Chapter Twelve -- File-System Implementation -- Lecture Notes

• 12.0 Objectives
  
  
  • Describe details of implementing local file systems and directory structures.
  • Describe the implementation of remote file systems.
  • Discuss block allocation and free-block algorithms and trade-offs.
  
  
• 12.1 File-System Structure
  
  
  • For efficiency reasons, I/O operations between disk and primary memory are performed in units of blocks. A block is N sector's worth of data, where N >= 1 is a (fixed) small positive integer.
    
    
  • Designers must decide on the logical view users will get of a file system - what file types there will be, what their attributes will be, what the directory structure will be, and what means users will have for organizing files.
    
    
  • Designers also must decide on the algorithms and data structures that will be used to implement the desired logical view of the file system.
    
    
  • At the lowest level (above the hardware) the file system implementation consists of I/O control - device drivers and interrupt handlers that implement commands like "retrieve block 123."
    
    
  • Next above the level of I/O control is the basic file system, which issues commands to the I/O control level to access physical blocks by giving drive, cylinder, track, and sector numbers as arguments. The basic file system also manages memory buffering and caching, of file data and metadata. Typical metadata consists of directory information and attributes.
    
    
  • The next level up is the file-organization module, which is concerned with free-space management and with translating logical block addresses (in some 1..N or 0..N-1 range) into physical addresses on disk (in other words, drive, cylinder, track, and sector numbers).
    
    
  • Next up comes the logical file system which manages the metadata, like directories, and per-file file control block structures that contain file attributes.
    
    
  • An operating system can support many different file systems, and typically multiple file systems can utilize the same I/O control module. Perhaps multiple file systems can also have large portions of their basic file system modules in common. Such sharing and avoidance of duplication of code is an advantage of a layered design. However overhead introduced at each layer can affect file system efficiency negatively, which is a major concern because of the fact that delays caused by access to secondary storage are often a major bottleneck in a computing system.
  
  
• 12.2 File-System Implementation
  
  
  • 12.2.1 Overview
    
    
    • Various metadata structures on secondary memory are utilized to implement file systems. Here are examples:
      
      
      • per volume boot control block - typically the first block on the volume. On a bootable volume it has info the computer uses to boot the OS from the volume. (examples: UFS boot block and NTFS partition boot sector)
      • volume control block - info like the number of blocks on the volume, the block size, the free-block count, pointer(s) to free blocks, free-FCB count, and pointer(s) to free FCBs (examples: UFS superblock and NTFS master file table)
      • directory structure
      • per-file file control block (FCB)
      
      
    • Other metadata structures are utilized in primary memory, for aspects of file system management, and to facilitate file access and enhance efficiency. Examples include:
      
      
      • mount table - info about each mounted volume
      • directory-structure cache - info on recently-accessed directories
      • system-wide open-file table - copies of the FCBs of open files, and other info
      • per-process open-file table - contains some per-process info and a pointer to an entry in the system-wide open-file table
      • Buffers to hold file blocks when read from or written to secondary memory
      
      
    • To create a file the OS must allocate an FCB and add an entry to the appropriate directory.
      
      
    • When a process opens a file, the OS adds an entry to the per-process open-file table. If no process already has the file open, the OS also creates a new entry in the system-wide open-file table. There is a counter in each entry of the system-wide open-file table to keep track of how many processes have the file open. The system call that opens a file returns a pointer to the appropriate entry in the per-process open-file table. (examples: unix file descriptor and Windows file handle)
      
      
    • When a process closes a file, the OS deletes the entry in the per-process file table and decrements the counter in the corresponding entry in the system-wide open-file table. If the counter goes to zero, the OS deletes the entry in the system-wide table, after copying any modified metadata back to disk.
      
      
    • The system-wide open-file table may also be used for managing objects with file-like interfaces, such as devices and network connections.
      
      
    • The caching of file system metadata is very important, since it helps greatly reduce the delays caused by file system interaction, which is a major bottleneck in most computing systems.
    
