Refresher on RAID
Before going any further, let’s take a brief look at RAID storage and its main modes (we speak of levels) used by the NAS tested here.
An acronym for Redundant Array of Inexpensive Disks, this technology consists of creating a single and unique storage space from multiple hard drives. More specifically, this means that when configured in RAID, all drives are seen as one by Windows. The fact of using several drives offers several advantages – notably in terms of performances – but the main one lies in the possibility of putting into place data security methods.
The most obvious, called level 1 Raid (or Raid 1 for short), functions with two drives. Data is simultaneously written on each unit. In the case of a drive breakdown, the second continues to function and no information is lost. On the other hand, the maximum capacity of Raid storage is only equal to half of the total capacity of the two drives (and this on the condition that the sizes are the same, otherwise the capacity is equal to that of the smaller one). In theory, performances in reading and writing are equivalent to those of a one drive system.
This is a simple solution, and assuredly efficient, but limited to two drives. Of course, we could create Raid 1 storage with four drives (or six, eight, etc.), but this solution isn’t optimal. At best you will always lose half of the total capacity because additional drives are only used to replicate the first. In short, you can do better.
A step above is Raid 5 which enables using a larger number of units while retaining the security of data. The only downside consists of ‘’sacrificing’’ the capacity of a hard drive which will be used to store parity information which would serve in reconstructing files in the case of one of the drives breaking down. Obviously, putting Raid 5 storage into place is only possible starting with three or more hard drives. So the storage capacity is equal to the total of the drives minus one (1 TB or three 500 GB drives, for example).
Another advantage is the fact that sharing data between several drives enables performance gains in both reading and writing. This is only logical because instead of writing 1 on a single drive in a given amount of time, for example, we can write 1, 2 and 3 on three drives in the same time (or almost) – at least in theory. In practice, calculation of (and to a lesser extent in reading) the parity information requires considerable resources. For this reason, while we can observe systematic performance gains compared to Raid 1, the opposite often occurs. And moreover, this is the main weakness of the NAS tested here: the integrated processors significantly hold back performances. The only exception is the Thecus with its Celeron M on its N5200BR Pro, which consequently produces higher performances in Raid 5 than in Raid 1.
In short, Raid 1 or 5 therefore enable the protection against hard drive breakdown. Except when this occurs, you have to be there to replace the faulty unit as soon as possible (or quickly buy a new drive). In the meantime, the system is no longer protected against another failure. If this is a risk you do not want to run, there are at least two other solutions:
– Some NAS offer the possibility of adding a «
spare » drive to Raid 1 and Raid 5 storage. Of course this is an empty drive that isn’t used except when there is a problem. When this does happen it is immediately used to reconstruct Raid 1 or Raid 5 storage and the security of data is assured. In replacing the defective unit, the new drive automatically becomes the new spare. Obviously, this involves using three drives for Raid 1 and a minimum of four for Raid 5 which otherwise doesn’t increase the total storage capacity.
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Raid 6 can handle a breakdown of two drives. Its principle is almost the same as that of Raid 5 except it sacrifices two drives for the storage of parity information (and whose calculation is much more complex). With small sized Raid clusters as is the case for the NAS tested here, Raid 6 does not offer any particular interest compared to a Raid 5 + spare solution. This is all the more true that performances are lower.
In practice and except in specific needs, the NAS tested here were conceived to be used in Raid 5. However, all support at least a third Raid level which is a bit different because it favors performances over security. Called
Raid 0, this mode shares data on all the drives. In theory, the increase in transfer rates is almost perfectly proportional: almost twice as fast with two drives, three times as fast with three drives, etc.
In short, it’s great but this mode does present a size problem to such an extent that we strongly recommend avoiding it. The breakdown of any drive actually means the loss of all data in storage. In other words, with four drives the chance of breakdown is four times higher than with one drive or non-negligible risk.
Finally, with some NAS you can create a
Raid 10 storage. More concretely, this is a two unit Raid 0 storage space, with each unit being composed of RAID 1 storage of two drives. Therefore, you will need four drives to create this system. It is safe from a two drive breakdown on the absolute condition that these two drives are not in the same Raid 1. If not, obviously everything is lost.
As for the
Jbod mode, this isn’t pedantically speaking a type of Raid. All drives are simply put end to end to create a single unit. There is no safety system (even if, contrary to Raid 0, the breakdown of a drive does mean the loss of all data) and performances aren’t improved.
Announced and real capacities
So it should be understood that available capacity is directly related to the level of desired security. And moreover – and this is no longer a secret for anyone – the Gigabytes announced by hard drive manufacturers never translate into the same capacity once the drive is formatted. Given the same number of physically available bytes, for example 500 billion, a manufacturer announces 500 GB and Windows roughly 465 GB.
The reason for this has been known for some time. Manufacturers count in « 1000’s » (1 KB = 1000 bytes, 1 MB = 1000 KB, etc.) while they should count in « 1024’s » to obtain the correct computer result. The rule should actually be that 1 KB equals 1024 bytes and not 1000 (and thus: 1 MB = 1024 KB, etc.). With large capacities, this small subtlety can actually result in a significant difference!
This difference is all the more great when added to the «loss» of space due to the use of Raid technology. Thus, an NAS configured in Raid 5 which proudly displays 2 TB actually only offers around 1.36 TB or more like 68.2% of the announced capacity!
Here is a table which summarizes the real available capacities by Raid level compared to announced hard drive capacities. Note that this is only a theoretical calculation. The exact capacity depends not just on the specific drive but also the amount of space used by the NAS itself for operating system files (several dozen or even hundreds of megabytes depending on the model).
