Dell PERC 4/SI - Manual (Page 72)

PERC 4e/Di provides reliability, high performance, and fault-tolerant disk subsystem management. It is an ideal RAID solution for the internal storage of Dell's
workgroup, departmental, and enterprise systems. PERC 4e/Di offers a cost-effective way to implement RAID in a server and provides reliability, high
performance, and fault-tolerant disk subsystem management.
Physical Disk
A hard drive. A hard drive consists of one or more rigid magnetic discs rotating about a central axle, with associated read/write heads and electronics. A
physical disk is used to store information, (data), in a non-volatile and randomly accessible memory space.
Physical Disk Roaming
The ability of adapters to detect when hard drives have been moved to different slots in the storage enclosure, such as after a hot swap.
Protocol
A set of formal rules describing how to transmit data, generally across a network or when communicating with storage sub-systems. Low-level protocols define
the electrical and physical standards to be observed, bit- and byte-ordering, and the transmission and error detection and correction of the bit stream. High-
level protocols deal with the data formatting, including the message syntax, the terminal to system dialogue, character sets, sequencing of messages, etc.
RAID
Redundant Array of Independent Disks (originally Redundant Array of Inexpensive Disks) is an array of multiple small, independent hard drives that yields
performance exceeding that of a Single Large Expensive Disk (SLED). A RAID disk subsystem can improve I/O performance relative to a system using only a
single drive. The RAID array appears to the controller as a single storage unit. I/O is expedited because several disks can be accessed simultaneously.
Redundant RAID levels (RAID levels 1, 5, 10, and 50), provide data protection.
RAID Level Migration
RAID level migration (RLM) changes the array from one RAID level to another. It is used to move between optimal RAID levels. You can perform a RLM while the
system continues to run, without having to reboot. This avoids downtime and keeps data available to users.
RAID Levels
A style of redundancy applied to an array. It can increase the performance of a logical drive though it may decrease usable capacity. Each logical array must
have a RAID level assigned to it.
Read-Ahead
A memory caching capability in some adapters that allows them to read sequentially ahead of requested data and store the additional data in cache memory,
anticipating that the additional data will be needed soon. Read-Ahead supplies sequential data faster, but is not as effective when accessing random data.
Ready State
A condition in which a workable hard drive is neither online nor a hot spare and is available to add to an array or to designate as a hot spare.
Rebuild
The regeneration of all data to a replacement disk from a failed disk in a logical drive with a RAID level 1, 5, 10 or 50 array. A disk rebuild normally occurs
without interrupting normal operations on the affected logical drive, though some degradation of performance of the disk subsystem can occur.
Rebuild Rate
The percentage of CPU resources devoted to rebuilding.
Reconstruct
The act of remaking a logical drive after changing RAID levels or adding a physical drive to an existing array.
Redundancy
The provision of multiple interchangeable components to perform a single function to cope with failures and errors. Common forms of hardware redundancy
are disk mirroring, implementations of parity disks or distributed parity.
Replacement Disk
A hard drive replacing a failed member disk in a RAID array.
Replacement Unit
A component or collection of components in a system or subsystem that is always replaced as a unit when any part of the collection fails. Typical replacement
units in a disk subsystem include disks, adapter logic boards, power supplies and cables.
SCSI
(Small Computer System Interface) A processor-independent standard for system-level interfacing between a system and intelligent devices, such as hard
disks, floppy disks, CD-ROM, printers, and scanners. SCSI can connect up to 15 devices to a single adapter (or host adapter) on the system's bus. SCSI
transfers 8, 16 or 32 bits in parallel and can operate in either asynchronous or synchronous modes. The synchronous transfer rate is up to 320 MB/s.
The original standard is now called SCSI-1 to distinguish it from SCSI-2 and SCSI-3, which include specifications of Wide SCSI (a 16-bit bus) and Fast SCSI (10
MB/s transfer.) Ultra 160M SCSI is a subset of Ultra320 SCSI and allows a maximum throughput of 160 MB/s, which is twice as fast as Wide Ultra2 SCSI.
Ultra320 allows a maximum throughput of 320 MB/s.
SCSI Channel