    
  • 12.2.2 Partitions and Mounting
    
    
    • A disk drive or similar device may have multiple partitions, each of which may be raw or contain a file system
      
      
    • A boot loader may be utilized as part of the boot sequence, thus providing the capability to boot a choice of operating systems.
      
      
    • Typically a root partition containing the OS is mounted immediately at boot time, and other partitions are mounted later.
    
    
  • 12.2.3 Virtual File Systems
    
    
    • A virtual file system design can be used to provide support for multiple types of file systems and integrate them into a unified directory structure.
      
      
    • At a high level, there is a file-system interface that supports generic open(), read(), write(), and close() calls on file descriptors.
      
      
    • Below that there is a virtual file system (VFS) layer that examines the object to which the descriptor refers, and then chooses the appropriate operation (method) for implementing the generic operation requested. The VFS can distinquish among local file objects belonging to different file system types, and between local and remote file objects.
  
  
• 12.3 Directory Implementation
  
  
  • 12.3.1 Linear List
    
    
    • The simplest kind of directory implementation to code would be a linear list of (file name, pointer to disk location) pairs.
      
      
    • However, it is well known that such a structure does not well support the complete set of symbol table operations. One is forced to settle either for sequential searching or large amounts of data movement during insertions and deletions. A balanced binary search tree with threading to support inorder and other common traversal orders would be good for larger directories, but not worth the overhead for smaller directories.
    
    
  • 12.3.2 Hash Table
    
    
    • One compromise is a hash table that uses chaining for collision resolution. That gives good performance for search, insertion, and deletion. The use of chaining mitigates problems stemming from the fixed size of the hash table.
      
      
    • Of course, the hash table structure does not support traversal in key order for listing operations, except brute-force methods.
  
  
• 12.4 Allocation Methods
  
  
  • The trick to disk space allocation for file systems is to get two things at once: good storage utilization and fast file access. Contiguous, linked, and indexed allocation are the three main methods in use.
    
    
  • 12.4.1 Contiguous Allocation
    
    
    • Contiguous allocation requires that a set of contiguous blocks on secondary memory be allocated for each file.
      
      
    • A major advantage is that all blocks of the file can be accessed with a minimal number of seeks, and with minimal seek time.
      
      
    • To keep track of which blocks are allocated to a file, the OS only has to store two numbers in the FCB, the address of the first block, and the number of blocks in the file.
      
      
    • When accessing the file, it is simple to compute the physical address of a file block from its logical address. Typically the logical block numbers are just a range of the form 0 .. N, and the physical block number of a logical block is the sum of the logical block number and the physical block number of the base block.
      
      
    • Because there is a quick constant-time method to calculate the physical location corresponding to any logical block address, contiguous allocation easily supports both sequential and direct file accesses.
      
      
    • Unfortunately contiguous allocation has a very serious drawback. The problem of allocating contiguous storage on secondary memory is an instance of the now-familiar dynamic storage allocation problem which means that a very significant amount of the storage could become unusable eternal fragmentation.
      
      
    • Compaction of hard disks can take hours.
      
      
    • It is also a problem to decide how much space should be allocated for a file. One often does not know how large a given file will need to grow. Adding more contiguous space to an existing file may seldom be possible.
      
      
    • If users routinely overestimate the storage requirements for files there will be significant amounts of internal fragmentation.
      
      
    • The limitations of contiguous allocation motivate efforts to use forms of linked or indexed allocation.
    
    
  • 12.4.2 Linked Allocation
    
    
    • With linked allocation, each file is a linked list of file blocks.
      
      
    • Linked allocation does not suffer from external fragmentation at all. Any block anywhere on the volume can be used in a file. It is easy to add more blocks to a file any time. Compaction is never necessary.
      
      
    • The directory contains pointers to the first and last blocks of the file.
      
      
    • One way of implementing the rest of the pointers is to place in each data block a pointer to the next block.
      
      
    • The major problem with linked allocation is that it supports direct access extremely poorly. Generally, in order to find the physical address of block #k, it is necessary to first traverse all k-1 pointers from the beginning of the file up to the pointer to block #k-1.
      
