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71 lines
1.9 KiB
Markdown
71 lines
1.9 KiB
Markdown
---
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title: "15-file-systems"
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aliases:
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tags:
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- cosc204
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- lecture
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---
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# what is a file (LINUX)
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an interface of OS to users
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os manages files just as files - it doesn't care what type of file it is - they are all bytes arranges logically in sequence.
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they are arrangges logically in sequence
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files need to be interpreted. THis is done by applications. e.g., powerpoint, ms paint, etc.
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- the OS provides a logical unit of storage for the user
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- the user refers to files
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- the operating system maps files onto regions of the storage
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- files are really an artifact of the dialogue between the user and the OS
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## how to define it
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- a collection of related data
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- e.g., the set of lines in a program
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structure:
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- a byte stream or a sequence of bytes, text file is a stream of ascii characgers
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what can the user do
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- create, write, read, reposition, delete, truncate (through system calls)
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## attributes
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- name
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- owner
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- Uid and Gid
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- file type
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- location of data
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- size of data
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- permission
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- "housekeeping" information
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- time and created modified etc
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- and more
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## where is the data stored
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- in an index node (inode) pointed by an entry of a directory. (you can think of a directory as just a set of <name, inode> entries. )
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internal fragmentation
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- dont know exactly how many bytes to allocate to a file
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- extra space is lost
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external fragmentaion TODO
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these issues have been solved using block allocaiton instead of continuous allocation
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## system calls for file systems
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- open close, read, write, ioctl, etc
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`ssize_t write(int fd, const, void *buf, size_t count)`
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- fd - file descriptor
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- write up to count butes from the buffer pointed buf to the file referred to by the file descriptor
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- returns -1 if error. and errno is set
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- should return number of bytes written if ok
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# file access pattersns
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# directory structure
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# file system implementation
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# disk allocation methods
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