RAID – Redundant Array of Independent Disks
- The storage which we are used in laptop computers are single disk but in servers, data centers, cloud computing we used multiple disk, means RAID technology.
- RAID saves data in multiple disk.
- RAID is a set of multiple hard disk, which are in the network.
- As additional memory in form of cache, can improve system performance, in the same way additional disks can also improve system performance.
- In RAID, we use an array (multiple) of disks. These disks operate independently.
- Since there are many disks, separate and multiple I/O requests can be handled in parallel if the data required is on separate disks.
- A single I/O operation can be handled in parallel if the data required is distributed across multiple disks
Benefits of RAID:
- RAID technology prevents data loss due to disk failure.
- RAID technology can be implemented in hardware or software.
- Servers make use of RAID technology.
- To prevent fail of Operating system RAID technology is beneficial.
There are 7 levels of RAID schemes.
- RAID 0,
- RAID 1,
- RAID 2,
- RAID 3,
- RAID 4,
- RAID 5,
- RAID 6.
Out of above 7 RAIDs, RAID Level 2, 3 and 4 are much complicated and outdated.
RAIDs 0,1,5 and 6 are used in servers now a days.
The common characteristic in all these levels is:
- A set of physical disk drives.
- The operating system views these separate disks as a single logical disk.
- Data is distributed across the physical drives of the array.
- Redundant disk capacity is used to store parity information.
- Parity information can help in recovering data in case of disk failure
PARITY CHECK: A parity check is the process in which accurate data transmission between nodes during communication is checked.
HOW TO PARITY CHECK:
- A parity bit is appended to the original data bits to create an even or odd bit number; the number of bits with value one.
- The source then transmits this data via a link to the destination.
- Transferred bits are checked and verified at the destination.
- If number of bits at destination are same as the number of bits at source than data is considered as accurate.
RAID Level 0:
- RAID 0 implementation requires minimum 2 disks.
- There is no redundancy in RAID 0.
- RAID level 0 divides data into block units and writes them across a number of disks.
- As data is placed across multiple disks, it is also called “data striping”.
- There is no parity checking of data.
- So if data in one drive gets corrupted then all the data would be lost. Thus RAID 0 does not support data recover.
- Spanning is another term that is used with RAID level 0 because the logical disk will span all the physical drives.
RAID Level 0- Advantages:
- Data process is very fast.
- Throughput (speed) is increased because :
- Multiple data requests probably not on same disk.
- Disks seek in parallel.
- A set of data is likely to be striped across multiple disks.
- Implementation is easy.
- No overhead of parity calculation
RAID Level 0 –Disadvantages.
- Not provide fault tolerant.
- The failure of just one drive will result in all data in an array being lost.
- Should not be used in mission critical environments
- No backup facility.
RAID LEVEL 1:
- The other name of RAID Level 1 is “mirroring” as it copies data onto two disk drives simultaneously.
- Minimum two hard disk are required.
- As same data is placed on multiple disks, it is also called “data mirroring”
- No striping its uses on mirroring.
- The automatic duplication of the data means there is little likelihood of data loss or system downtime.
- Data striping is used as in RAID 0, but in RAID 1 each logical strip is mapped to two separate physical drives
- Thus every disk in the array has a mirror disk that contains the same data
- Data can be read from either disk but is written on both disks
- A read request can be executed by either of the two disks
- A write request means that both the disks must be updated. This can be done in parallel
- There is no overhead of storing parity information
- Recovery from failure is simple. If one drive fails we just have to access data from the second drive
- Generally RAD 1 is used for Operating system.
RAID Level 1 – Advantages:
- Main advantage is RAID 1 provides fault tolerance.
- Provides data redundancy.
- If one disk fails, the other automatically takes over, so continuous operation is maintained.
- RAID 1 is used to store systems software (such as drivers, operating systems, compilers, etc) and other highly critical files.
RAID Level 1 – Disadvantages:
- Main disadvantage is cost. Since data is duplicated, storage costs increase.
RAID Level 2:
- In RAID 2 mechanism, all disks participate in the execution of every I/O request.
- The spindles of individual disk drives are synchronized so that each disk head is in the same position on each disk at any given time.
- Data striping is used.
- Error correcting code is also calculated and stored with data
- Not implemented in practice due to high costs and overheads
RAID Level 3:
- RAID 3 required minimum 3 disk.
- RAID 3 perform Byte-level striping with dedicated parity.
- Data is divided into byte units and written across multiple disk drives.
- Parity information is stored for each disk section and written to a dedicated parity drive.
- All disks can be accessed in parallel Data can be transferred in bulk. Thus high speed data transmission is possible
- In case of drive failure, the parity drive is accessed and data is reconstructed from the remaining devices.
- Once the failed drive is replaced, the missing data can be restored on the new drive
- RAID 3 can provide very high data transfer rates
RAID Level 4:
- RAID Level 4 is same as RAID Level 3.
- RAID 4 required minimum 3 disk.
- RAID 4 perform block-level striping with dedicated parity.
RAID Level 5:
- RAID 5 is very important RAID, which is used in servers, datacenters or large computing centers.
- RAID 5 required minimum 3 disk.
- RAID 5 perform Byte-level striping with distributed parity.
- RAID 5 include striping and mirroring.
- Minimum 3 hard disk are required in RAID 5.
- 1 hard disk is used to store parity bits, when data gets corrupted.
- RAID 5 also do error checking which is also known as parity checking.
How RAID 5 do error checking?
When data in a disk gets corrupted or errored, than what the content that errored hard disk contain gets copied into the new hard disk, which we replaced.
RAID Level 6:
- RAID 6 is same concept of RAID 5, but its have a extra parity bit storage than RAID 5.
- RAID 6 required minimum 4 disk.
- RAID 6 perform Byte-level striping with double distributed parity.
- In RAID 6, backup of parity storage will get store in another storage too. That means there are 2 parity backup