RAID stands for Redundant Array of Inexpensive (or sometimes "Independent") Disks. RAID is a method of combining multiple hard disks in a single logical unit to offer high availability, performance or a combination of both. This provides better resilience and performance than a single disk drive.
benefits of RAID
software RAID
Many operating systems provide functionality for implementing software based RAID systems . The software RAID systems generate the RAID algorithms using the server CPU, this can severely limit the RAID performance. Should a server fail the whole RAID system is lost. Cheap to implement and only need a single SCSI controller.
hardware RAID
All RAID algorithms are generated on the RAID controller board, thus freeing the server CPU. Allows full benefits and data protection of RAID. More robust and fault tolerant than software RAID. Requires dedicated RAID controller to work.
Various RAID levels exist these are:
| 0 | 1 | 2 | 3 | 4 | 5 | 6 | 10 | 50 | 0+1 |
The levels of RAID protection varies with the RAID level selected RAID levels 0 is not technically RAID as they have no redundancy in the event of drive failure.
JBOD Subsystems
These JBOD subsystems are high-density storage enclosures that can be cond to specific applications including changes to disk drive form factor and number of drives per enclosure.
This includes configurations of 8, 12, 14, 16 disk drives per enclosure.
RAID Features:
Write Through Cache
With Write Through Cache the data is written to both the cache and drive once the data is retrieved.
As the data is written to both places, should the information be required it can be retrieved from the cache for faster access.
The downside of this method is that the time to carry out a Write operation is greater the time to do a Write to a non cache device. The total Write time is the time to write to the cache plus the time to Write the disk.
Write Back Cache
With Write Back Cache the write operation does not suffer from the Write time delay.
The block of data is initially written to the cache, only when the cache is full or required is the data written to the disk.
The limitation of this method is that the storage device for a period of time does not contain the new or updated block of data.
If the data in the cache is lost due to power failure the data cannot be recovered.
When using Write Back Cache a battery backup module would prevent data loss in a RAID power failure.
Battery Backup
Provides data recovery in the event of power failure. Should the RAID controller fail the battery backup module can be transported to the replacement RAID controller and everything will continue.
A downside of battery backup modules is they loose their capacitance over time, need replacing every 12-24 months, add to the RAID cost and only hold the information for up to 72 hours.
Hot Swap
Whenever a RAID system mentions hot swap, the components can replaced while the RAID continues to operate.
Online Hot Spare
Should a drive fail within the RAID it will automatically utilise the hot spare and carry out a RAID re-build.
These can be of two types a). Local hot spare is available only to a specified RAID set.
b). Global hot spare can be available to multiple RAID sets.
Read Ahead Caching
A buffering technique used by hard disk drives and other disk access devices, in which extra data beyond that requested by the system is read and stored in cache memory.
There is a strong chance, especially when dealing with sequential data, that this subsequent information will also be requested by the computer.
Reading from cache memory is much faster than reading from the disk or media, so read-ahead caching speeds increase overall system performance to a degree. Also called look-ahead caching.
Online Capacity Expansion
The primary reason for Online Capacity Expansion is that it allows disk drives to be added to RAID systems whilst operating.
These disk drives can then be used to grow the overall RAID capacity, without taking anything offline.
The traditional method would be to backup the information and then destroy the RAID set and build a new RAID from scratch.
SAF-TE
This specification defines a set of SCSI commands for setting drive status information, including status for RAID arrays, into a disk drive array enclosure.
The drive array enclosure may be a separate enclosure, or the same enclosure.
The specification also defines commands for managing hot-swap drive slots and returning environmental health information for a drive enclosure.
The status commands are typically used by the enclosure manufacturer to assert lights or other indicators that provide information to the user about the state of the drives in the array.
This can include status such as 'rebuilding', 'fault', and 'hot spare'. The SAF-TE status setting commands are typically issued either by an intelligent disk controller, or by software, e.g. RAID software , running under the operating system.
Other parties on the SCSI bus may elect to access the status information as a means of determining the state of the physical drives in the array.
In addition, SAF-TE commands can be used to report certain environmental information about the enclosure, such as temperature, voltage, power supply, and fan health.
SMART
SMART - Self-Monitoring Analysis and Reporting Technology
The fundamental principle behind SMART is that many problems with hard disks don't occur suddenly.
They result from a slow degradation of various mechanical or electronic components.
SMART evolved from a technology developed by IBM called Predictive Failure Analysis or PFA. PFA divides failures into two categories: those that can be predicted and those that cannot. Predictable failures occur slowly over time, and often provide clues to their gradual failing that can be detected.
An example of such a predictable failure is spindle motor bearing burnout: this will often occur over a long time, and can be detected by paying attention to how long the drive takes to spin up or down, by monitoring the temperature of the bearings, or by keeping track of how much current the spindle motor uses.
An example of an unpredictable failure would be the burnout of a chip on the hard disk's logic board: often, this will "just happen" one day.
Clearly, these sorts of unpredictable failures cannot be planned for.
RAID Stripe
As a rule a Stripe size can be anything from 2k to 512k or greater (depends on controller support).
There is no rule to the recommended Stripe size, it depends entirely on the application and performance needs.
For example large numbers of small reads and writes are probably better off with small stripe sizes, applications where smaller numbers of larger files need to be read quickly will likely prefer large stripes.
At the end of the day it is trial and error.
RAID Rebuild
This is when a RAID system suffers a drive. During a RAID re-build performance is degraded, in order for it to work out where the Data and Parity bit should be written.
