【正文】 (2)rotation delay or latency time (half the time required for the disk to make a plete rotation, which is the average amount of time required for the desired data to rotate around to the read/write head once the head has been positioned over the desired track)。（以符合邏輯記錄（讀）或物理記錄（寫）的要求）也 就是說，在主存儲器與大容量存儲系統(tǒng)之間傳輸?shù)臄?shù)據(jù)應該符合物理記錄的要求。 現(xiàn)代的流式磁帶機都將磁帶劃分為許多段，每段的標記是格式化過程中磁化形成的，類似于磁盤驅(qū)動器。 這種設(shè)計思想就使得 CD 存儲系統(tǒng)在對付長而連續(xù)的數(shù)據(jù)串時有最好的表現(xiàn)，如音樂復制。通過在它們反射層上創(chuàng)建反射偏差的方法在上面記錄信息，這種信息可以借助激光束檢測出來，因為在 CD 旋轉(zhuǎn)時激光束監(jiān)視它的反射面上的反射偏差。它平均為盤片旋轉(zhuǎn)一圈時間的一半 。因此 , 如果一個磁盤上的信息被損壞了磁盤能被再格式化，雖然這一過程會破壞所有的先前在磁盤上被記錄的信息。每一組磁道稱為一個柱面。聯(lián)機意味著，設(shè)備或信息已經(jīng)與計算機連接，計算機不需要人的干預，脫機意味著設(shè)備或信息與機器相連前需要人的干預，或許需要將這個設(shè)備接通電源，或許包含有 該信息的介質(zhì)需要插到某機械裝置里。 大量儲存器系統(tǒng)的主要缺點是他們典型地需要機械的運動因此需要較多的時間，因為主存儲器的所有工作都由電子器件實現(xiàn) 。 因為一個磁道能包含的信息可能比我們一次操作所需要得多，所以每個磁道劃分成若干個弧區(qū)，稱為扇區(qū)，記錄在每個扇區(qū)上的信息是連續(xù)的二進制位串。 磁盤儲存器系統(tǒng)的容量取決于所使用盤片的數(shù)目和所劃分的磁道與扇區(qū)的密度。 （ 3）存取時間，尋道時間和等待時間之和。 CD 技術(shù)原來用于音頻錄制，采用稱為 CDDA（光盤數(shù)字音頻）的記錄格式，今天作為計算機數(shù)據(jù)存儲器使用的 CD 實際上使用同樣的格式。相反，當一個應用需要以隨機的方法存取數(shù)據(jù)時，那么磁盤存儲器所用的方法（獨立的、同心的磁道）就勝過 CD 所用的螺旋形方法。每一段含有若干條縱向相互平行的磁道，這些磁道可 以獨立地存取，因而可以說，磁帶是由許多單獨的二進制位串組成的，好比磁盤的扇區(qū)。 同時 位于主存儲器區(qū)域的數(shù)據(jù) 按照 邏輯 記 錄可以 被查閱。 (3)access time (the sum of seek time and rotation delay)。 sectors of 512 bytes or 1024 bytes are mon. The location of tracks and sectors is not a permanent part of a disk’s physical structure. Instead, they are marked magically through a process called formatting (or initializing ) the disk. This process is usually performed by the disk’s manufacturer , resulting in what are known as formatted disks. Most puter systems can also perform this task. Thus, if the format information on a disk is damaged, the disk can be reformatted, although this process destroys all the information that was previously recorded on the disk. The capacity of a disk storage system depend on the number of number of disks used and the density in which the tracks and sectors are placed. Lowercapacity systems consist of a single plastic disk known as a diskette or, in those cases in which the disk is flexible, by the less prestigious title of floppy disk. (today’ s 3 1／ 2inch diameter floppy disks are housed in rigid plastic cases, which do not constitute as flexible a package as their older 5 1／ 4inch diameter cousins that were housed in paper sleeves.) Diskettes are easily inserted and removed from their corresponding read/write units and are easily stored. As a consequence, diskettes are often used for offline storage of information. The generic 3 1／ 2inch diskette is capable of holding of data but nongeneric diskettes are available with much higher capacities. An example is the Zip disk system from Iomega Corporation, which provides storage capacities up to several hundred MB on a single rigid diskette. Highcapacity disk systems, capable of holding many gigabytes, consist of perhaps five to ten rigid disks mounted on a mon spindle. The fact that the disks used in these systems are rigid leads them to be known as harddisk systems, in contrast to their floppy counterparts. To allow for faster rotation speeds, the read/write heads in these systems do not touch the disk but instead “float”just off the surface. The spacing is so close that even a single particle of dust could bee jammed between the head and disk surface, destroying both (a phenomenon known as a head crash), Thus harddisk systems are housed in cases that are sealed at the factory. Several measurements are used to evaluate a disk system’s performance:(1)seek tome (the time required to move the read/write heads from one rack to another)。 邏輯記錄的大小很少能夠與大容量存儲系統(tǒng)的物理記錄相匹配，因此，可能許多個邏輯記錄可以存放在一個物理記錄中，也可能一個邏輯記錄分成幾個物理記錄，因此，從大容量存儲系統(tǒng)中存取數(shù)據(jù)時需要一定的整理工作，對于這個問題的常用解決方法是，在主存儲系統(tǒng)里設(shè)置一個足夠大的存儲區(qū)域，它可以存放若干個物理記錄并可