1 A Typical File-system Organization

2 Directories Record information about groups of files Management of files Single-Level directory: most simple; all files in the same directory Two-Level directory: separate directory for each user Tree-Structured (hierarchical) directories: most common Operations on directories: Search for a file Create a file (directory entry) Delete a file (directory entry) List a directory Rename a file Traverse the file system (recursive)

3 Single-Level Directory A single directory for all users Naming problem Grouping problem

4 Two-Level Directory Separate directory for each user Path name Can have the same file name for different user Efficient searching No grouping capability

5 Tree-Structured Directories

6 Directory Structure A collection of nodes containing information about all files F 1 F 2 F 3 F 4 F n Directory Files Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes

7 Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

8 Organize the Directory (Logically) to Obtain Efficiency – locating a file quickly Naming – convenient to users Two users can have same name for different files The same file can have several different names Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

9 Tree-Structured Directories (Cont) Efficient searching Grouping Capability Current directory (working directory) cd /spell/mail/prog type list

10 Tree-Structured Directories (Cont) Absolute or relative path name Creating a new file is done in current directory Delete a file rm rm Creating a new subdirectory is done in current directory mkdir mkdir Example: if in current directory /mail mkdir count mkdir count mail progcopyprtexpcount Deleting “mail”  deleting the entire subtree rooted by “mail”

11 Acyclic-Graph Directories Have shared subdirectories and files

12 Acyclic-Graph Directories (Cont.) Two different names (aliasing) If dict deletes list  dangling pointer Solutions: Backpointers, so we can delete all pointers Variable size records a problem Backpointers using a daisy chain organization Entry-hold-count solution New directory entry type Link – another name (pointer) to an existing file Resolve the link – follow pointer to locate the file

13 General Graph Directory

14 General Graph Directory (Cont.) How do we guarantee no cycles? Allow only links to file not subdirectories Garbage collection Every time a new link is added use a cycle detection algorithm to determine whether it is OK

15 UNIX Directories Fully hierarchical, tree-structured Directories are represented as files Problem: Truncation Processes have a current working directory pwd command Each user has a home directory cd; echo $HOME – commands to obtain info about the home dir. The file system has a single root directory cd / - command changes working directory to root directory Special names identify neighbors in the directory tree./ - the current directory../ - the directory one level above the current directory

16 Linking Names and File Content UNIX separates file names and file content file content may have multiple (different) names ln command associates new name with existing file File content identified by: (Device, File system on device, i-node) i-node contains references to all blocks making up a file a free-node list is maintained for each file system Information contained in a UNIX i-node

17 File Protection Access rights can be independently defined for: (u) user – Owner (creator) of a file (g) group – Group (o) other – all other users of the UNIX system Example: luna test ( 48 )-% ls -lisa total drwxr-xr-x 2 apolze 1024 Jan 7 15: drwxr-xr-x 13 apolze 2048 Jan 7 15: rw apolze 0 Jan 7 15:05 Mail.txt rw-rw-rw- 1 apolze 0 Jan 7 15:05 test.c

18 File Protection (contd.) Access rights for a file: (r) Read access right; List right for directorisy (w) Write access right; includes delete/append rights (x) Execute access right; Traverse right for directories Binary representation: (x): Bit 0 (+1) (w): Bit 1 (+2) (r): Bit 2 (+4) Rights can be combined Read+Write access right: 6 Read+Execute access right: 3 Read-only: 2

19 Protection File owner/creator should be able to control: what can be done by whom Types of access ReadWriteExecuteAppendDeleteList

20 Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access 7  RWX b) group access 6  RWX c) public access1  Ask manager to create a group (unique name), say G, and add some users to the group. For a particular file (say game) or subdirectory, define an appropriate access. ownergrouppublic chmod761game Attach a group to a file chgrp G game

21 Windows XP Access-control List Management

22 A Sample UNIX Directory Listing

23 File Sharing Sharing of files on multi-user systems is desirable Sharing may be done through a protection scheme On distributed systems, files may be shared across a network Network File System (NFS) is a common distributed file-sharing method

24 File Sharing – Multiple Users User IDs identify users, allowing permissions and protections to be per-user Group IDs allow users to be in groups, permitting group access rights

25 File Sharing – Remote File Systems Uses networking to allow file system access between systems Manually via programs like FTP Automatically, seamlessly using distributed file systems Semi automatically via the world wide web Client-server model allows clients to mount remote file systems from servers Server can serve multiple clients Client and user-on-client identification is insecure or complicated NFS is standard UNIX client-server file sharing protocol CIFS is standard Windows protocol Standard operating system file calls are translated into remote calls Distributed Information Systems (distributed naming services) such as LDAP, DNS, NIS, Active Directory implement unified access to information needed for remote computing

26 File Sharing – Failure Modes Remote file systems add new failure modes, due to network failure, server failure Recovery from failure can involve state information about status of each remote request Stateless protocols such as NFS include all information in each request, allowing easy recovery but less security

27 File Sharing – Consistency Semantics Consistency semantics specify how multiple users are to access a shared file simultaneously Similar to process synchronization algorithms Tend to be less complex due to disk I/O and network latency (for remote file systems Andrew File System (AFS) implemented complex remote file sharing semantics Unix file system (UFS) implements: Writes to an open file visible immediately to other users of the same open file Sharing file pointer to allow multiple users to read and write concurrently AFS has session semantics Writes only visible to sessions starting after the file is closed

28 Distribution – Network File Systems Various approaches towards distributed file systems: SUN Network File System (Standard) UNIX United Andrew File System SpriteLocus SUN NFS Client/Server-System (based on remote procedure call (RPC)) File system operations are forwarded from client to server Server executes actual file system operations, returns results Client has access to remote resources Stateless operation (Reliability !)

29 Operation of a Network File System A set of operations is implemented as RPC-callable functions: Searching for a file in a directory Reading a set of directory entries Manipulating links and directories Accessing file attributes Reading and writing files Logical connection between client and server has to be established mount protocol NFS works in heterogeneous environments Machine-independent protocol for data representation (XDR) Stateless protocol Network file system may tolerate client crashes (reboots)

30 Mounting a Remote File System Computer "sun" exports the "/local"-file system to computer "moon" Exported to moon Exported to all computers on the net

31 Windows – UNIX File System Interoperability Windows uses per default the Server Message Block (SMB) protocol to implement distributed file systems SMB/CIFS (remote) resource sharing: Samba – de.samba.org POSIX libraries/tool chains for Windows API: Cygwin – sources.redhat.com/cygwin/ U/WIN – www. research.att.com/sw/tools/uwin/ NuTCracker / MKS Toolkit – Interix, SFU –

32 What's Samba all about? Free SMB and CIFS client/server for UNIX & other OS Functionality: a SMB server, to provide Windows NT and LAN Manager- style file and print services to SMB clients such as Windows, Warp Server, smbfs and others. a NetBIOS (rfc1001/1002) nameserver, which amongst other things gives browsing support. Samba can be the master browser on your LAN if you wish. a ftp-like SMB client so you can access PC resources (disks and printers) from UNIX, Netware and other operating systems