RAID 0
Characteristics & Advantages
RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive I/O performance is greatly improved by spreading the I/O load across many channels and drives
Best performance is achieved when data is striped across multiple controllers with only one drive per controller
No parity calculation overhead is involved
Very simple design
Easy to implement
Disadvantages
Not a "True" RAID because it is NOT The failure of just one drive will result in all data in an array being lost Should never be used in mission critical environments Recommended Applications For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair. RAID Level 1 requires a minimum of 2 drives to implement Characteristics & Advantages One Write or two Reads possible per mirrored pair Twice the Read transaction rate of single disks, same Write transaction rate as single disks 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk Transfer rate per block is equal to that of a single disk Under certain circumstances, RAID 1 can sustain multiple simultaneous drive failures Simplest RAID storage subsystem design Disadvantages Highest disk overhead of all RAID types(100%) - inefficient Typically the RAID function is done by system software, loading the CPU/Server and possibly degrading throughput at high activity levels. Hardware implementation is strongly recommended May not support hot swap of failed disk when implemented in "software" Recommended Applications Each bit of data word is written to a data disk drive (4 in this example: 0 to 3). Each data word has its Hamming Code ECC word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors. Characteristics & Advantages "On the fly" data error correction Extremely high data transfer rates possible The higher the data transfer rate required, the better the ratio of data disks to ECC disks Relatively simple controller design compared to RAID levels 3,4 & 5 Disadvantages Very high ratio of ECC disks to data disks with smaller word sizes - inefficient Entry level cost very high - requires very high transfer rate requirement to justify Transaction rate is equal to that of a single disk at best (with spindle synchronization) No commercial implementations exist / not commercially viable The data block is subdivided ("striped") and written on the data disks. Stripe parity is generated on Writes, recorded on the parity disk and checked on Reads.
RAID Level 3 requires a minimum of 3 drives to implement Characteristics & Advantages Very high Read data transfer rate Very high Write data transfer rate Disk failure has an insignificant impact on throughput Low ratio of ECC (Parity) disks to data disks means high efficiency Disadvantages Transaction rate equal to that of a single disk drive at best (if spindles are synchronized) Controller design is fairly complex Very difficult and resource intensive to do as a "software" RAID Recommended Applications Each entire block is written onto a data disk. Parity for same rank blocks is generated on Writes, recorded on the parity disk and checked on Reads. RAID Level 4 requires a minimum of 3 drives to implement. Characteristics & Advantages Very high Read data transaction rate Low ratio of ECC (Parity) disks to data disks means high efficiency High aggregate Read transfer rate Disadvantages Quite complex controller design Worst Write transaction rate and Write aggregate transfer rate Difficult and inefficient data rebuild in the event of disk failure Block Read transfer rate equal to that of a single disk Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads. RAID Level 5 requires a minimum of 3 drives to implement. Characteristics & Advantages Highest Read data transaction rate Medium Write data transaction rate Low ratio of ECC (Parity) disks to data disks means high efficiency Good aggregate transfer rate Recommended Applications Disadvantages Disk failure has a medium impact on throughput Most complex controller design Difficult to rebuild in the event of a disk failure (as compared to RAID level 1) Individual block data transfer rate same as single disk. Two independent parity computations must be used in order to provide protection against double disk failure. Two different algorithms are employed to achieve this purpose. RAID Level 6 requires a minimum of 4 drives to implement. Characteristics & Advantages RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (dual parity) Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives; RAID 6 provides for an extremely high data fault tolerance and can sustain multiple simultaneous drive failures Perfect solution for mission critical applications Recommended Applications Disadvantages More complex controller design Controller overhead to compute parity addresses is extremely high Write performance can be brought on par with RAID Level 5 by using a custom ASIC for computing Reed-Solomon parity Requires N+2 drives to implement because of dual parity scheme RAID Level 10 requires a minimum of 4 drives to implement Characteristics & Advantages RAID 10 is implemented as a striped array whose segments are RAID 1 arrays RAID 10 has the same fault tolerance as RAID level 1 RAID 10 has the same overhead for fault-tolerance as mirroring alone High I/O rates are achieved by striping RAID 1 segments Under certain circumstances, RAID 10 array can sustain multiple simultaneous drive failures Excellent solution for sites that would have otherwise gone with RAID 1 but need some additional performance boost Disadvantages Very expensive / High overhead All drives must move in parallel to proper track lowering sustained performance Very limited scalability at a very high inherent cost Recommended Applications RAID Level 50 requires a minimum of 6 drives to implement Characteristics & Advantages RAID 50 should have been called "RAID03" because it was implemented as a striped (RAID level 0) array whose segments were RAID 3 arrays(during mid-90s) Most current RAID 50 implementation is illustrated above RAID 50 is more fault tolerant than RAID 5 but has twice the parity overhead High data transfer rates are achieved thanks to its RAID 5 array segments High I/O rates for small requests are achieved thanks to its RAID 0 striping Maybe a good solution for sites who would have otherwise gone with RAID 5 but need some additional performance boost Disadvantages Very expensive to implement All disk spindles must be synchronized, which limits the choice of drives Failure of two drives in one of the RAID 5 segments renders the whole array unusable RAID Level 0+1 requires a minimum of 4 drives to implement Characteristics & Advantages RAID 0+1 is implemented as a mirrored array whose segments are RAID 0 arrays RAID 0+1 has the same fault tolerance as RAID level 5 RAID 0+1 has the same overhead for fault-tolerance as mirroring alone High I/O rates are achieved thanks to multiple stripe segments Excellent solution for sites that need high performance but are not concerned with achieving maximum reliability Disadvantages RAID 0+1 is NOT to be confused with RAID 10. A single drive failure will cause the whole array to become, in essence, a RAID Level 0 array Very expensive / High overhead All drives must move in parallel to proper track lowering sustained performance Very limited scalability at a very high inherent cost Recommended Applications Call us now We would welcome the opportunity to discuss your requirements further. If we can be of any assistance to you, please contact us or one of our ad sponsors. :: :: :: :: RAID 2
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