NAS stands for Network Attached Storage. It means any stand-alone hard drive or storage unit that is connected to an Ethernet network, and can be shared by the devices on that network.
This is distinct from a USB or Firewire or eSATA hard drive that you plug into a computer, which would be called direct-attached. A NAS drive contains a tiny fileserver computer that shares the storage space of its hard drive(s) out to the network. It is similar to using a server computer for file sharing, but without the power, space, expense and complexity requirements of setting up a file server.
The two main purposes for a NAS drive are to
- Make backups from any of the computers on the network, and
- Share files and media libraries between two or more computers or media devices
In addition, some NAS drives, depending on their internal software capabilities can
- stream media files to smart TVs and media players,
- host libraries of photos, music and movies,
- host web, ftp and mail servers,
- store video from networked security cameras,
- upload files to cloud (internet) based storage for offsite backup and more.
NAS drives usually have their own hard drives built in, anywhere from one to four (or more drives in larger commercial units). Some NAS units have USB or eSATA ports to chain additional hard drives externally. Note: Some internet routers also have USB ports for attaching a shared hard drive, so that they can function as an entry level NAS – the low powered processors in routers mean that shared drive performance can be quite slow, however, and there are additional security concerns.
NAS performance is dependent on the speed of the network, the speed of the hard drives, and the power of the internal processor inside the NAS. For networking, it is a real advantage to have Gigabit Ethernet capability throughout the system, which means not only the NAS unit, but the routers, switches, cabling and computer Ethernet interfaces all have to be Gigabit (1000BaseT) compatible. A NAS can function on a 10/100 BaseT Ethernet network, but file transfer performance will be limited.
A NAS is actually a small computer running file server software (typically Linux or Unix based), it runs on a quite low powered processor (CPU) compared to a regular computer, and the performance of the NAS varies with how powerful its internal CPU is and how much memory it has. Increased CPU power and drive capacity are the things that you are buying with increasing price.
NAS units that have more than one hard drive can be set up to do a Redundant Array of Drives (RAID) for additional security against drive failure. RAID 1 (mirroring) uses two drives, and mirrors whatever is written to the main drive immediately to a second drive. RAID 5, 6 and 10 are other ways of using 4 or more hard drives to provide higher performance. The point of a RAID is that every hard drive will fail at some point, and with redundancy of a RAID your data can survive the failure of any one hard drive. Once you have a failure, you need to replace the drive ASAP and rebuild the array, because if you have a second drive fail, chances are all of your data will be lost.
RAID 0 is an anomaly, it divides or ‘stripes’ the data between two drives for higher speed, but has no redundancy and thus is at a higher risk of failure. RAID 0 should not be used on drives that are intended as backups.
One thing that we look for in a NAS unit is the maturity of the software that runs it. Our favorite brands are QNAP and Synology for small to medium sized NAS units, because they have a mature software environment and have proven their reliability. They offer good support for both Windows and Mac OSX environments. When we start looking at business- and enterprise-class NAS units, then the price and the options open up more. Lenovo/EMC, Western Digital Sentinel series and others deserve some consideration alongside the larger QNAP and Synology units.
There are many different models of each of these brands, so the correct model can be chosen for your requirements. Some of the models have external expansion chassis available, so you can grow the NAS from four disks up to as high as 15 disks if required.
One question we get a lot is whether one drive can be taken out of a NAS unit for offsite storage. This is an extraordinarily bad idea. When you remove a member of a RAID set, you degrade the RAID, and it has to spend often hours reconstituting the RAID structure on the replacement drive. During this period, you are at risk of total RAID data loss if there is a power failure or a drive failure, and the performance of the NAS unit as a networked drive will be severely compromised. The second reason for not routinely removing and reinserting drives is that the physical SATA connector is not rated for very many insertions – in fact the service life of the SATA connection on the drive and the backplane of the NAS unit is, according to the SATA spec, only 50 insertions. Keep in mind that if the SATA connector inside the NAS unit breaks, you are royally hooped until you can get a replacement unit.
www.wdc.com/en/products/resources/drivecompatibility/
If you want offsite storage, my strong recommendation is to choose a NAS unit with an external USB or eSATA port and software support for backing up the data you need to the external drive. Then unplug that drive and carry it offsite for security, swapping in a second offsite drive for backing up the next evening. Yes, I realize that you are limited in offsite backup to the capacity of the external drive, but you can get drives that are reasonably portable with up to 10 TB of storage. If you have more than this, then you need to start thinking about a tiered storage strategy, dividing your archived data between archival, seldom accessed and frequently accessed data. An example would be cloned images of hard drive installs, which do not change after they have been created. These do not have to be copied every week and taken offsite – a static archive of them would be sufficient.
Along with the NAS unit, you will also need consider which hard drives to install in it (assuming you choose an unpopulated unit), and you should be installing a battery back up power supply to protect it from AC power events. No backup drive should be without a power backup.
As far as hard drive mechanisms go, you do NOT want to install regular desktop hard drives, even if they are about 20% cheaper. Drives that are specially tuned to work in NAS units are more reliable. Seagate NAS HDD and Western Digital Red series drives are made to run cooler and more reliably, have error correction and seek algorithms which are tailored to be appropriate to NAS units rather than desktop computers, and can tolerate a range of vibration and heat that desktop drives are not designed to encounter.
www.custompcreview.com/articles/guides/selecting-best-hard-drive-network-attached-storage-nas/19177/
www.anandtech.com/show/7258/battle-of-the-4-tb-nas-drives-wd-red-and-seagate-nas-hdd-faceoff
QNAP links
www.qnap.com/en/index.php?sn=1833&lang=en
www.qnap.com/en/index.php?lang=en&sn=822&c=351
Synology links
www.synology.com/en-us/dsm/index/overview
www.synology.com/en-us/products/index
Lenovo/EMC links
shop.lenovo.com/us/en/servers/network-storage/lenovoemc/index.html
Western Digital links
www.wd.com/en/products/business/networkstorage/
The biggest issue with RAID are the unrecoverable read errors.
If you loose the drive, the RAID has to read 100% of the remaining drives even if there is no data on portions of the drive. If you get an error on rebuild, the entire array will die.
http://www.enterprisestorageforum.com/storage-management/making-raid-work-into-the-future-1.html
A UER on SATA of 1 in 10^14 bits read means a read failure every 12.5 terabytes. A 500
GB drive has 0.04E14 bits, so in the worst case rebuilding that drive in a five-drive
RAID-5 group means transferring 0.20E14 bits. This means there is a 20% probability
of an unrecoverable error during the rebuild. Enterprise class disks are less prone to this problem:
http://www.lucidti.com/zfs-checksums-add-reliability-to-nas-storage