RAID (Redundant Array of Independent/Inexpensive Disks) is a formation which consists of a certain number of disks unified into one logical element. All the disks are managed by one or several controllers. As a rule, RAIDs are made up of hard drives, although nowadays they are likely to be formed from SSDs. Such an assembly is aimed to increase speed and improve the safety of stored data.
The arrays differ in both the number of disks and the methods of data distribution among the disks. A data distribution method is called the type or level of a RAID. In addition to technical characteristics, this level defines array properties, the speed of operation and its ability to resist hardware failures.
There are several different array levels, and each has its own specific properties and appliance. Besides, almost every array controller may have the JBOD mode (Just a Bunch of Disks), i.e. a conventional set of disks that do not act as a whole.
Most RAID levels (except RAID0) can operate when one or even more disks fail, in the so-called degraded mode or the mode of limited functionality, in which data protection is constrained (or absent) and the speed is limited.
RAID0 is based on the technology of block data striping and its even distribution among all disks of the array (stripe). Such distribution significantly increases the operational speed of the entire system, because several disks can perform reading/writing simultaneously.
Unfortunately, the technology doesn't focus much on reliability. If one of the disks fails, the data of the entire array will be practically irrecoverable.
RAID0 is perfect for storages in which speed is more important than reliability, for example, for servers used for video streaming or photo editing.
The highest productivity among all RAID types.
High-speed data writing/reading.
The user can utilize the entire array capacity for writing data.
The cheapest option from the perspective of expenses per 1 GB of data storage.
Relatively poor reliability. Any failure may lead to data loss.
The array employs the technology of mirrored copying (mirroring), i.e. each disk with data has its identical twin which completely reproduces this disk. Therefore, RAID1 is composed of two and more drives – exact twins.
This means that in case of simple mirroring (on two components), only a half of the whole capacity of the array is available to the user increasing the price for data storing by two times, i.e. for a RAID1 array of 500 GB you need to buy 2 disks 500 GB each. On the other hand, if one disk fails, you can quickly replace it with a new one. At the same time, the system copies the data from the remaining disk to the new one instantly. To configure RAID1 you need to have minimum two disks at hand.
RAID1 features high reliability often achieved at the cost of reduction of the operational speed of data writing. The information is written to one disk first and then to the other. However, the reading speed may be high due to parallel data reading. These traits depend completely on the hardware (or software) RAID configuration.
RAID1 is perfect for storing critical data, for example, for accounting systems as well as for small data servers.
High failure resistance.
The array will continue working even after the failure of one of the data disks (if a working mirror is present).
High reading speed (depending on the controller).
The most wide-spread configuration with the highest support among all RAID types.
Low writing speed (without independent controllers).
Only half of the disk space is available to the user.
High price per gigabyte.
RAID2 and RAID3
RAID Level 2 also employs the technology of data striping, but it divides its data into bytes instead of blocks. To achieve failure resistance RAID2 provides space for a Hemming code, i.e. to assemble an array at least three disks are required.
RAID3 cuts data into bytes and distributes it among the disks. There is a separate disk for failure correction in the array.
Due to byte-to-byte data distribution, all drives of the array work as one unit. This means they can do only one operation at a time. Arrays of this type are rarely used.
RAID4 applies the data striping method as well and has a separate disk for error correction. However, in comparison to Levels 2 and 3 the data is divided into blocks. This allows the disks to work independently and perform several reading operations at a time. This means that the array has a high reading speed, the same as RAID0. The writing speed is reduced due to the necessity to keep parity information on an additional disk for error correction purposes, because its data needs to be updated every time when new information is being written.
To assemble RAID4 you need at least three disks. On the other hand, this modification has found a broad use due to a completely suitable compromise between speed and reliability. If one of the blocks is lost, the system can recover it by itself using the neighboring blocks and parity information.
Fairly high fault tolerance, the ability to perform self-recovery.
Ability to work in the mode of limited functionality.
High data-reading speed.
Low data writing speed as one disk is dedicated to error correction.
Considerably lowered reading speed in the mode of limited functionality.
In case of the loss of one disk, data recovery on a new disk may take much time. If during this process one more disk fails, data can be lost irreversibly.
RAID5 is probably the most popular RAID configuration used in NAS storages. At present, RAID5 is an ideal combination of price, speed and quality.
