Overview of SCSI and RAID Setups

The SCSI (Small Computer Systems Interface) enables to attach hardware peripherals to the system. The SCSI systems contains SCSI controller that communicates with the system. The SCSI cables are used to connect SCSI drives to the SCSI controller and the SCSI devices. They are mainly used to connect hard disk and tape storage devices.

Working with SCSI drives

SCSI devices transfer data between the device and the system at greater speeds. This ensures that you can fully utilize the increased speeds that the device can reach. SCSI enables you to connect multiple hard disks together to form a single unit by implementing RAID. You can connect a device to the SCSI chain without switching OFF the system, this enables you to attach and remove devices such as external hard disks from the system while the system is ON.

Redundant Array of Inexpensive Disks (RAID)

RAID is a technology that uses collection of hard disks to share and replicate data. The operating system recognizes multiple hard drives as a single logical unit. RAID is performed on multiple hard drives of identical sizes. But different sizes of disks can be used. RAID increases the data retrieval speed as the system reads different blocks of data from two different disks. It also keeps your data safe by enabling you to recover data lost due to hard disk crashing.

RAID Level 0

RAID 0 splits data across two or more disks. RAID 0 increases the performance of the disks. Data is written and retrieved from the hard drive in an efficient manner. Data is broken into blocks and each block is written to a separate disk. It is known as stripe sets because data is striped or distributed, across all the drives in the array. It is non-fault tolerant.

RAID Level 1

This level uses mirroring. One disk stores data whereas mirror disk stores the copy of this data. This ensures that if one disk fails the data can be retrieved from the mirrored disk. This technology is known as disk mirroring. In this method, one disk acts as a mirror disk, that is, it stores copy of data from the other disk. The total disk space available is half of the total capacity of the disks.
RAID Level 3:
RAID 3 stripes the data into blocks of bytes. Parity stripes are generated while writing data onto the disk and are checked while reading them from the disk. This method needs a minimum of 3 disks.

RAID Level 5

This is frequently used method to RAID hard disks. In this level data as parity information is stored on multiple disks. If one drive fails, the failed disk can be recreated after it is replaced. It is preferable to use hard disks of same storage capacity to implement RAID level 5. Available disk space is the sum of the size of all disks minus the size of one disk. RAID level 5 requires a minimum of 3 disks to be implemented

Linear RAID

Linear RAID combines multiple hard disks together to form one drive. While storing data, it first fills the first hard disk in the series and after the first disk is full it stores data to the next disk in the series. This RAID level performs no benefit, as it is unlikely that any I/O operations will be split between member drives. It also offers no redundancy and, in fact, decreases reliability —- if any one member drive fails, the entire array cannot be used. The capacity is the total of all member disks. It is sometimes referred to as Spanned volume.

Hot Swappable drives

Hot swap drives enable you to connect and disconnect devices to the system without shutting down or restarting the system. To connect the device to the system, you just plug the device into the system. The operating system automatically recognizes the device, assigns it a SCSI ID and loads the driver required by the system to communicate with the device. Hot swap drives are ideal for use on servers that must ideally not be shutdown.