P-S

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Partition

A reserved part of a disk that is set aside for a specific purpose. A physical disk can be divided into one or more partitions. Partitions are created before a disk is formatted.

Partitioning (Initializing)

Reserves a part of a disk for a certain function; the operating system considers each partition as a separate drive. Even if the entire disk is to be a single partition, that must be specified. When using dynamic disks in UNIX or Windows 2000, the term volume is used rather than partition.

Physical Cluster Number

Each file has a physical cluster number (PCN) and a logical cluster number (LCN). When no bad clusters are present, the PCNs and LCNs match exactly, but when the system discovers a PCN that is bad, the LCN is directed to point to another PCN. Thus, two consecutive LCNs may be widely separated on the disk.

Platters

The element that actually stores the data. It consists of a substrate coated with magnetic media. The substrate, made out of a non-magnetic material, is there to act as a rigid support for the magnetic media. Hard drives typically contain multiple disks, or platters, stacked on top of each other. PC hard drives generally have one to five platters, while servers will have up to a dozen.

Quota Threshold

A threshold is a point at which something is detectable or observable. In disk management, this can be a point at which a warning is issued (such as X% of the disk quota has now been used) or the point where the limit of disk space usage is reached. Different disk quota management tools permit different types and levels of thresholds. A threshold's being reached triggers specific actions such as e-mail and pop-up alerts, event log notifications, and restrictions.

RAID-0

Uses striping for added performance but has no data redundancy. RAID-0 writes data to more than one drive so as to speed the process. Information is striped to one disk, then others, back and forth, so the data is distributed among multiple disks. If one drive fails, the data is lost because no redundancy is built into RAID-0.

RAID-1

Uses full disk mirroring, with the exact same data being put on two or more disks. Read speeds are higher than a single disk, as more than one disk can be read at the same time. Write speeds, however, are the same as a single disk, as all the data is written to each of the disks. RAID-1 is expensive because twice the storage space must be used to achieve redundancy. In comparison, RAID-3 and RAID-5 are less expensive.

RAID-2

Uses striping for performance and stores error checking codes (ECCs), which allow data errors to be detected and corrected. This is a relatively rare method.

RAID-3

Combines the speed advantage of striping, as in RAID-0, but with the addition of redundancy. Rather than doing full mirroring as for RAID-1, however, it takes the data from two disks, XORs it, and places the result on a third disk, thus it requires half as many disks as RAID-1 to store the redundant data (XOR stands for exclusive-OR). A RAID system will take one bit from each of the two data disks. If both bits are the same (i.e., both 0 or both 1) it will record a 0 on the redundant disk. If the two bits are different (i.e., one is 0 and the other 1), it records a 1. This is also known as parity.

RAID-4

Does not use striping; instead, stores parity bits on a separate disk. Like RAID-2, this method is rarely used.

RAID-5

Similar to RAID-3 in terms of striping and replication. With RAID-5, however, the redundant data is interspersed with the user data on the same disks, rather than being stored on separate disks. The read speed is similar to RAID-0 and RAID-3, but the write speed is slower because both the user data and the redundant data must be written to the same disks. This is the most widely used RAID level.

RAID-10

Actually RAID-1,0 as it is a combination of striping (RAID-0) and mirroring (RAID-1). It combines the improved performance of striping with the fault tolerance of mirroring.

Redundant Arrays of Independent Disks (RAID)

A disk subsystem that increases performance and/or provides some means of fault tolerance. It is composed of two or more disk drives as well as a controller. RAID is used mainly on servers but is now starting to find its way into PCs, also.

Restore

Retrieve a file from backup; if data loss occurs and the data has been backed up, that data must then be restored from the backup.

Rotational Latency

It is much quicker to access data that is immediately under the head. All the head has to do is move toward or away from the center of the disk to access it. If, however, the head has just passed over the point on the disk where the data resides, it has to rotate almost a complete circle in order to access the data. This is known as rotational latency. In a disk spinning at 7500 rpm, it takes a little over 8 milliseconds for the head to complete a 360-degree rotation on the disk; therefore, rotational latency can vary from 0 to 8 ms on such a disk and averages around 4 ms.

Sector

A portion of one of the concentric rings on a hard disk. On PCs, a sector is usually 512 bytes in size. Tracks are broken down into sectors, which are the smallest units of storage on a disk.

Seek

Moving the head from one track to another to locate data on a specific part of a disk or to be in the proper position to retrieve data, program instructions, etc.

Seek Time

The time it takes for the head to move from one track to another. It is the most important factor in determining the speed of a disk and can be broken down into three elements: the time it takes to move the head from a stationary position until it is moving at full speed; the time expended moving the head from one track to another; and the time required to stop the head. Note that if a file is fragmented, many additional seeks are necessary to open the file. Also note that, in its truest sense, seek time really only means the time it takes for the head to move across the disk perpendicular to the tracks; however, some people use seek time to mean the time it takes for one I/O, which includes rotational latency.

Self-Monitoring Analysis and Reporting Technology (SMART)

A technology designed to provide data about the health of a disk. The controller takes data from sensors and provides that data, upon request, to the BIOS, operating system, or other software designed to monitor drives. The exact items monitored vary from one manufacturer to another but can include such things as head flying height (predicts future head crashes), disk spin-up time and temperature (disk motor problems), or the number of errors corrected.

Simple Network Management Protocol (SNMP)

A widely used network monitoring and control protocol; SNMP agents pass data about devices to the console that is monitoring them.

Simple Volume

Uses free space from a single disk. It can be a single region on a disk or consist of multiple regions that are joined together. A simple volume can be extended within the same disk or onto additional disks. Simple volumes contain no partitions or logical drives and cannot be accessed by DOS or Windows 9x.

Small Computer Systems Interface (SCSI)

Used widely for a variety of devices, not just hard drives.

Snap-In

A module of MMC that provides certain management capabilities for a specific device. Every administrative tool in Windows 2000 is a snap-in.

Soft Quota

Usage changes are monitored, but quota limits are not enforced. This soft quota mode means that quota violation events are not generated and file operations do not fail due to violation of a disk quota parameter.

Spanned Volume

A spanned volume is created from free disk space that is linked together from multiple disks. A spanned volume can be extended onto a maximum of 32 disks. A spanned volume cannot be mirrored and is not fault tolerant.

Spindle

The platters of a hard drive are attached at the center to a spindle that is directly attached to the shaft of the motor.

Striped Volume

A volume for which the data is distributed across two or more physical disks. The data on this type of volume is allocated alternately and evenly to each of the physical disks. The data on each disk is divided into blocks and data is spread onto all the disks at the same time. Thus, reads and writes are faster — sometimes as much as 300 percent faster. A striped volume cannot be mirrored or extended and is not fault tolerant. Striping is also known as RAID-0.



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Server Disk Management in a Windows Enviornment
Server Disk Management in a Windows Enviornment
ISBN: N/A
EAN: N/A
Year: 2003
Pages: 197

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