RAID5 is almost identical to RAID4, however, it doesn't write the parity information onto a separate disk and rather evenly spreads it over all disks. This means that RAID5 can resist the failure of one disk without losing data and access to it, because the data and the parity information located on the disks serve to recover the lost blocks.
Depending on the retailer, production capacity and objective, RAID systems of Level 5 may differ in data distribution methods and parity information.
To build an array of Level 5 you need at least three disks.
High reading and writing speed.
If one of the disks fails, the user still has access to the data (in the mode of restricted functionality).
Resistance to failures and errors.
The breakdown of a disk reduces productivity.
In case of the loss of one disk, data recovery on a new disk may be time-consuming. If during this process one more disk fails, data can be lost irreversibly.
RAID6 is similar to RAID5 with one difference – a double Reed-Solomon code, which is written onto two disks, is used instead of the parity information. Therefore, the minimum number of disks rises up to four and the system may continue working even in the case of the loss of two disks.
There are several popular complex array levels, e.g. RAID10 (1+0), RAID01(0+1), RAID50(5+0), etc.
RAID10 is a RAID0 array which consists of RAID1 arrays, while RAID01 is a RAID1 array composed of RAID0 arrays. Similar to that, RAID50 is a RAID0 array which comprises RAID5 arrays.
Such combinations serve to increase productivity and failure resistance, but all this is achieved at the expense of a significant rise in price. For instance, to build RAID10 you need to have at least four hard drives.
The combination of two and more RAID5 arrays into RAID0 notably speeds up performance due to parallel data use.
RAID10 and RAID50 have become very popular due to their simplicity from the implementation perspective and the combination of sufficient speed with backup options, even though a high price does not allow to employ them in small systems. RAID01 is not in big favor because it is practically identical to RAID10 in its features.
Data lost due to a RAID failure can be recovered.
Possible symptoms of a damaged RAID system include:
The array is in the limited mode, but all the files can still be accessed.
RAID was recovered incorrectly after data loss, settings or disk sequence numbers were changed; the access to files is limited or absent.
The status of the array is "not active", "not found", "switched off", at the same time the system does not display it and the access to files is blocked accordingly.
RAIDs have the same data loss problems that are typical of usual hard disks: accidental deletion, software failures, overwriting, file system damages, etc. Therefore, if RAID works as usual but the data cannot be accessed, the loss may be caused by another problem, like, an operator's mistake or computer viruses.
Besides, RAID failures may be caused by the breakdown of a controller or disk defects. Depending on the RAID level, this may result in some difficulties and restrictions during the process of data recovery.
|RAID LEVEL||CONTROLLER BREAKDOWNS||DISK FAILURE|
|RAID0||Data may be easily retrieved with information about disk order.||If recovery of any disk is impossible, data recovery is neither possible.|
|RAID1||Data is easily recoverable from any component.|
|RAID4, RAID5||Data is easily recoverable. The information about the disk order number, the block size and the method of parity information distribution is required for recovery.||Data is easily recoverable on condition that there is only one damaged disk. If more disks are damaged, recovery is impossible.|
|RAID6||Data is easily recoverable. Information about the disk order number, the block size and the method of Reed-Solomon distribution is required for recovery.||Data is easily recoverable on condition that damaged are one or two disks. If more disks are damaged, recovery is impossible.|
|Complex arrays||It’s required to perform reconstruction of each component of the complex array and then assemble an array of components (of an upper level).||Data is recoverable on condition of a sufficient number of components being in order. For instance, RAID50 needs each of RAID5 components working, even in a mode of limited functioning.|
Storage interpretation as a part of RAID
It is commonly known that every data storage has its own properties, such as type, size, model, retailer, etc. This information resides inside the disk and is usually programmed by the disk retailer with a slight possibility of any modifications.
If a disk becomes a part of a RAID, it receives a corresponding label from the RAID controller, whether the one of a NAS or of the motherboard. This label contains the information about the number of disks in the group, RAID configuration, stripe size, etc. If a user decides to utilize the disk from a RAID system, this label can remain there even after the disk was taken out the system and formatted. Despite being used in RAID, the disk may be ejected and successfully used for other purposes – as a basic hard drive of a computer, in another data storage medium, as a removable storage, etc.
In case when data recovery is required and the disk is opened in Recovery Explorer, the program scans its properties to define the type of a device. If the disks are or used to be a part of a RAID, Recovery Explorer would read their labels and suggest assembling the disks into a single system. At the same time, the program performs this assembly in the automatic mode as a response to the user's permission.
