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CompTIA® Server+ : Storage : Raid Technologies, Features And Benefits

3. Storage

3.1 Describe RAID technologies and its features and benefits

Important Disk RAID levels:

  1. a. RAID 0: Disk Striping without parity. Minimum 2 disks are required. RAID 0 doesn't provide any fault tolerance, but read/ write to the disk will improve

  2. b. RAID 1: Disk Mirroring and Disk Duplexing. Disk Mirroring requires at least two partitions of same size. Each partition should be on a different physical drive. In other words, minimum 2 disks are required. For disk duplexing minimum 2 disks and 2 controllers are required. RAID 1 provides good fault tolerance, though disk access is slower compared to disk striping

  3. c. RAID 2: Disk Striping with Error correction.

  4. d. RAID 3: Disk striping with error correction code stored as parity

  5. This takes a striped array as in RAID 0, then adds a parity hard disk drive to the array. This parity information is vital if one drive fails. If a drive should fail in this situation, the parity drive can be used to restore blocks that have been broken due to data corruption.

  6. e. RAID 4: Disk striping with large blocks allocation.

  7. f. RAID 5: Disk Striping with Parity. Disk Striping with Parity requires at least three partitions of same size, and each partition should be on a different physical drive. In other words, minimum 3 disks are required for disk striping with parity. Fault tolerant, less expensive than disk mirroring. If more than one disk fails, data can't be recovered. You still need to depend on tape back up for multiple disk failures.

  8. RAID 5 is similar to RAID 3, except the parity information is spread across all drives, this allows all drives to be able to rebuild the array if a drive fails. In RAID 3 parity is written to only one drive.

  9. Note: compared to disk striping or disk striping with parity; Disk mirroring has more overhead, as the entire disk get copied to another disk.

  10. RAID 10 describes Mirrored Striping. It is a mode 0 array, plus a mode 1 array, striped. For example, if you have striped data into two drives, each drive is mirrored in RAID 10. That is a total of 4 drives.

  11. RAID is utilized across several platforms: SCSI, FIBRE-SCSI and IDE are the most common implementations of RAID technologies.

Hardware RAID v/s Software RAID

The following are the benefits of hardware RAID:

  1. 1. Hardware RAID provides better performance compared to Software RAID.

  2. 2. Software RAID is usually limited to RAID levels 0, 1 and 5. It is possible to implement several other RAID combinations with Hardware RAID.

  3. 3. Hardware RAID allows multiple operating systems to access the RAID array, whereas Software RAID usually do not allow any other operating system other than the one used to create it.

  4. 4. Boot Volume Limitations: Since the operating system has to be running to enable the array, the operating system cannot boot from the RAID array! Therefore, it requires a separate, non-RAID partition to be created for the operating system.

  5. 5. One disadvantage of hardware RAID is that it is expensive.

Hot swapping/ hot spare:

A hot spare is a spare unit that is ready to take on the job of the main unit in the even of failure. Hot spare refers to the physical unit such as a disk drive or even a standby computer, such as in a clustered configuration. The failure may or may not be noticed by clients. For example, if the RAID system is configured with RAID 0, a failure may be noticed by the clients, whereas, if configured with RAID 5, the failure may not be noticed (in case of single point failure).

Hot-Swapping is the ability to add and remove devices to a computer while the computer is running and have the operating system automatically recognize the change. Hot plugging is also called hot swapping

Cache types

  • Write-Through cache, each write operation to the cache is accompanied by a write operation to the main memory. That means, the data is written to the cache and the main memory at the same time.

  • With Write-Back cache, the CPU writes to the cache first. But the actual updating of main memory takes place at a later time.

3.2 Given a scenario, select the appropriate RAID level

RAID benefits:

  1. 1. Higher Data Security: Most RAID levels provide data protection by using redundancy which means even if one hard disk fails completely it will not require any data to be restored from backs. Except RAID level 0 all levels provide some degree of data protection.

  2. 2. Fault tolerance: Redundancy provide reliable overall storage so there are very less chances of entire storage system failing due to any hardware failure.

  3. 3. High Availability: RAID allow recovery from hardware fault without any disruption and high fault tolerance makes availability of data much easier.

  4. 4. High capacity :It turns number of smaller drives into a larger array combining their capacity and increasing integrated capacity.

  5. 5. Improved performance: RAID systems improve performance by allowing the controller to exploit the capabilities of multiple hard disks to get around performance-limiting mechanical issues that plague individual hard disks

3.3 Install and configure different internal storage technologies

LUN stands for Logical Unit Number. LUN is used to identify a logical device that is an independent functional part of a SCSI device. SCSI-2 specification allows 8 logical units for each SCSI device address. The logical unit addresses are numbered 0 to 7.

SCSI Type Transfer speed Bus
SCSI-1 5MBPS 8 bit bus
Fast Wide SCSI 20MBPS 16 bit bus
Wide Ultra SCSI 40MBPS 16 bit bus
Ultra2 SCSI 40MBPS 8 bit bus
8 bit bus 80MBPS 16 bit bus
Ultra3 SCSI or Ultra 160 160MBPS 16 bit bus
Ultra320 320MBPS 16 bit bus

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