RAID is an acronym for Redundant Array of Inexpensive Disks, or Redundant Array of Independent Disks.
RAID
RAID was developed to improve hard drive storage reliability and performance. Originally RAID systems consisted of clustering small, "inexpensive"
disk drives together into an array so that the array would appear to the system as a single logical drive. During initial testing, it was discovered that
an array of drives actually delivers performance exceeding that of single, more expensive hard drives.
The Mean Time Before Failure (the average time before a failure will occur) in a RAID, was reduced due to the probability of any one drive in the array failing.
Consequently, five levels of RAID were originally developed to provide a balance between performance and data protection.
RAID1
The furnace computer utilizes RAID1 disk mirroring which provides data protection by duplicating all data from a primary drive on a secondary drive. The benefit
of this system is it offers the highest data protection.
Figure 1. RAID1, data from one hard drive is mirrored onto a second drive
Since RAID 1 employs the mirroring technique, it offers the advantage of 100% redundancy. If one disk fails, rebuild the lost data from the mirror.
RAID 1 requires at least 2 hard drives, and additional hard drives must always be added in pairs. It is ideal for applications where uptime is important and/or
data is critical.
In RAID 1 (mirroring without parity or striping), data is written identically to multiple disks (a "mirrored set"). While any number of disks
may be used, the furnace system requires only two. The array continues to operate as long as at least one drive is functioning. With appropriate operating
system support, there can be increased read performance, and only a minimal write performance reduction. Implementing RAID 1 with a separate controller for
each disk in order to perform simultaneous reads (and writes) is sometimes called multiplexing (or duplexing when there are only 2 disks).
As shown in Figure 1, in a RAID 1 array, data from one hard drive is mirrored onto a second hard drive, so that there are two identical copies of the data. Data
from a third drive is mirrored to a fourth drive. RAID 1 drives must always be added in pairs.
Verifying That RAID Is Working on a Dell Furnace Computer
Your computer displays information pertaining to your RAID configuration at start-up, before loading the operating system. If RAID is not configured, the
message none defined appears under RAID Volumes, followed by a list of the physical drives installed in your computer. If a RAID volume is identified, you can then
check the Status field to determine the current state of your RAID configuration. The Status field contains information about the following conditions:
Normal — Your RAID configuration is functioning properly.
Degraded — One of your hard drives has failed. The computer is still bootable; however, RAID is not functioning and data is not being copied to the
other drive.
Rebuild — Following a degraded condition, the computer has detected the replacement/connection of a secondary hard drive and will automatically restore
the RAID configuration the next time the operating system loads.
RAID Data Recovery
RAID Rebuilding
With their built-in redundancy, RAID systems are able to continue functioning even if a hard drive fails. When this happens however, performance is negatively
affected, and the RAID is said to be operating in a degraded, or critical state. This occurs because the lost information must be regenerated "on the fly"
from the parity data.
When a failed drive is replaced, the data that was removed from the array with its departure must be regenerated on the new disk by the RAID controller, a process
called rebuilding. An array is vulnerable while it is running in a degraded state. Until the failed drive is replaced and its data rebuilt, the array provides no
redundancy. Consequently, it's not a good idea to put off rebuilding a degraded array; you're putting your data at risk!
RAID Data Recovery Process
RAID systems are susceptible to the same ailments that plague single hard drives, such as viruses, logical problems, human error, and physical damage. Moreover,
due to their complexity, they may suffer from additional points of failure, such as lost server registry configurations, accidental RAID drive reconfigurations,
RAID controller failures and multiple drive failures.
If multiple drives fail, or other serious problems occur in a RAID, your data may be compromised.
Under such circumstances, if you fail to make a proper backup, you may have to call a data recovery company. Many have high success rates when it comes to RAID
data recovery. Using specialized facilities, equipment, and software, they can even recover data from a moribund hard drive.
Recovering From a Single Hard Drive Failure Using the Intel Matrix Storage Manager on a RAID1 Dell Furnace Computer
NOTE: Perform the following steps only after you have replaced the failed hard drive
(see the appropriate "Drives" section for your computer). Remember to check the warranty. If the system is still in warranty contact Dell with the
Service Tag and/or Express Service Code of the furnace computer. Dell will provide next day on-site installation of the failed drive.
1. Turn on or restart your computer.
2. Press <Ctrl><i> when you are prompted to enter the Intel RAID Option ROM utility.
3. Under DEGRADED VOLUME DETECTED, confirm that the new (non-RAID) drive is listed and then press <Enter>.
4. Under Disk/Volume Information confirm that the volume status is Rebuild.
NOTE: Volumes
with a status of Rebuild are rebuilt withing the operating system.
5. Use the up- and down-arrow keys to select Exit, and then press <Enter>.
Your computer boots to the operating system and begins rebuilding the RAID volume automatically. A dialog box appears and displays the progress
of the rebuild.
NOTE: You can use your computer while the computer is rebuilding the
RAID level 1 volume.
RAID Levels
Typical features of most common RAID levels are depicted in Table 1.
Block-level striping with double distributed parity.
When more than one drive fails, the array is compromised.
RAID TERMS
Striping
Striping is a technique which offers the best performance of any RAID configuration. In a striped array, data is interleaved across all the drives
in the array.
Striping works by splitting up the data and distributing it across multiple drives to increase performance. Performance in a striped array
is dependent on the stripe width (the number of drives in the array) and the stripe size (the size of the chunks of data being written across
the array).
Striping can occur at two different levels: byte level and block level. Byte level striping involves breaking up the data
into bytes and storing them sequentially across the hard drives. Block level striping involves breaking up the data into a given block size.
These blocks are then distributed in the same way across the array as in byte level striping.
The stripe size should recommend to bring the most performance from a RAID system depends on the type of application you're using it for. Larger
stripes mean fewer accesses to the disk. For this reason, larger stripes are useful for I/O-intensive (Input/Output) applications such as database servers. Smaller
stripes on the other hand, mean that data can be accessed more quickly because data chunks are smaller. Consequently, smaller stripes are better suited for throughput-intensive
applications such as video production and editing.
Mirroring
Although a striped array may offer the best performance of any RAID configuration, it provides no redundancy. If one drive in the array fails, all
of your data will be lost and you may need to consider RAID data recovery options. That's where mirroring comes in. With mirroring, whatever
is written to one drive, gets written simultaneously to another. Thus, there is always have an exact duplicate of your data on the second drive. This is one
of the two data redundancy techniques used in RAID to protect you from data loss. The advantage of this technique is that when one hard drive in the array fails,
the system can still continue to operate since there are two copies of the data. Downtime is minimal and rebuilding data from the good copy is relatively easy.
Mirroring also provides a small performance boost over a single non-arrayed drive. Since the mirrored pairs contain the same data, the RAID controller
can read data from one drive while simultaneously requesting data from the other. Of course, write speeds will be slower than with other techniques because data
must be written twice, once on each drive.
Parity
Parity is an error correction technique commonly used in certain RAID levels. It is used to reconstruct data on a drive that has failed in
an array. The RAID controller adds a parity byte to all binary information being written to the array which is an extra byte of data added onto the actual
data. These parity bytes are summed by the controller to equal either an even or an odd number. By analyzing this value, the controller can determine whether
the information has been compromised in any way. To the extent it has, it can replace the data automatically with data from the other drive.
Parity data is created using a logical operation called eXclusive OR (XOR). The controller analyzes the series of 0's
and 1's which make up the data, and returns either a TRUE (for even numbers) or FALSE (for odd ones). By using this data, it can "fill in the holes. The
XOR logic is used in this way to rebuild corrupted data on the array, thus maintaining integrity.