      
    • Also, since each file block can be anywhere on the volume, the average seek time required per block access can be much greater than is the case with contiguous allocation.
      
      
    • If we link contiguous clusters of blocks instead of blocks there will be fewer pointers to follow and the proportion of space on disk used by pointers will be smaller. On the other hand, the average amount of internal fragmentation will increase.
      
      
    • Reliability is a problem, since the consequences of a lost or corrupted pointer are potentially great.
      
      
    • The concept of a file allocation table (FAT) is a useful variation on linked allocation. The FAT is a table stored at the beginning of the volume, having an entry for each physical block. It is in effect an array indexed by physical block number. The pointers used to link files together are stored in the FAT instead of in the data blocks of the file.
      
      
    • As an example of how the FAT is utilized, suppose we want to access logical block #2 of file X. First we consult X's FCB to learn the physical block number of X's logical block 0. Let's say that number is 123. We then examine the contents of entry 123 in the FAT. Let's say 876 is stored there. That means that 876 is the physical block number of X's logical block 1. We then go to entry 876 of the FAT, and find there the physical address of X's logical block #2. Let's say that number is 546. All that is required now is to request physical block 546.
      
      
    • The FAT may also be used to keep track of free blocks, by using a special value in its entry, instead of a physical block number.
      
      
    • The use of the FAT makes for less seeking during file access, because all the pointers are relatively close to each other. Also, parts of the FAT may be cached in primary memory, and so traversing those parts of the FAT would not require any seeks. It is also more straight forward to enhance the reliability of the pointers by making backup copies of the FAT.
    
    
  • 12.4.3 Indexed Allocation
    
    
    • Indexed allocation utilizes a per-file table where all the physical addresses of the data blocks of the file are stored.
      
      
    • This table, like a FAT, is basically an array of physical block numbers. However, the indexes into the array are logical block numbers. (The indexes into a FAT are physical block numbers.)
      
      
    • To find logical block #k of the file, we simply consult the kth entry in the table and read off the physical address of the block.
      
      
    • The table is usually called "the index"
      
      
    • The directory contains the address of the index.
      
      
    • The indexing scheme works pretty much the same way that paging works in primary memory management.
      
      
    • With indexed allocation, there is no external fragmentation and both sequential and direct access are supported efficiently.
      
      
    • Internal fragmentation occurs in the last data block of files and in the unused portions of the indexes.
      
      
    • The entire index of a file can be cached in a relatively small amount of memory and the physical address of any block in the file can be found by looking at a single entry of the index. In contrast, it is likely that there will not be enough memory to cache an entire FAT, and the average number of FAT entries that must be probed to find the physical address of a file block is proportionate to the size of the file.
      
      
    • When using indexed allocation, each file block can be anywhere on the volume, so there can be a long average seek time required for accessing a series of blocks of the file, whether sequentially or randomly.
      
      With contiguous allocation, the blocks of a file are close together, so when accessing a series of blocks, the average seek time tends to be shorter than is the case with either sequential allocation or indexed allocation.
      
      
    • To accommodate large files, the system may resort to using a linked list of index blocks, or a multilevel index in which one master index points to multiple second-level indexes that point to file data blocks.
      
      
    • The unix inode utilizes a variation on multilevel indexing. The first few entries in the inode point to file data blocks. One entry of the inode points to a single indirect block, which is a block of pointers to file blocks. Another entry of the inode points to a double indirect block - a block of pointers to single indirect blocks. A third entry of an inode points to a triple indirect block, which is a block of pointers to double indirect blocks. "Under this method, the number of blocks that can be allocated to a file exceeds the amount of space addressable by the 4-byte pointers used by many operating systems" (4 GB).
    
    
  • 12.4.4 Performance
    
    
    • Sequential allocation is appropriate for files that will be accessed sequentially, but not for files to be accessed directly.
    • Contiguous allocation performs well supporting both direct access and sequential access.
    • The performance of indexed allocation depends on implementation details - usually it is somewhat better than the performance of sequential allocation, but not as good as that of contiguous allocation.
    • Some systems use more than one allocation method and try to match the allocation method to the size of files and/or the ways that the files are used.
  
