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20091110 - File Systems The filesystem portion of an OS...

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Unformatted text preview: File Systems The filesystem portion of an OS organizes mass storage into files , which are named collections of data. Traditional re- sponsibilities are: • Providing device independence . • Implementing symbolic names for data. • Checking access permissions . • Guaranteeing file integrity . • Supporting access methods (sequential, direct). • Supporting file backup and recovery . Role of Filesystem in the OS The filesystem provides to application programs an abstract view of the mass storage devices. • Device drivers present a logical unit/block view of devices. • Filesystem presents abstractions files , directories , etc. The Filesystem Component of an OS Files and Directories Files are the basic objects implemented by the filesystem. Files are often organized into directories . The filesystem supports various operations on files: • open – prepare a file for access. • close – terminate file access. • create – make a new file. • destroy – delete a file. • rename – change the name of a file. • read – get data from a file. • write – put data into a file. File Types Some operating systems implement files of different types , such as: • text – a sequence of lines. • binary – uninterpreted bytes of data. • executable – binary machine code. Unix traditionally has had just one file type: a sequence of 8-bit bytes. There are advantages and disadvantages to each approach. Data and Metadata To implement the file and directory abstractions and support efficient access, the filesystem reserves some blocks on the storage device for various index structures and other infor- mation. • Contents of the reserved blocks are called metadata . • File contents are called simply data . • The process of initializing index structures is usually called formatting . In-RAM and On-disk Data The OS usually caches (in RAM) information associated with currently active files. • The master copy of the filesystem data and metadata is on the disk. • Cached copies of active information are maintained in RAM. • Opening a file usually brings information into RAM. • Closing a file allows cached data to be released. The filesystem portion of the OS must ensure that changes made in RAM are eventually written back to the disk. The NACHOS Filesystem “Stock” NACHOS has two filesystem implementations: • “Stub” filesystem , which uses the underlying OS filesys- tem to implement file operations. • “Real” filesystem , which implements a rudimentary filesys- tem using the NACHOS disk device. Both filesystems are accessed through the abstract class FileSystem and the OpenFile interface. The FileSystem Class The FileSystem abstract class provides methods for creating, deleting and opening files....
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20091110 - File Systems The filesystem portion of an OS...

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