MicroNet Technology RAIDBank4 manual Glossary

D-Glossary

Logical Unit (LUN) a SCSI protocol entity which may be addressed by the actual input/ output (I/O) operations of a Logical Drive. Each SCSI-type target provides one or more logical units.

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 self-contained in the sense that it is capable of withstanding a disk failure within its group and reconstructing the data in the failed disk from parity information contained within that group.

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 plug-in PCI cards per bus. Since PCs can have two or more PCI buses, the number of PCI cards they can support are a multiple of four. The current PCI bus implementation (version 2.2) incorporates two 64-bit slots at 66 MHz. Consequently, the highest throughput achievable using such a bus is 528 MB/sec.

PCI-XAn enhanced version of PCI version 2.2. It supports one PCI slot per bus when running at 133 MHz, two slots when running at 100 MHz and four slots when running at 66 MHz. It is intended to provide throughputs in excess of 1 GB/sec using a 64-bit wide 133 MHz implementation.

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