If you are sure that the disk to be recovered used to be a part of a RAID but no longer is, refuse the automatic RAID assembly offered by the program and ignore the warnings before the scan start. In contrast, if the disk was taken out of a RAID, you should make sure that all other disks – the components of this RAID – are also attached, and then assemble the RAID. You can do it manually or allow the program to perform this process automatically.
How can I recover data from a RAID?
As a RAID is a complex storage device that consists of several disks arranged into one logical system, it usually needs to be re-assembled before data recovery. Recovery Explorer automatically detects the metadata on disks which describes the RAID and assembles the disks into one virtual data storage. Still, if any of the metadata responsible for the RAID has been lost, we recommend using data recovery tools which specialize in recovery and reconstruction of complex RAID systems.
The process of data recovery from a RAID system includes the following steps:
Switch off the RAID;
Take out all the RAID disks;
Connect them to a stationary computer for recovery;
Start retrieving data with Recovery Explorer by switching to the Complex storages tab;
After the scanning is over, find and copy the recovered data to a safe place.
Manipulations with RAID
RAID systems are usually intended to store important information, the loss of which is undesirable. Therefore, users who utilize RAID systems need to know all the precautions to be taken to prevent it from happening.
- Are there enough coolers to cool the storage down?
Any hard drive gets hot during its operation, and if there are several of them, the negligence of cooling regulations and requirements may cause their breakdown. If the array overheats, it can fail, so you need to ensure adequate cooling. Before configuring the storage, you need to make sure that a working cooler is in and that there is enough space around the array and to prevent it from overheating.
- Use a UPS
If a power outage occurs during the process of writing data, then the data will fail to be written. Moreover, this increases the possibility of logical and mechanical disk failures. To eliminate this, use an uninterruptable power supply (UPS).
First, it will save the equipment from the effects of a voltage flicker. Second, in case of a power outage, the user will have time to save files and safely switch the equipment off.
- Take disks of different production groups
Most RAIDs, except RAID0, are optimized to work in the mode of limited functionality. If one disk (or up to two in RAID6) breaks down, the system will continue working with a reduced read/write speed.
If more than one disk (two disks) break down while the array is working in the degraded mode, the data will be lost. For this reason, it's recommended to arrange the array of disks of different production groups. In this way, you can reduce the likelihood of concurrent disk failures.
Disk order may be important
Each disk in a RAID has its sequence number and data is written and read in a certain order. Some controllers define the order of disks by the order in which they are connected. If the disk order is changed, depending on the configuration, there might occur some typical problems:
RAID0: the system is not able to initialize the disk data correctly because the block sequence is changed;
any RAID with a parity code: the system may not be able to understand the data. There is a high possibility that any further manipulation (e.g. data writing) will lead to the loss of some blocks on the messed-up disks. The recovery of this data requires an additional software application and specialized knowledge.
Most advanced RAID systems may define the order by themselves if the disks are connected in the right order. This can be helpful for diagnostics.
To format or not to format?
Let's assume that the user had to eject a drive from a RAID system and to connect it to a PC for some reason (e.g. diagnostics). In such a case, it's necessary to read the operating system messages carefully.
The system may offer to format the device. It's not recommended to accept this offer. The problem is that formatting may lead to the loss of valuable information located on the disk followed by a long and time-consuming recovery process.
Connecting and disconnecting disks
If the data storing system does not offer the possibility of a spare disk, it's required to disconnect it from a power supply before any physical operations with the RAID system. If you don't do that, incautious actions may lead to mechanical disk damages or logical data damages.
If you use the hot spare, please, be aware of the following precautions:
You cannot use this function for disks that are a part of a RAID which operates in the degraded mode;
Do not disconnect more than one disk at a time. After disconnecting a disk and replacing it, make sure that RAID rebuild is over. Only after that you can disconnect other disks.
Transportation of a RAID requires enhanced caution to prevent the disks from mechanical damages. Do not throw or shake the array. Pack it in antistatic plastic and aluminum foil and protect it against vibrations using a special package.
What is not allowed to do with RAID: checklist
Before you start any operations with RAID, please, get familiar with the following checklist:
1. Do not connect a RAID to a device which does not have a sufficient number of coolers.
2. Do not change the order of disks.
3. During the diagnostics of disks, read all the operating system messages.
4. Before performing any physical operations switch the disks off from the net.
5. Eliminate the mechanical damages of the disk.
6. Do not expose the array to vibrations and other loads during its transportation.