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Bài giảng Nhập môn Hệ điều hành: File system (Hard Link - Soft Link) - Nguyễn Xuân Vinh

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Chương này sẽ giới thiệu về hệ thống file trong hệ điều hành. Thông qua chương này người học sẽ nắm bắt được các thành phần của file, về hardlinked và soft link (symbolic link, symlink),... Mời các bạn cùng tham khảo để nắm bắt các nội dung chi tiết.

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Nội dung Text: Bài giảng Nhập môn Hệ điều hành: File system (Hard Link - Soft Link) - Nguyễn Xuân Vinh

GV: NGUYỄN XUÂN VINH NHẬP MÔN HỆ ĐIỀU HÀNH INTRODUCTION TO OPERATING SYSTEMS [214242] FILE SYSTEM NHẬP MÔN HỆ ĐIỀU HÀNH Hard Link - Soft Link Nguyễn Xuân Vinh nguyenxuanvinh@hcmuaf.edu. 1/15/16 vn /XX 1
GV: NGUYỄN XUÂN VINH File Parts • Unix files consist of two parts: – Data part: associated with Inode which carries the map of where the data is, the file permissions, … NHẬP MÔN HỆ ĐIỀU HÀNH – Filename part : carries a name and an associated inode number. 1/15/16 /XX 2
GV: NGUYỄN XUÂN VINH Hardlinked • More than one filename can reference the same inode number – These files are said to be “hard linked” together NHẬP MÔN HỆ ĐIỀU HÀNH The specific location of physical data. 1/15/16 /XX 3
GV: NGUYỄN XUÂN VINH Soft link (symbolic link, symlink) • Special file type whose data part carries a path to another file • OS recognizes the data as a path, and redirects opens, reads, and writes so that, instead of accessing the data within the special file NHẬP MÔN HỆ ĐIỀU HÀNH 1/15/16 A symbolic path indicating /XX the abstract location of 4
GV: NGUYỄN XUÂN VINH Directory • The directory, as a file, is just an array of filename parts of other files • When a directory is built, it is initially populated with the filename parts of two special files: the “.” and “..” files. – The filename part for the “.” file is populated with the inode# NHẬP MÔN HỆ ĐIỀU HÀNH of the directory file in which the entry has been made. • “.” is a hardlink to the file of the current directory. – The filename part for the “..” file is populated with the inode# of the directory file that contains the filename part of the current directory file. • “..” is a hardlink to the file that implements the immediate 1/15/16 parent of the current directory. /XX 5
GV: NGUYỄN XUÂN VINH Restrictions • Hardlinked: – Both links must reside on the same filesystem. – The source file must exist. – Hard links shouldn’t link directories. NHẬP MÔN HỆ ĐIỀU HÀNH q Softlinks: – Source and target can be on separate file systems. – Source does not have to exist. – Additional I/O necessary to complete file access – Additional storage taken up by softlink file’s data 1/15/16 /XX 6
GV: NGUYỄN XUÂN VINH Example • Let's start off with an empty directory, and create a file in it NHẬP MÔN HỆ ĐIỀU HÀNH q Now, let's make a hardlink to the file 1/15/16 /XX a.hardlink.file shares the same inode (73478) as basic.file 7 c.hardlink.file shares the same data as basic.file
GV: NGUYỄN XUÂN VINH Example • If we change the permissions on basic.file: then the same permissions change on hardlink.file. NHẬP MÔN HỆ ĐIỀU HÀNH q Let's now make a softlink to the original file: Here, we see that although softlink.file accesses the same data as 1/15/16 basic.file and hardlink.file, it does not share the same inode (73479 vs 73478), nor does it exhibit the same file permissions. It does show a new permission bit: the 'l' (softlink) bit /XX 8
GV: NGUYỄN XUÂN VINH File System • If we delete basic.file: NHẬP MÔN HỆ ĐIỀU HÀNH then we lose the ability to access the linked data through the softlink: However, we still have access to the original data through the hardlink: You will notice that when we deleted the original file, the hardlink didn't vanish. 1/15/16 Similarly, if we had deleted the softlink, the original file wouldn't have vanished. /XX 9
GV: NGUYỄN XUÂN VINH File System • When deleting files, the data part isn't disposed of until all the filename parts have been deleted • There's a count in the inode that indicates how many filenames point to this file – That count is decremented by 1 each time one of those NHẬP MÔN HỆ ĐIỀU HÀNH filenames is deleted – When the count makes it to zero, the inode and its associated data are deleted – By the way, the count also reflects how many times the file has been opened without being closed (in other words, how many references to the file are still active) 1/15/16  Can delete a file so that no "filename" part points to the inode, without releasing the space for the data part of the file, because the /XX file is still open 10
GV: NGUYỄN XUÂN VINH /var/log/messages • notice that /var/log/messages (or some other syslogowned file) has grown too big, and you NHẬP MÔN HỆ ĐIỀU HÀNH to reclaim the space, but the used space doesn't reappear? This is because, although you've deleted the filename part, there's a process that's got the data part open still (syslogd), and the OS won't release the space for the data until the process closes it. In order to complete your space reclamation, you have to to get syslogd to close and reopen the file. 1/15/16 /XX 11
GV: NGUYỄN XUÂN VINH Advantage in programs • use this to your advantage in programs: have you ever wondered how you could hide a temporary file? Well, you could do the following: NHẬP MÔN HỆ ĐIỀU HÀNH 1/15/16 /XX 12
13 /XX 1/15/16 NHẬP MÔN HỆ ĐIỀU HÀNH GV: NGUYỄN XUÂN VINH HỎI ĐÁP

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