Chapter 11: Implementing File Systems

11.2 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Chapter 11: Implementing File Systems Chapter 11: Implementing File Systems File-System Structure File-System Implementation Directory Implementation Allocation Methods Free-Space Management Efficiency and Performance Recovery Log-Structured File Systems 11.9 NFS (skip) Example: WAFL File System (skip)

11.3 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Objectives To describe the details of implementing local file systems and directory structures To describe the implementation of remote file systems (11.9, skip) To discuss block allocation and free-block algorithms and trade-offs

11.4 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 11.1 File-System Structure Disk characteristics for storing multiple files Can be rewritten in place Can access directly any block of information it contains File structure Logical storage unit Collection of related information File system resides on secondary storage (disks) Allow the data in disk to be stored, located, and retrieved easily  How the file system should look to the user  How to map the logical file system to the physical secondary storage devices File system organized into layers File control block (FCB) – storage structure consisting of information about a file

11.5 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Layered File System Device driver Translates logical block addresses to physical block addresses Manages metadata information System calls like create( ), open( ), close( )

11.6 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 11.2 File-System Implementation On-disk and in-memory structures are used to implement a file system On disk A boot control block A volume control block A per-file FCB: file permissions, ownership, size, and location of the data blocks  In UNIX File System, it is called inode  In Windows NTFS, it is stored as a record in master file table A directory structure  In Unix File System, this include file names and associated inode numbers  In NTFS, it is stored in the master file table

11.7 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 11.2 File-System Implementation In-memory information is used for file-system management and performance improvement via caching In-memory mount table In-memory directory-structure cache System-wide open-file table  A copy of the FCB for each open file Per-process open-file table  A pointer to the appropriate entry in system-wide open-file table  In Unix, it is called a file descriptor  In Windows, it is called a file handler

11.8 Silberschatz, Galvin and Gagne ©2005 Operating System Principles A Typical File Control Block

11.9 Silberschatz, Galvin and Gagne ©2005 Operating System Principles In-Memory File System Structures The following figure illustrates the necessary file system structures provided by the operating systems. Figure 11-3(a) refers to opening a file.

11.10 Silberschatz, Galvin and Gagne ©2005 Operating System Principles In-Memory File System Structures Figure 11-3(b) refers to reading a file.

11.11 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Partitions and Mounting A disk can be sliced into multiple partitions. A volume can span multiple partitions on multiple disks (RAID, Section 12.7) Raw disk: no file system. Used in Unix swap space, and database management systems Boot information has its own format, and is usually a sequential series of blocks, loaded as an image into memory Allow dual-booted for installing multiple OS The root partition, containing the OS kernel and other system files, is mounted at boot time Other volumes can be automatically mounted at boot time or manually mounted later OS maintains a mount table for mounted file systems Skip

11.12 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 11.3 Directory Implementation Linear list of file names with pointer to the data blocks. simple to program but time-consuming to execute To create a new file, directory must be searched to be sure that no existing file has the same name. To delete a file, we search the directory for the named file, then release the space allocated to it To reuse the directory entry, several options  Mark the entry as unused by – Assigning it s special name – Or with a used-unused bit  Attach it to a list of free directory entries  Copy the last entry in the directory into the freed location and decrease the length of the directory Disadvantage: finding a file requires a linear search  Make a list sorted would complicate the creating and deleting of files

11.13 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Directory Implementation Hash Table – linear list stores the directory entries with a hash table The hash table takes a value computed from the file name and returns a pointer to the file name in the linear list decreases directory search time Some provisions must be made for collisions – situations where two file names hash to the same location Difficulties:  fixed size (because it is a table)  The dependence of the hash function on that size Alternatively, a chained-overflow hash table can be used instead  each hash entry is a linked list instead of an individual value  Collisions resolved by adding the new entry to the linked list

11.14 Silberschatz, Galvin and Gagne ©2005 Operating System Principles 11.4 Allocation Methods An allocation method refers to how disk blocks are allocated for files. How to allocate space to these files so that disk space is utilized effectively and files can be accessed quickly Three major methods Contiguous allocation Linked allocation Indexed allocation

