enterprise NAS device, there are two things to consider:
- Capacity. If your capacity needs are relatively modest (up to approximately 12 TB spread across four disks), you might be able to use an SMB NAS. Higher capacity requirements typically require a higher end product.
- Redundancy. SMB-oriented NAS devices often consist of a network interface card (NIC), a storage controller, a power supply and not much more. Although such devices can be configured to protect against a disk failure, the failure of any other component will usually take the appliance offline. In contrast, enterprise NAS devices may provide redundant power supplies, redundant (and possibly bonded) NICs, and often also include a replication feature that allows the appliance's contents to be replicated to a standby appliance.
As you can see, a NAS device may not be a good fit for every organization. The key to using NAS effectively is to purchase a network-attached storage device with a feature set that is well matched to your organization's size and functionality requirements.
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