8. Files and File Systems

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1 8. Files and File Systems 8. Files and File Systems File Storage Structure File System Implementation Kernel Abstraction Communication Through a Pipe 146 / 303

2 8. Files and File Systems Disk Operation Disk Structure Plates, tracks, cylinders, sectors Multiple R/W heads Quantitative analysis Moderate peak bandwidth in continuous data transfers E.g., up to 160MB/s on a modern SATA, 320MB/s on a modern SCSI Plus a read (and possibly write) cache in DRAM memory Very high latency when moving to another track/cylinder A few milliseconds on average, slightly faster on SCSI Block Granularity Access and Request Handling Algorithms Coarse-grain I/O: block-based I/O to amortize latency Scheduling Queue pending requests and select them in a way that minimizes head movement and idle plate rotation Variants of the elevator algorithm Strong influence on process scheduling and preemption: disk thrashing 147 / 303

3 8. Files and File Systems File Storage Structure 8. Files and File Systems File Storage Structure File System Implementation Kernel Abstraction Communication Through a Pipe 148 / 303

4 8. Files and File Systems File Storage Structure Storage Structure: Inode Index Node UNIX distinguishes file data and information about a file (or meta-data) File information is stored in a structure called inode Attached to a particular device Attributes File Type Number of hard links (they all share the same inode) File length in bytes Device identifier User identifier (UID, file owner) Group identifier (GID, user group of the file) Timestamps: last status change (e.g., creation, modification, and access) Access rights Possibly more attributes, depending on the file system 149 / 303

5 8. Files and File Systems File Storage Structure Inode: Access Rights Classes of file accesses user: owner group: users who belong to the file s group, excluding the owner others: all remaining users Classes of access rights read: directories: controls listing write: directories: controls file status changes execute: directories: controls searching (entering) Additional file modes suid: with execute, the process gets the file s UID directories: nothing sgid: with execute, the process gets the file s GID directories: created files inherit the creator process s GID sticky: (not portable) directories: files owned by others cannot be deleted or renamed 150 / 303

6 8. Files and File Systems File Storage Structure File System Storage General Structure Boot block Boot flag (bootable or not) Link to data blocks holding boot code Super block File system status (mount point) Number of allocated and free nodes Link to lists of allocated and free nodes Inode table Data blocks Note: directory = list of file names in a data block Boot block Super block Inode table Data blocks Simplified file system layout 151 / 303

7 8. Files and File Systems File Storage Structure Inode: Data Block Addressing Every Inode has a table of block addresses Addressing: direct, one-level indirect, two-levels indirect, Meta-data Direct block addressing Inode Third-level Indirect block addressing Second-level First-level Data blocks 152 / 303

8 8. Files and File Systems File Storage Structure File Meta-Data Protection Modes for chmod Command Mode Octal value Comment u±rwx mask for file owner permissions u±r owner has read permission u±w owner has write permission u±x owner has execute permission g±rwx (or g±rwx) mask for group permissions g±r group has read permission g±w group has write permission g±x (or g±x) group has execute permission o±rwx mask for permissions for others o±r others have read permission o±w others have write permission o±x (or o±x) others have execute permission u±s SUID bit g±s SGID bit ±t restricted deletion flag (sticky bit) 153 / 303

9 8. Files and File Systems File System Implementation 8. Files and File Systems File Storage Structure File System Implementation Kernel Abstraction Communication Through a Pipe 154 / 303

10 8. Files and File Systems File System Implementation File Systems Virtual File System Mounting multiple file systems under a common tree $ mount options device directory Superset API for the features found in modern file systems Software layer below system calls Full support of UNIX file systems Integration of pseudo file systems /proc, /sys, /dev, /dev/shm, etc. Supports virtual devices Loopback devices: /dev/loop0, /dev/loop1 for encryption, compression, swap files, etc. Support foreign and legacy file systems: FAT, NTFS, ISO9660, etc. 155 / 303

11 8. Files and File Systems File System Implementation Modern File Systems Features Transparent defragmentation Unbounded file name and size Minimize down-time with journaling Maximal protection (default): support logging of all data and meta-data blocks Minimal overhead: logging of meta-data blocks only Atomic (transactional) file operations Access control Lists (ACL) 156 / 303

