【正文】 序不是打開磁帶上的一個文件，而是作為 raw設備打開整個磁帶。 ? Usually the tape drive is reserved for the exclusive use of that application. 通常磁帶設備是由一個應用程序獨占使用的。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts Application Interface (Cont.) ? Since the OS does not provide file system services, the application must decide how to use the array of blocks. 由于操作系統(tǒng)在磁帶上不提供文件系統(tǒng)服務，應用程序必須決定如何使用數(shù)據(jù)塊的數(shù)組。 ? Since every application makes up its own rules for how to anize a tape, a tape full of data can generally only be used by the program that created it. 由于每個應用程序對于如何組織一個磁帶都建立了自己的規(guī)則，一個裝滿數(shù)據(jù)的磁帶通常只能由創(chuàng)建它的應用程序來使用。 ? The basic operations for a tape drive differ from those of a disk drive. 磁帶驅動器的基本操作與磁盤驅動器是不同的。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts Tape Drives磁帶驅動器 ? locate positions the tape to a specific logical block, not an entire track (corresponds to seek). 定位操作指向磁帶的一個特定邏輯塊，而不是一個完整的磁道（與尋道操作對比） ? The read position operation returns the logical block number where the tape head is. 讀位置操作返回磁帶頭所在的邏輯塊號。 ? The space operation enables relative motion. 間隔操作允許相關的運動。 ? Tape drives are ―appendonly‖ devices。 updating a block in the middle of the tape also effectively erases everything beyond that block. 磁帶驅動器是“只能附加”的設備；更新磁帶中間的一個塊，需要先清除那個塊上的所有數(shù)據(jù)。 ? An EOT mark is placed after a block that is written. 寫完的一塊后面需要加上一個 EOT標記。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts File Naming 文件命名 ? The issue of naming files on removable media is especially difficult when we want to write data on a removable cartridge on one puter, and then use the cartridge in another puter. 當我們想向一臺計算機的一個可移動盤碟寫入數(shù)據(jù)、然后在另一臺計算機中使用的時候，命名文件的問題在可移動媒介上更加困難。 ? Contemporary OSs generally leave the name space problem unsolved for removable media, and depend on applications and users to figure out how to access and interpret the data. 現(xiàn)代操作系統(tǒng)通常沒有解決可移動媒介上的命名空間問題，而是依靠應用程序和用戶來指出如何訪問解釋數(shù)據(jù)。 ? Some kinds of removable media (., CDs) are so well standardized that all puters use them the same 可移動介質（比如 CD） 相當?shù)臉藴驶?，所有的計算機都以同樣的方式使用它們。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts Hierarchical Storage Management (HSM) 層次存儲管理 ? A hierarchical storage system extends the storage hierarchy beyond primary memory and secondary storage to incorporate tertiary storage — usually implemented as a jukebox of tapes or removable disks. 一個層次存儲系統(tǒng)擴展了存儲層次，從主存、二級存儲到一體化的三級存儲 ——通常是一個磁帶的自動播放機或者可移動磁盤。 ? Usually incorporate tertiary storage by extending the file 。 ? Small and frequently used files remain on disk. 經常使用的小文件仍然存在磁盤上。 ? Large, old, inactive files are archived to the jukebox. 不使用的大的舊文件存在自動播放機上。 ? HSM is usually found in superputing centers and other large installations that have enormous volumes of data. HSM在超級計算中心和其他有龐大數(shù)據(jù)量的大設備中比較常見。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts Performance Issues有關性能的問題 Speed 速度 ? Two aspects of speed in tertiary storage are bandwidth and latency. 三級存儲速度的兩個方面是帶寬和延遲。 ? Bandwidth is measured in bytes per second. 帶寬用每秒字節(jié)數(shù)來衡量。 ?Sustained bandwidth – average data rate during a large transfer。 of bytes/transfer time. Data rate when the data stream is actually flowing. 持續(xù)的帶寬 ——大量傳輸過程中的平均數(shù)據(jù)率；單位傳輸時間的字節(jié)數(shù)。數(shù)據(jù)流實際流動時的數(shù)據(jù)率。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts Speed ? Effective bandwidth – average over the entire I/O time, including seek or locate, and cartridge switching. Drive’s overall data rate. 有效帶寬 ——整個 I/O時間的平均，包括尋道或者定位，以及盤碟選擇。驅動器的全面數(shù)據(jù)率 ? Access latency – amount of time needed to locate data. 訪問延遲 ——定位數(shù)據(jù)需要的時間。 ? Access time for a disk – move the arm to the selected cylinder and wait for the rotational latency。 35 milliseconds. 磁盤的訪問時間 ——移動磁臂來選擇柱面，并且等待旋轉延遲； 35毫秒。 ? Access on tape requires winding the tape reels until the selected block reaches the tape head。 tens or hundreds of seconds. 訪問磁帶需要把所選的塊倒到磁帶頭的位置；數(shù)十甚至數(shù)百秒。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts Speed (Cont.) ? Generally say that random access within a tape cartridge is about a thousand times slower than random access on disk. 一般來說，對磁帶的隨機訪問比對磁盤的隨機訪問要慢一千倍。 ? The low cost of tertiary storage is a result of having many cheap cartridges share a few expensive drives. 三級存儲成本低，這是許多便宜的盤碟使用少量昂貴驅動器的結果。 ? A removable library is best devoted to the storage of infrequently used data, because the library can only satisfy a relatively small number of I/O requests per hour. 一個可移動的庫對于很少使用的數(shù)據(jù)的存儲是很好的，因為庫每個小時只需要滿足相對很少的 I/O請求。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts Reliability 可靠性 ? A fixed disk drive is likely to be more reliable than a removable disk or tape drive. 固定磁盤驅動器比可移動磁盤或磁帶驅動器更可靠。 ? An optical cartridge is likely to be more reliable than a magic disk or tape. 光介質比磁介質的磁盤或磁帶更可靠。 ? A head crash in a fixed hard disk generally destroys the data, whereas the failure of a tape drive or optical disk drive often leaves the data cartridge unharmed. 對于固定的硬盤，磁頭撞擊通常會破壞數(shù)據(jù)，然而磁帶或光盤驅動器的錯誤通常對數(shù)據(jù)盤碟是無害的。 Silberschatz, Galvin and Gagne ?2022 Operating System Concepts Cost 成本 ? Main memory is much more expensive than disk storage 主存比磁盤存儲要貴很多。 ? The cost per megabyte of hard disk storage is petitive with magic tape if only one tape is used per drive. 硬盤存儲的每兆字節(jié)成本與磁帶不相上下，如果每個驅動器只用一條磁帶。 ? The cheapest tape drives and the cheapest disk drives have had about the same storage capacity over the ，最便宜的磁帶驅動器和最便宜的磁盤驅動器的存儲容量幾乎一樣。 ? Tertiary storage gives a cost savings only when the number of cartridges is considerably larger than the number of drives. 只有當盤碟的數(shù)量遠大于驅動器數(shù)量的時候，三級存儲才能節(jié)約成本。 Silberschatz, Galvin and Gagne ?2022 Oper