More RAID Levels and RAID Data Recovery
RAID 3
RAID 3 has a lot in common with its younger brother RAID 2 in that
it also uses byte level striping
and a dedicated parity disk. Where the siblings part company however,
is in their error correcting methods. While RAID 2 uses Hamming
code ECC, RAID 3 uses the more effective XOR
algorithm to generate parity.
Unlike the previous levels we've seen, RAID 3 is a practical solution
that delivers good performance and fault tolerance. The dedicated
parity disk does slow down write speeds though, because the parity
information has to be written to the parity drive whenever a write
occurs. RAID data recovery however, is not as big an issue with this
implementation.
RAID 3 requires at least 3 hard drives.
Figure 3. Under RAID 3, data is striped at the byte level, across
multiple disks. The parity information is sent to a dedicated parity
disk, but the failure of any disk in the array can be tolerated.
RAID 4
RAID 4 is very similar to RAID 3. In fact, it's so similar that
people often confuse the two. There is one major difference between
them however: RAID 4 uses block
level striping. The advantage of block level striping is that
you can change the stripe size to suit your application needs.
RAID 4 requires at least three hard drives. Like RAID 3, it offers
good performance and fault tolerance, and RAID data recovery isn't as
much of a concern. The dedicated parity disk however, remains the
bottleneck.
Diagram 4. RAID 4 improves performance by striping data across
many disks in blocks. It provides fault tolerance through a dedicated
parity disk.
RAID 5
The most popular member of the RAID family, RAID 5 combines block
level striping with distributed parity for good performance, fault
tolerance and storage efficiency. This level minimizes the write
bottlenecks of RAID levels 3 and 4, by distributing parity stripes
over a series of hard drives. In doing so, it provides relief to
the concentration of write activity on a single drive, which in
turn enhances overall system performance.
RAID 5 is often used as an all-purpose RAID solution, but it is
also used for database and file server applications.
RAID 5 requires a minimum of three hard drives, but often costs
less to implement than RAID 3 or 4. RAID recovery may be necessary
if more than one disk fails.
Diagram 5. In RAID 5, data and parity information are striped
in blocks across all the drives in the array. Fault tolerance is
maintained by ensuring that the parity information for any given
block of data is placed on a separate drive from those used to store
the data itself.
Below is a summary of some of the features of the RAID levels we've
discussed. There are of course, many
more RAID levels that exist, which aren't listed.
Table 1. RAID Level Summary
Level |
Techniques |
Description |
Min. Drives |
Failure Conditions |
Pros/Cons |
Uses |
RAID O |
Disk striping
(no fault tolerance) |
Data is broken
into stripes which
are sent to each disk in the array. |
2 |
When one drive
fails, the entire array is compromised. |
Offers Best performance
No fault tolerance. |
Video editing
and production |
RAID 1 |
Disk mirroring |
Data on one drive
is mirrored on another. |
2 |
If one drive fails,
data is not lost. If both drives fail, the data is lost. |
100% redundancy
of data/Slower performance and 50% loss of storage space. |
Accounting, payroll,
financial |
RAID 2 |
Byte level striping
with Hamming code ECC |
Data is split
at the bit level over a number of data and ECC disks. |
Up to 14+ |
Only one drive
may fail and still be recoverable "on the fly". |
On the fly data
error correction/Extremely high cost. |
No commercial
uses |
RAID 3 |
Byte level striping
with dedicated parity |
Data is striped
at the byte-level, across multiple disks. |
3 |
When more then
one drive fails, the array is compromised. |
High read/write
data transfer rates/Complex controller design |
Image and video
editing |
RAID 4 |
Block level striping
with dedicated parity |
Data is striped
in blocks across data disk, with parity store on a separate
disk. |
3 |
When more then
one drive fails, the array is compromised. |
High Read/Low
Write data transaction rates. |
General purpose |
RAID 5 |
Block level striping
with distributed parity |
Data and parity
are striped in blocks across all disks. |
3 |
When more then
one drive fails, the array is compromised. |
High Read data
transaction rates/ Complex controller design |
Web, database
or file servers |
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RAID Data Recovery
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