12 8. Files and File Systems File System Implementation Modern File Systems Features Transparent defragmentation Unbounded file name and size Minimize down-time with journaling Maximal protection (default): support logging of all data and meta-data blocks Minimal overhead: logging of meta-data blocks only Atomic (transactional) file operations Access control Lists (ACL) Notes About Linux EXT3 Compatible with EXT2 Journalization through a specific block device Use a hidden file for the log records 156 / 303

13 8. Files and File Systems File System Implementation Modern File Systems Features Transparent defragmentation Unbounded file name and size Minimize down-time with journaling Maximal protection (default): support logging of all data and meta-data blocks Minimal overhead: logging of meta-data blocks only Atomic (transactional) file operations Access control Lists (ACL) Notes About Windows NTFS Optimization for small files: resident data Direct integration of compression and encryption 156 / 303

14 8. Files and File Systems Kernel Abstraction 8. Files and File Systems File Storage Structure File System Implementation Kernel Abstraction Communication Through a Pipe 157 / 303

15 8. Files and File Systems Kernel Abstraction I/O Kernel Structures One table of file descriptors per process: System.in, System.out, System.err Table of open files (status, including opening mode and offset) Inode table (for all open files) File locks opening mode inode pointer lock pointer # of descriptors offset # of openings File locking linked lists Descriptor tables Open file table Inode table 158 / 303

16 8. Files and File Systems Kernel Abstraction I/O Kernel Structures Example: file descriptor aliasing E.g., process creation (e.g., via the ProcessBuilder class in Java) opening mode inode pointer lock pointer # of descriptors offset # of openings File locking linked lists Descriptor tables Open file table Inode table 158 / 303

17 8. Files and File Systems Kernel Abstraction I/O Kernel Structures Example: open file aliasing E.g., multiple processes opening the same file opening mode inode pointer lock pointer # of descriptors offset # of openings File locking linked lists Descriptor tables Open file table Inode table 158 / 303

18 8. Files and File Systems Kernel Abstraction File Permissions Process and File Permissions Every file I/O is conditioned to read/write/execute access rights, relative to UID (user) and GID (group) Java Interface File class checkread(), checkwrite(), checkexec(), etc. checkpermission() FilePermission class More precise control and interaction with security policies 159 / 303

19 8. Files and File Systems Kernel Abstraction Application: I/O Redirection Example No redirection opening mode inode pointer No redirection stdin stdout stderr Terminal file 1 file 2 # of descriptors offset # of openings Descriptor table Open file table Inode table 160 / 303

20 8. Files and File Systems Kernel Abstraction Application: I/O Redirection Example Standard input redirection: System.setIn() opening mode inode pointer < file_1 stdin stdout stderr Terminal file 1 file 2 # of descriptors offset # of openings Descriptor table Open file table Inode table 160 / 303

21 8. Files and File Systems Kernel Abstraction Application: I/O Redirection Example Standard output redirection: System.setOut() opening mode inode pointer > file_2 stdin stdout stderr Terminal file 1 file 2 # of descriptors offset # of openings Descriptor table Open file table Inode table 160 / 303

22 8. Files and File Systems Kernel Abstraction Application: I/O Redirection Example Standard error redirection: System.setErr() (with sh/bash) 2> file_2 opening mode inode pointer stdin stdout stderr Terminal file 1 file 2 # of descriptors offset # of openings Descriptor table Open file table Inode table 160 / 303

23 8. Files and File Systems Communication Through a Pipe 8. Files and File Systems File Storage Structure File System Implementation Kernel Abstraction Communication Through a Pipe 161 / 303

24 8. Files and File Systems Communication Through a Pipe FIFO (Pipe) Principles Channel to stream data among processes Data traverses the pipe first-in (write) first-out (read) Blocking read and write by default (bounded capacity) Illegal to write into a pipe without reader (delivers UNIX signal PIPE = 13) A pipe without writer simulates end-of-file opening mode inode pointer out in Terminal FIFO inode # of descriptors offset # of openings Descriptor table Open file table Inode table 162 / 303

25 8. Files and File Systems Communication Through a Pipe FIFOs and I/O Redirection Question Implement $ ls more Solution new PipeInputStream() new PipeOutputStream() Process to become ls System.setOut() new ProcessBuilder().start() (which calls getruntime().exec() on "ls") Process to become more System.setIn() new ProcessBuilder().start() (which calls getruntime().exec() on "more") out in Descriptor table opening mode inode pointer # of descriptors offset # of openings Open file table Inode table Terminal FIFO inode 163 / 303

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