11.15 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Contiguous Allocation Each file occupies a set of contiguous blocks on the disk Simple – only starting location (block #) and length (number of blocks) are required Random access (next page) Problems Dynamic storage-allocation problem  First-fit, best-fit, worst-fit  Repacking off-line or on-line Determining how much space is needed for a file when it is created.  If we allocate too little space to a file, it cannot be extended  Pre-allocation may be inefficient

11.16 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Contiguous Allocation Mapping from logical to physical Logical Address/512 Q (Quotient) R (Remainder) Block to be accessed = Q + starting address Displacement into block = R

11.17 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Contiguous Allocation of Disk Space

11.18 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Extent-Based Systems Many newer file systems (I.e. Veritas File System) use this modified contiguous allocation scheme Extent-based file systems allocate disk blocks in extents A contiguous chunk of space is allocated initially If that amount is not large enough later, another chunk of contiguous space, called extent, is added A file consists of one or more extents.  The location of a file’s blocks is recorded as a location and a block count, plus a link to the first block of the next extent

11.19 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Linked Allocation Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk. The directory contains for each file a pointer to the first and last blocks of the file Pointer to the next block Contents of a block

11.20 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Linked Allocation (Cont.) Simple – need only starting address Free-space management system – no waste of space No random access Mapping Block to be accessed is the Q-th block in the linked chain of blocks representing the file. Displacement into block = R + 1 File-allocation table (FAT) – disk-space allocation used by MS-DOS and OS/2. Logical Address/511 Q (Quotient) R (Remainder)

11.21 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Linked Allocation Disadvantages: 1. Can be used effectively only for sequential-access files 2. The space required for the pointers. Solution: collect blocks into clusters, and allocate clusters rather than blocks. Cost is increased internal fragmentation. 3. Reliability: disaster if pointers were lost or damaged. Solution: doubly linked lists or store file name and relative block number in each block

11.22 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Linked Allocation Linear list of file names with pointer to the data blocks simple to program time-consuming to execute Hash Table – linear list with hash data structure decreases directory search time collisions – situations where two file names hash to the same location fixed size

11.23 Silberschatz, Galvin and Gagne ©2005 Operating System Principles File-Allocation Table (MS-DOS and OS/2) end-of-file Unused block: a 0 table value Allocating a new block to a file: Finding the first 0-valued table entry and replacing the previous end-of-file value with the address of the new block. The 0 table entry is then replaced by the end-of-file value.

11.24 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Indexed Allocation Brings all pointers together into the index block Each file has its own index block Logical view. index table

11.25 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Example of Indexed Allocation

11.26 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Indexed Allocation Need index table Random access Dynamic access without external fragmentation, but have overhead of index block With only 1 block for index table and a block size of 512 words, mapping from logical to physical in a file of maximum size of 256K (=0.5K * 512) words. Logical Address/512 Q (Quotient) R (Remainder) Q = displacement into index table R = displacement into block

11.27 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Indexed Allocation If the index block is too small, it will not be able to hole enough pointers for a large file. Mechanisms to handle this issue: Linked scheme  The last word in the index block is nil (for a small file) or is a pointer to another index block (for a large file) Multilevel index  Use first-level index block to point to a set of second-level index blocks, which point to the file blocks Combined scheme  Example: In Unix File System, for the 15 pointers of the index block in the file’s inode – The first 12 point to data of the file – The next three pointers point to (single, double, triple) indirect blocks

11.28 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Combined Scheme: UNIX (4K bytes per block)

11.29 Silberschatz, Galvin and Gagne ©2005 Operating System Principles Performance Before selecting an allocation method, we need to know how the system would be used Contiguous allocation requires only one access to get a disk block. Linked allocation is only good for sequential access.  Some system supports both, but require the declaration of the type of access in file creation. Keeping index block in memory requires considerable space. The performance of indexed allocation depends on the index structure (how many level), on the size of the file, and on the position of the block desired.  Some system uses contiguous allocation for small files and automatically switching to an index allocation if the file grows large Many other optimizations are in use  It is reasonable to add (hundreds of) thousands of instructions to save a few disk-head movements