  
• 12.5 Free-Space Management
  
  
  • Some sort of data structure is required to keep track of which file blocks are allocated, and which are free.
    
    
  • 12.5.1 Bit Vector
    
    
    • A bit map or bit vector is a sequence of bits. the ith bit represents the ith physical block. If the ith physical block is free, the ith bit in the vector is 1, else it is 0.
      
      
    • It is simple to implement bit vectors and devise algorithms for locating free blocks and runs of contiguous free blocks. Instructions that might be used: "ISZERO" and bit-shift.
      
      
    • However, bit vectors are not efficient to use unless they are cached entirely in primary memory. It is fairly common nowadays (the year 2016) for a laptop computer to have a terabyte disk and 8GB of primary memory. If the disk has 4KB blocks or clusters, the bit vector would need about 32 MB of physical memory, which is about 0.4% of the 8GB.
    
    
  • 12.5.2 Linked List
    
    
    • If contiguous blocks are not needed, then simply storing a link to another free block in each free block is a reasonable approach.
    
    
  • 12.5.3 Grouping
    
    
    • In this variant of linking, the first block in the free list structure contains n-1 pointers to free blocks, and finally one pointer to another block like itself, which points to n-1 more free blocks and another block like itself, and so on.
      
      
    • This structure makes it possible to find a large number of free blocks quickly.
    
    
  • 12.5.4 Counting
    
    
    • Make a list of contiguous runs of free blocks by storing pairs of the form (base address, # of blocks)
      
      
    • The list will be compact if most runs are longer than 1 block.
      
      
    • Store these records in a balanced tree for efficient search, insertion, and deletion.
    
    
  • 12.5.5 Space Maps
    
    
    • ZFS uses a scheme that divides the volume into areas with separate free lists.
      
      
    • ZFS logs allocation and freeing activity and then uses the log to update in-memory copies of free lists with batches of changes.
  
  
• 12.6 Efficiency and Performance
  
  
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  • 12.6.1 Efficiency
    
    
    • A unix optimization is to locate file data blocks close to the inode to reduce seek times for file accesses.
      
      
    • Also unix uses variably sized clusters. Smaller clusters are for the last cluster of a file to lessen internal fragmentation.
      
      
    • "Generally, every data item associated with a file needs to be considered for its effect on efficiency and performance."
    
    
  • 12.6.2 Performance
    
    
    • A unified buffer cache (process pages and file data) enhances performance, using virtual memory techniques, and avoiding double-caching.
      
      
    • Frame allocation must be balanced so that processes can't take away too many frames that are needed for file caching, or vice-versa.
      
      
    • When writes to file blocks are synchronous, processes must wait for data to be flushed to secondary storage before continuing.
      
      
    • On the other hand, when processes write asynchronously, delays tend to be short - often significantly shorter than delays for reads, since the OS can return control to the process immediately after caching the write data in primary memory.
      
      
    • When a file is accessed sequentially it is often a good idea to use free-behind and read-ahead.
      
      
    • Free-behind: After page buffers are accessed, they should be freed almost immediately because they will probably not be accessed again. This is contrary to LRU, but makes sense in this context.
      
      
    • Read-ahead: Similarly, when reading a block into buffers from a file which is being accessed sequentially, several of the blocks that follow in the file should also be brought in from disk because they will probably be accessed very soon and it saves on disk access time to batch them this way.
  
  
• 12.7 Recovery
  
  
  • A crash or other failure can interrupt the OS while it is making changes to meta-data, which can leave a file system in a damaged and/or inconsistent state. Such damage can also occur if the OS crashes before it is able to flush cached meta-data changes to non-volatile secondary storage. Other things, such as bugs in software and hardware, can result in damage to a filesystem.
    
    The OS must take measures to protect the system from loss of data and to recover from failures.
    
    
  • 12.7.1 Consistency Checker
    
    
    • The OS may utilize a consistency checker such as the unix fsck to resolve inconsistencies between the directory structure and the data blocks.
      
