When discussing NAS (network attached storage) there is nothing like a TLA (three letter acronym) or FLA (four letter acronym) to confuse a person and leave one misinformed. I thought a glossary of NAS terms might be appropriate so we can all know EXACTLY what we are talking about.
Let’s start off with RAID terminology
I will discuss RAID levels and definitions, for the most part, as pertains to network attached storage devices reviewed on the pages of this website. Other RAID levels exist but are typically not seen in the types of network storage devices seen here.
Redundant Array of Independent Disks. It may have initially referred to Redundant Array of Inexpensive Disks, but when it was implemented with SCSI disks, one could hardly argue that the disks were inexpensive.
As per Wikipedia and my own experience, RAID is now an "umbrella" term for computer disk storage arrays of about any type. As you can see with RAID0 below, striping is considered "RAID" even though your data is NOT protected by redundant disks.
One very important point to understand is that RAID is NOT DATA BACKUP! Data backup is a data set from a particular point in time, and is best kept at a separate location from the data source. RAID is a protection against hardware failure, not a way to restore data that has been deleted, changed or corrupted.
You can get additional information on what RAID means here.
RAID Level 0: Data is “striped” between 2 disks or sets of disks for performance purposes. Unless striping is combined with mirroring (RAID 0+1 or 1+0) there is NO DATA REDUNDANCY. If 2 drives are in a RAID0 stripe, total storage is equal to the sum of both drives less overhead. If either drive fails, data will be lost!
RAID Level 1: Otherwise known as “mirroring”. With RAID1 you must have even pairs of drives, storage available will equal the total formatted disk space of the lesser capacity drive less overhead. Ideally, drives in a RAID1 mirror should be identical or at least of identical performance specifications.
RAID1 has the advantage of quicker disk “reads” as data can be read from either drive. “Writes” can take longer, however, since data must be written to both drives; performance loss depends on RAID controller hardware capability.
RAID1 is disparaged by some since the cost per megabyte is quite high since 50% of the drive space purchased is required for redundancy and data protection. Either drive can fail in a 2 drive RAID1 mirror and the network storage device will continue to function and no data is lost as long as the failed drive is replaced prior to failure of the remaining drive and subsequent resyncing operation.
RAID Level 5: Probably the most popular RAID level, RAID5 distributes a “parity” bit that allows any 1 drive to fail and still allow the RAID array to function and prevent data loss. 3 hard drives are the minimum requirement for a RAID5 array.
Some NAS devices allow a 4 drive configuration of a 3 drive RAID5 array and 1 drive designated as a "hot spare" to immediately replace (upon resynchronization) any failed drive of the 3. Data is at risk until the failed drive is replaced and resynchronized, or in the case of a hot spare, until resynchronization.
With today’s large capacity hard drives, synchronization can times can be lengthy (many hours) particularly if the NAS device continues to have data written to it during the resyncrhonization. RAID6 was developed as a solution to the data loss risk during that time period.
RAID Level 6: Builds on RAID5 by having "double" "distributed parity" that allows for up to 2 hard drives to fail in the array without data loss. RAID6 is considered a better alternative to RAID5 with a hot spare since there is no resynchronization time where data is a risk after just 1 hard drive failure.
RAID Level 10: This hybrid, nested RAID level is typically thought of as RAID1 + RAID0; i.e., mirrored sets of drives that are then striped for performance purposes. While performance gains may be present with a RAID10 system, it comes at a very high cost per megabyte due to the number of drives required.
X-RAID is a proprietary RAID system found in Netgear ReadyNAS network storage devices that improves upon RAID5 by allowing dynamic expansion of the RAID5 volume without having to backup the data prior to volume expansion and restore the data afterward, a process that can result in data loss if something in the backup and restore process goes wrong but also requires extensive downtime for the process to be completed.
X-RAID2 is a newer proprietary RAID system found in some newer x86 (Intel CPU) Netgear ReadyNAS network storage devices allows dynamic expansion of RAID5 & RAID6 volumes by replacing only some of the drives in the array.
X-RAID needs all the drives replaced with larger drives before it can expand the volume whereas X-RAID2 only needs enough drives replaced to allow redundancy (i.e. 2) before expansion.
Netgear allows for “dual redundancy” similar to RAID6 in X-RAID2 but ONLY if you have a 6 drive (total capacity) device like the Pro Business Edition or Ultra 6. Need to decide to implement BEFORE you load up data, a switch to dual redundancy does wipe out your data.
JBOD stands for Just a Bunch Of Disks (hard drives). Only certain vendors offer this option with their network storage devices. It basically allows each hard drive in the NAS device to act as an independent, non-redundant, network accessible external hard drive.
While some may find this advantageous, the lack of redundancy means a JBOD system is most valuable for backing up other data and NOT to be used for storage of data that should be backed up itself.
512 Byte Sector Size
A sector is the building block of a hard drive. Currently, hard drives are manufactured to hold 512 bytes of data. If a file is 513 bytes in size, you need 2 hard disk sectors to store the data, wasting 511 bytes. So the smaller the sector size, the more efficiently you can store files of a small size.
Unfortunately, Murphy’s Law dictates that an empty space will be filled and since hard drives get significantly larger over time, programmers have no incentive to store data efficiently. Yet maybe that is irrelevant today since so much of our data is in the form of multimedia, which is efficiently compressed.
512 byte sectors have a limiting factor on the overall size of the hard drive with 32 bit operating systems, and require more overhead with today’s larger file sizes.
4k Byte Sector Size
As a result, new hard drives seeking to break through the 2 terabyte size barrier and catch up with the capabilities of 64 bit operating systems are using 4k byte sectors that will also be more efficient (speedy) at retrieving large files.
Unfortunately, older operating systems like Windows XP in 32 bit will not be able to use these larger drives, but that’s progress. Drives with this larger sector size can be "programmed" to emulate 512 byte sectors for compatibility, but do so at a performance cost of about 10%.
Understanding NAS Comparisons
Buying network storage devices can be confusing enough since every vendor does not provide the same level of detail on each of their OWN products, let alone anything close to a standard by which you could compare competing NAS products.
Hopefully this glossary of NAS terms has helped you understand how each network storage device can benefit you in caring for your data.