C-Glossary
Logical Drive A logical drive is comprised of spaces from one or more physical disks and presented to the operating system as if it were one disk.
Logical Unit (LUN) a SCSI protocol entity which may be addressed by the actual input/output (I/O) operations of a Logical Drive. Each
Native Command Queuing (NCQ) a technology designed to increase performance of SATA hard disks by allowing the disk firmware to internally optimise the order in which read and write commands are executed. This can result in increased performance for workloads where multiple simultaneous read/write requests are outstanding, which occurs most often in server- type applications.
Online Capacity Expansion The ability to add space to an existing RAID array within a session while preserving the RAID type and data within the array is known as online capacity expansion. The availability of this feature enables the user to add space to a RAID array as and when required without rebooting, thereby obviating the need for precise forecasts of capacity requirements for the future.
Parity A mathematical function that serves as a method for error verification and correction. In strict technical terms the parity of a group is set to 1 if the number of bits in the group that are set to 1 is odd, and 0 otherwise. For instance, the parity of N bytes of data is obtained by determining the number of ith bits in the N bytes that are set to 1. If that number is odd, then the ith bit of the result is set to 1. This may sound complicated, but in reality the result can be obtained by simply evaluating the XOR of the N bytes. Parity allows one error in a group (of bytes) to be corrected.
Parity Group Complex RAID types such as RAID 10 or RAID 50 are built using two levels of hierarchy. For instance, consider a RAID 50. A RAID 50 array is comprised of a group of RAID 5 arrays at the first tier. Each RAID 5 array in the first tier is used just like a hard disk in creating a RAID 0 at the next tier. The result is a RAID 50. In this example, each RAID 5 array at the first tier is denoted as a parity group. Each parity group is
Partition The space contributed to each array on a physical drive is referred to as a partition.
PCI An acronym for “Peripheral Component Interconnect”. It is Intel’s local bus standard that supports up to four
PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe (or
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