      
    • Details of the file system implementation, such as the allocation algorithm and free-space management algorithm, determine what kinds of problems a consistency checker can detect and correct.
      
      
    • For example, if there is linked allocation, a file can be reconstructed from its data blocks.
    
    
  • 12.7.2 Log-Structured File System
    
    
    • The use of log-based recovery algorithms is becoming common, enhancing protection of file systems.
      
      
    • The basic idea is to write changes to metadata to a log first, and then replay the log entries across the actual file system metadata structures that need to be changed.
      
      
    • If the system crashes, the OS can use the log to resolve any inconsistencies during a recovery phase.
    
    
  • 12.7.3 Other Solutions
    
    
    • WAFL and ZFS file systems never overwrite blocks with new data.
      
      
    • Changes are written to new blocks and then pointers to old blocks are updated to point to new blocks.
      
      
    • Old pointers and blocks may be saved in order to provide snapshots of previous states of the file system.
      
      
    • ZFS employs checksums for all metadata and file blocks, further reducing chances of inconsistency.
    
    
  • 12.7.4 Backup and Restore
    
    
    • Backup schedules involve some combination/rotation of full and incremental backups.
      
      
    • Some backups should be saved "forever," in case a user discovers a file was damaged or deleted long ago.
      
      
    • Protect backup media by storing it where it will be safe, and make sure to always have backups on media that is in good condition, and not worn out.
      
      
    • Verify the condition of backups on a careful schedule.
  
  
• 12.8 NFS
  
  
  • 12.8.1 Overview
    
    
    • NFS allows a client to mount an arbitrary directory on a remote server over a directory (a mount point) in its local file system.
      
      
    • After the client system performs the mount operation, users on the client can access the remote files transparently (without needing to make any reference to the network, the client, or the server) with normal file accesses that utilize the pathname of the mount point.
      
      
    • If a collection of clients mounts home directories from a server via NFS, users can access their homes by logging in to any of the client machines.
      
      
    • The implementation of NFS uses RPC and XDR.
    
    
  • 12.8.2 The Mount Protocol
    
    
    • There is a separate protocol for mounting a remote directory.
    
    
  • 12.8.3 The NFS Protocol
    
    
    • NFS is essentially a stateless protocol. The server does not keep track of what the client is doing with the file - nothing like an open file table. Files are not opened or closed by the NFS protocol. Each client request has to identify the file and the byte offset being accessed.
      
      
    • An advantage of the statelessness of NFS is that it is robust across server crashes. The client need only repeat a request to a server that has rebooted.
      
      
    • For protection of data and metadata, NFS is required to flush writes to the server secondary memory synchronously. The server must wait until a write is complete before sending the return value of a client write requests.
      
      
    • The NFS protocol does not provide for the locking required for concurrent file acceses. The OS may provide that.
      
      
    • NFS is integrated into the virtual file system of Solaris, as one of the layers below the VFS interface.
    
    
  • 12.8.4 Path-Name Translation
    
    
    • When a path name is given as an argument of an NFS operation, each component must be checked individually. Any of them may be a mount point. Once a mount point is crossed, the appropriate server must be consulted, via an NFS operation, to look up the vnode of the directory or file.
    
    
  • 12.8.5 Remote Operations
    
    
    • NFS utilizes client-side caching of inode-information and file blocks.
      
      
    • Clients check with servers to determine whether cached contents remain valid.
      
      
    • NFS does not preserve the unix semantics that call for writes to be visible immediately to all processes that have a file open.
      
      
    • The semantics of NFS are not the session semantics of the Andrew file system either.
  
  
• 12.9 Example: The WAFL File System
  
  
  • WAFL is a file system optimized for random writes, and designed for servers exporting files under NFS or CIFS.
    
    
  • WAFL is similar to the Berkely Fast File system, but with many differences.
    
    
  • All metadata is stored in ordinary files.
    
    
  • WAFL never writes over blocks, and can provide snapshots of previous states using old root nodes. Going forward the changes in the file system are recorded in copy-on-write fashion.
    
    
  • Writes can always occur at the free block nearest the current head location.