Logical storage unit file physical storage media disk

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" Logical storage unit file . " Physical storage media disk, tapes, ! The operating system maps files onto physical media and access these files via the storage devices. 58 File-System Management ! A file is a collection of related information defined by its creator. " Represent programs and data . # Data: numeric, alphabetic, or binary. free-form or non-free form. ! Files usually organized into directories to make them easier to use. ! Multiple user can access to the same file. " Access control on most operating systems to determine who can access what. ! OS activities include (chapters 10 and 11) : " Creating and deleting files and directories. " Primitives to manipulate files and directories. " Mapping files onto secondary storage . " Backup files onto stable (non-volatile) storage media.
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Introduction to OS 30 59 Mass-Storage Management ! Computer system must provide secondary storage to back up main memory. " Disks are used as the principal storage medium for programs and data. ! Disks are frequently used as the source and destination of program processing. " Proper management is of central importance. " Speed of computer operation hinges on disk subsystem. ! OS activities: " Free-space management. " Storage allocation. " Disk scheduling. 60 Mass-Storage Management (cont’d) ! Tertiary storage : " Storage that is slower and lower in cost than secondary storage. # Optical storage, magnetic tape. " Backup disk data. " Still must be managed.
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Introduction to OS 31 61 Caching ! Important principle of computer systems. " Information is normally kept in some storage system (large, slow, and cheap). " As it is used, it is copied into a faster (small, and expensive) storage system — the cache . ! When we need a particular piece of information, we first check faster storage (cache) to determine if information is there. " If it is, information used directly from the cache (fast) . " If not, data copied to cache and used there. ! Because cache is smaller than storage being cached, cache management is an important design problem. " Careful selection of the cache size and replacement policy. 62 Caching (cont’d) ! Performance of Various Levels of Storage: ! Movement between levels of storage hierarchy can be explicit or implicit: " For example, transfer of data from disk to memory is usually controlled by the operating system (explicit).
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Introduction to OS 32 63 Caching (cont’d) ! In a hierarchical storage structure, the same data may appear in different levels of the storage system. ! An integer A that is to be incremented by 1 is located in file B on disk. " Once the increment takes place in the register, the value of A differs in the various storage system!! ! In an environment where only one process executes at a time. " An access to A will always to be the copy at the highest level of the storage hierarchy. 64 Caching (cont’d) ! In multitasking environments: " each process (has its memory space) must obtain the most recently updated value of A. ! In multiprocessor environment: " Each CPU also contains a local cache. " A copy of A may exist simultaneously in several caches. " Since CPUs can execute concurrently , an update of A in one cache must be reflected in all other caches where A resides cache coherency .
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  • Spring '12
  • GwangS.Jung
  • OS, main memory

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