UNIX Internals – The New Frontiers Chapters 8 & 9 File Systems 1 Contents The User Interface to Files File System File System Framework The Vnode/VFS Architecture Implementation Overview File-System-Dependent Objects Mounting a File System Operations on Files The System V File System(s5fs) S5fs Kernel 2 8.2 The User Interface files, directory, file descriptor, file systems File & Directories File: logically a container for data A hierarchical, tree-structured name space Pathname: all the components in the path from the root to the node, by “/” “.” & “..” Link: a directory entry for a file. 3 Directory tree 4 Operation on directory 5 dirp = opendir(const *filename); direntp = readdir (dirp); rewinddir(dirp); status = closedir(firp); struct dirent { int_t d_ino; char d_name[NAME_MAX +1]; }; File Attributes Kept in the inode: index node File attributes: File type Number of hard links File size Device ID Inode number User and Group Ids of the owner of the file. Timestamps Permissions and mode flags 6 Permissions and mode flags 7 0wner, group, others (3 x 3 bits) Read, write, execute (3 bits) Mode flags - apply to executable files - suid, sgid – to set the user’s effective UID to that of the owner of the file, - stick – to retain file in swap area System calls 8 link, unlink – to create and delete hard links utimes – to change the access and modify timestamps, chown – to change the owner UID and GID, Chmode – to change permissions and mode flags. File Descriptors 9 fd = open (path, oflag, mode); fd is a per-process object. File descriptors 10 File I/O Random and sequential access – random access nread = read(fd, buf, count); Write has similar semantics Operations are serialized In append mode offset pointer set to the end of the file lseek 11 Scatter-Gather I/O 12 nbytes = writev(fd, iov, iovcnt); File Locking Read and write are atomic. Advisory locks: protect from cooperative processes, flock() in 4BSD; in SVR3 chmod must be enabled first SVR4: r/w locks. Mandatory locks:kernel C library function lockf 13 8.3 File systems 14 Mount-on - a directory is covered by the mounted file system. - mount table (original) & vfs list (modern) Restrictions - file cannot span file system, - each file system must reside on a single logical disk 15 Logical Disks 16 A logical disk is a storage abstraction that the kernel sees as a linear sequence of fixed sized, randomly accessible blocks. newfs, mkfs, Traditional: partition – physical storage of a file system Modern configurations: Volume (several disks combined), Disk mirroring Stripe sets RAID(Redundant Array of Inexpensive Disks) Special files 17 Generalization to include all kinds of I/O related objects such as directories, symbolic links, hardware devices (disks, terminals, printers, psuedodevices such as the system memory, and communications abstractions such as pipes and sockets; Problems with hard links – may not span file systems,can be created by superuser only, ownership problems, Special files 18 Symbolic links – special file that points to another file (linked-to file); the data portion of the file contains the pathname of the linked-to file; may be stored in the I-node of the symbolic link ( more on this in Practical UNIX Programming pp.90-96); Pipes – created by pipe system call, deleted by the kernel automatically FIFOs - created by mknod system call, must be explicitly deleted; 8.5 File System Framework Traditional UNIX can not support >1 types of FS. The new developments (DOS, file sharing, RFS, NFS) require the framework to change. AT&T: file system switch Sun Microsystem: vnode/vfs DEC: gnode SVR4:(AT&T+ standard 19 vnode/vfs+NFS)-> de facto 8.6 The Vnode/Vfs Architecture Objectives Support several file system types simultaneously. Different disk partitions may contain different types of file systems. Support for sharing files over a network. Vendors should be able to create their own file system types and add them to the kernel. 20 Lessons from Device I/O Devices: block & character Character device switch: struc cdevsw { int (*d_open)(); int (*d_close)(); int (*d_read)(); int (*d_write)(); } cdevsw[ ]; 21 Major device number: as the index read system call(in traditional UNIX) 1) 2) 3) 4) 5) 6) 7) 8) 9) 22 Use the file descriptor to get to the open file object; Check the entry to see if the file is open for read; Get the pointer to the in-core inode from this entry; Lock the inode so as to serialize access to the file; Check the inode mode field and find that the file is a character device file. Use the major device number to index into a table of character devices and obtain the cdevsw entry for this device; From the cdevsw, obtain the pointer to the d_read routine for this device; Invoke the d_read operation to perform the devicespecific processing of the read request. Unlock the inode and return to the user. Lessons from Device I/O It is necessary to separate the file subsystem code into file-systemindependent code and file-systemdependent code The interface between these two parts is defined by a set of generic functions that are called by the file systemindependent code 23 Object Oriented Design 24 Overview of the Vnode/Vfs Interface Vnode represents a file in the UNIX kernel. Vfs represents a file system 25 ) 26 base class data and operations pointers v_data: inode(s5fs), rnode(NFS), tmpnode(tmpfs), v_op: vnodeops Example: to close the file associated with the vnode 27 #define VOP_CLOSE(vp,…) (*((vp)->v_opclose))(vp,…) VFS base class 28 8.7 Implementation Overview Objectives Each operation must be carried out on behalf of the current process. Certain operations may need to serialize access to the file. The interface must be stateless and reentrant. FS implementation should be allowed to use global resources, such as buffer cache. The interface should be usable by the server side The use of fixed-size static tables must be avoided. 