Operating Systems CMPSCI 377 Spring Mark Corner University of Massachusetts Amherst

Transcription

1 Operating Systems CMPSCI 377 Spring 2017 Mark Corner University of Massachusetts Amherst

2 Clicker Question #1 For a sequential workload, the limiting factor for a disk system is likely: (A) The speed of light (B) The disk/controller bandwidth/transfer time (C) The seek and rotation time (D) The seek time

3 Answer on Next Slide

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5 Basic FS Abstraction File systems are made up of files and directories A file is a linear array of bytes, just like a programming array A directory is a collection of files and directories The top is the root directory

6 OS role The OS provides access to files, but it does not know/care what is in them. Thy are just bytes. The OS provides syscalls that can be used to access files You often use an abstraction on top of syscalls (like fopen in C)

7 Opening a file int fd = open("foo", O_CREAT O_WRONLY O_TRUNC, S_IRUSR S_IWUSR); Options include creating the file if it doesn t exist, truncating any existing content etc. The file descriptor is a number that describes the open file for later use. It is private per process (like virtual memory addresses).

8 Using strace On Unix systems, you can use strace to find out what system calls a process makes Three fds are automatic: 0 (stdin), 1 (stdout), 2 (stderr) > strace cat foo... open("foo", O_RDONLY O_LARGEFILE) = 3 read(3, "hello\n", 4096) = 6 write(1, "hello\n", 6) = 6 hello read(3, "", 4096) = 0 close(3) = 0...

9 Random Access Since a file is a big array, you can access any part of it (without sequentially reading) Open files track a current offset within the file off_t lseek(int fildes, off_t offset, int whence); If whence is SEEK_SET, the offset is set to offset bytes. If whence is SEEK_CUR, the offset is set to its current location plus offset bytes. If whence is SEEK_END, the offset is set to the size of the file plus offset bytes.

10 Buffering When you write to a file (call write), it will return before it has actually written to disk. Perhaps tens of seconds This can create problems in systems where we need to guarantee data has made it to disk (for instance a database) You can force the filesystem to persist the data to disk int fd = open("foo", O_CREAT O_WRONLY O_TRUNC, S_IRUSR S_IWUSR); assert(fd > -1); int rc = write(fd, buffer, size); assert(rc == size); rc = fsync(fd); assert(rc == 0);

11 Metadata The file system stores metadata about each file and directory You can get this data from stat() or fstat() ~ $ stat -x foo File: "foo" Size: 0 FileType: Regular File Mode: (0644/-rw-r--r--) Uid: ( 502/ mcorner) Gid: ( 20/ staff) Device: 1,4 Inode: Links: 1 Access: Tue Apr 18 11:57: Modify: Tue Apr 18 11:57: Change: Tue Apr 18 11:57:

12 Deleting You can remove files of course, but the syscall is unlink() prompt> strace rm foo... unlink("foo") = 0...

13 What about directories? What happens when you run mkdir and ls? There are special syscalls to deal with directories (mkdir(), dirents(), rmdir(), etc) Directories are lists of directory entries that point to files and other directories. (The pointers are inodes, which we will discuss later) As with most things, it is more convenient to use an abstraction on top of syscalls

14 Example: Reading a Dir int main(int argc, char *argv[]) { DIR *dp = opendir("."); assert(dp!= NULL); struct dirent *d; while ((d = readdir(dp))!= NULL) { printf("%d %s\n", (int) d->d_ino, d->d_name); } closedir(dp); return 0; } This prints out the contents of a directory, including the name of the file and the pointer to the file (the inode)

15 Clicker Question #2 (A) 0 (B) 1 If this is the directory entry, and we can get all of the directory entries in one syscall, how many syscalls will be needed to get the size of all of the files in the directory? (C) ~N, where N is the number of files in the dir (D) N^2 struct dirent { /* filename */ char d_name[256]; /* inode number */ no_t d_ino; /* offset to the next dirent */ off_t d_off; /* length of this record */ unsigned short d_reclen; /* type of file */ unsigned char d_type; };

16 Answer on Next Slide

17 Why does rm use unlink? When you create a file, you are actually creating an inode in the file system and then adding a link (a directory entry) The directory entry is a link between the human name foo, and the inode number You can get the inode number if you like mcorner@csreg031 ~ $ ls -i foo foo

18 Hard links That is a hard link You can link multiple human names to the same inode (just like in Java with object and references!) mcorner@csreg031 ~ $ ln foo foo2 mcorner@csreg031 ~ $ ls -i foo foo foo foo2

19 Unlink Unlink removes the link But when does the actual inode get removed? We keep a reference count in the inode and the OS can remove the inode and its associated storage at a reference count of 0 mcorner@csreg031 ~ $ stat -x foo File: "foo" Size: 0 FileType: Regular File Mode: (0644/-rw-r--r--) Uid: ( 502/ mcorner) Gid: ( 20/ staff) Device: 1,4 Inode: Links: 2

20 Soft Links If we have hard links, we should have soft ones Soft links are more like a special file that links from one filename to another name (and not the inode) mcorner@csreg031 ~ $ ln -s foo foo3 mcorner@csreg031 ~ $ stat -x foo3 File: "foo3" Size: 3 FileType: Symbolic Link Mode: (0755/lrwxr-xr-x) Uid: ( 502/ mcorner) Gid: ( 20/ staff) Device: 1,4 Inode: Links: 1

21 Soft links cont. But there is no reference counting for soft links So you can delete the underlying file and leave yourself with a dangling link mcorner@csreg031 ~ $ rm foo mcorner@csreg031 ~ $ cat foo3 cat: foo3: No such file or directory

22 Mounting file systems We must provide a mapping of directories to underline storage systems This mapping is done by mount But we can map a file system stored on a disk to any point in our global filesystem mount -t ext3 /dev/sda1 /home/users

23 Example ~ $ mount /dev/disk1 on / (hfs, local, journaled) devfs on /dev (devfs, local, nobrowse) map -hosts on /net (autofs, nosuid, automounted, nobrowse) map auto_home on /home (autofs, automounted, nobrowse)