29 Vnodes and Open Files The vnode is the fundamental abstraction that represents an active file in the kernel. access to a vnode: by a file descriptor by file-system-dependent data structures 30 Data structures Reference count 31 The Vnode struct vnode {u_short v_flag; u_short v_count; struct vfs *vfsmountedhere; struct vnodeops *v_op; struct vfs *vfsp; … }; // p242 32 Vnode Reference Count 33 It determines how long the vnode must remain in the kernel. Reference versus lock: Acquire a reference: Open a file A process holds a reference to its current directory. When a new file system is mounted Pathname traversal routine file is deleted physically when reference count becomes zero. The Vfs Object struct vfs { }; 34 struct vfs *vfs_next; struct vfsops * vfs_op; struct vnode *vfs_vnodecovered; int vfs_fstype; caddr_t vfs_data; dev_t vfs_dev; … //p243 35 8.8 File-System-Dependent Objects The Per-File Private Data Vnode 36 is an abstract objects. The vnodeops Vector struct vnodeops{ int (*vop_open)(); int (*vop_close)(); … }; //p245 For ufs: struct vnodeops ufs_vnodeops = { ufs_open; ufs_close; … }; //p246 37 38 File-System-Dependent Parts of the Vfs Layer struct vfsops { int (*vfs_mount)(); int (*vfs_unmount)(); int (*vfs_root)(); int (*vfs_statvfs)(); int (*vfs_sync)(); … }; //p246 39 40 8.9 Mounting a File System mount(spec, dir, flags, type, dataptr, datalen) //SVR4 Virtual File System Switch - a global table containing one entry for each file system type. struct vfssw{ char *vsw_name; int (*vsw_init)(); struct vfsops * vsw_vfsops; …. } vsfsw[]; 41 mount Implementation Adds the structure to the linked list headed by rootvfs. Sets the vfs_op field to the vfsops vector specified in the switch entry. Sets the vfs_vnodecovered field to point to the vnode of the mount point directory. 42 VFS_MOUNT processing Verify permissions for the operation. Allocate and initialize the private data object of the file system. Store a pointer to it in the vfs_data field of the vfs object. Access the root directory of the file system and initialize its vnode in memory. 43 8.10 Operations on Files Pathname Traversal lookuppn(): u_cdir 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. v_type is of a directory “..” & system root – move on “..” & a mounted system root – access the mount point VOP_LOOKUP Not found, last one - success, else – error ENOENT A mount point - go to the mounted vfs root A symbolic link – translate it and append Release the directory Go back to the top of the loop Terminate, do not release the reference of the final vnode //p250 44 Opening a file fd = open(pathname, mode) 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 45 Allocate a descriptor Allocate an open file object Call lookuppn() Check the vnode for permissions Check for the operations Not exist, O_Creat, VOP_CREAT; ENOENT VOP_OPEN If O_TRUNC, VOP_SETATTR Initialize Return the index of the file descriptor //p252 Other topics 46 File I/O File attributes User credentials Analysis Drawbacks of the SVR4 Implementation The 4.4 BSD Model Chapter 9 File System Implementations 47 9.2 The System V File System(s5fs) The layout of s5fs partition: B S inode list Directories: 48 s5fs directory is a special file containing a list of files and subdirectories. data blocks Inodes The inode contains administrative information,or meta data. The node list contains all the inodes. On-disk inode - see Tab. 9-1 In-core inode have more fields 49 Inode Fields 50 di_mode Bit-fields 51 Block array of inode—di_addr inode 10, 10K 256, 256K 256*256=65K, 65M 52 256*256*256=16M, 16G The superblock Size in blocks of the file system Size in blocks of the inode list Number of free blocks and inodes Free block list Free inode list 53 Free block list 54 9.3 s5fs Kernel Organization In-core Inodes The vnode Device ID Inode number of the file Flags for synchronization and cache management Pointers to keep the inode on a free list Pointers to keep the inode on a hash queue. Block number of last block read 55 Allocating and Reclaiming Inodes Inode table(LRU) containing the active inodes Reference count of a vnode ==0 the reclaim the inode as free Iget()(allocating): 56 Inode lookup s5lookup() Checks the directory name lookup cache Directory name lookup cache Miss? Reads the directory one block at a time, searching the entries for the specified file name:Get it If the file is in the directory, get the inode number, use iget() to locate the inode, Inode in the table?get it: allocate a new inode, initialize, copy, put in the hash queue, also initialize the vnode(v_ops, v_data, vfs) Return the pointer to the inode 57 File I/O (1) Read(to Fd-> 58 a user buffer address) the open file object, verify mode-> vnode-> get the rw-lock->call s5read() Offset -> block number & the offset -> uiomove()-> call copyout() The page not in memory?page fault->the handler>s5getpage()->call bmap() logical to physical mapping, search vnode’s page list, not in?allocates a free page and call the disk driver to read the data from disk Sleeps until the I/O completes. Before copying to user data space, verifies the user has access s5read() returns, unlock, advances the offset, returns the number of bytes read File I/O (2) Write: Not immediately to disk May increase the file size May require the allocation of data blocks Read the entire block, write relevant data, write back all the block 59 Allocating and reclaiming Inodes When the reference count drops to 0.. When a file becomes inactive…. It is better to reuse inodes………… 60 Analysis of s5fs Reliability concern : super block Performance: 2 disk I/Os Blocks randomly located Block size: 512(SVR2), 1024(SVR3) Name: 14 characters Inodes limit: 65535 61 The Berkeley Fast File System Hard disk structure On-disk organization - Blocks and fragments - Allocation policy FFS functionality enhancements – long file names, - symbolic links, - other enhancements; Analysis 62 Other file systems Temporary file systems - RAM disk, mfs, tmpfs) The Specfs File System The /proc File System 63 Linux Virtual File System Uniform file system interface to user processes Represents any conceivable file system’s general feature and behavior Assumes files are objects that share basic properties regardless of the target file system 64 65 66 Primary Objects in VFS Superblock object Represents Inode object Represents Dentry a specific directory entry object Represents process 67 a specific file object Represents File a specific mounted file system an open file associated with a