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If table is partitioned, then all partitions or subpartitions, as well as the LOB data and LOB index segments for each partition or subpartition, are truncated.
You cannot individually truncate a table that is part of a cluster. You must either truncate the cluster, delete all rows from the table, or drop and re-create the table.
You cannot truncate the parent table of an enabled foreign key constraint. You must disable the constraint before truncating the table.
An exception is that you can truncate the table if the integrity constraint is self-referential. You cannot truncate the parent table of a reference-partitioned table.
You must first drop the reference-partitioned child table. This clause permits materialized view master tables to be reorganized through export or import without affecting the ability of primary key materialized views defined on the master to be fast refreshed.
To support continued fast refresh of primary key materialized views, the materialized view log must record primary key information.
This is the default. This space can subsequently be used by other objects in the tablespace. Oracle Database also sets the NEXT storage parameter to the size of the last extent removed from the segment in the truncation process.
Storage values are not reset to the values when the table or cluster was created. This space can subsequently be used only by new data in the table or cluster resulting from insert or update operations.
This clause leaves storage parameters at their current settings. If you have specified more than one free list for the object you are truncating, then the REUSE STORAGE clause also removes any mapping of free lists to instances and resets the high-water mark to the beginning of the first extent.
Frequently the default allocation may provide reasonable usage. Choosing an allocation size that is too small results in excessive overhead if the file system will contain mostly very large files.
File system fragmentation occurs when unused space or single files are not contiguous. As a file system is used, files are created, modified and deleted.
When a file is created the file system allocates space for the data. Some file systems permit or require specifying an initial space allocation and subsequent incremental allocations as the file grows.
As files are deleted the space they were allocated eventually is considered available for use by other files. This creates alternating used and unused areas of various sizes.
This is free space fragmentation. When a file is created and there is not an area of contiguous space available for its initial allocation the space must be assigned in fragments.
When a file is modified such that it becomes larger it may exceed the space initially allocated to it, another allocation must be assigned elsewhere and the file becomes fragmented.
A filename or file name is used to identify a storage location in the file system. Most file systems have restrictions on the length of filenames.
In some file systems, filenames are not case sensitive i. Most modern file systems allow filenames to contain a wide range of characters from the Unicode character set.
However, they may have restrictions on the use of certain special characters, disallowing them within filenames; those characters might be used to indicate a device, device type, directory prefix, file path separator, or file type.
File systems typically have directories also called folders which allow the user to group files into separate collections. This may be implemented by associating the file name with an index in a table of contents or an inode in a Unix-like file system.
Directory structures may be flat i. The first file system to support arbitrary hierarchies of directories was used in the Multics operating system.
Other bookkeeping information is typically associated with each file within a file system. The length of the data contained in a file may be stored as the number of blocks allocated for the file or as a byte count.
A file system stores all the metadata associated with the file—including the file name, the length of the contents of a file, and the location of the file in the folder hierarchy—separate from the contents of the file.
Most file systems store the names of all the files in one directory in one place—the directory table for that directory—which is often stored like any other file.
Many file systems put only some of the metadata for a file in the directory table, and the rest of the metadata for that file in a completely separate structure, such as the inode.
Most file systems also store metadata not associated with any one particular file. Such metadata includes information about unused regions— free space bitmap , block availability map —and information about bad sectors.
Often such information about an allocation group is stored inside the allocation group itself. Some file systems provide for user defined attributes such as the author of the document, the character encoding of a document or the size of an image.
Some file systems allow for different data collections to be associated with one file name. These separate collections may be referred to as streams or forks.
Some file systems maintain multiple past revisions of a file under a single file name; the filename by itself retrieves the most recent version, while prior saved version can be accessed using a special naming convention such as "filename;4" or "filename -4 " to access the version four saves ago.
See comparison of file systems Metadata for details on which file systems support which kinds of metadata. In some cases, a file system may not make use of a storage device but can be used to organize and represent access to any data, whether it is stored or dynamically generated e.
File systems include utilities to initialize, alter parameters of and remove an instance of the file system. Some include the ability to extend or truncate the space allocated to the file system.
Directory utilities may be used to create, rename and delete directory entries , which are also known as dentries singular: Directory utilities may also include capabilities to create additional links to a directory hard links in Unix , to rename parent links "..
File utilities create, list, copy, move and delete files, and alter metadata. They may be able to truncate data, truncate or extend space allocation, append to, move, and modify files in-place.
Depending on the underlying structure of the file system, they may provide a mechanism to prepend to or truncate from, the beginning of a file, insert entries into the middle of a file or delete entries from a file.
Utilities to free space for deleted files, if the file system provides an undelete function, also belong to this category.
Some file systems defer operations such as reorganization of free space, secure erasing of free space, and rebuilding of hierarchical structures by providing utilities to perform these functions at times of minimal activity.
An example is the file system defragmentation utilities. Some of the most important features of file system utilities involve supervisory activities which may involve bypassing ownership or direct access to the underlying device.
These include high-performance backup and recovery, data replication and reorganization of various data structures and allocation tables within the file system.
There are several mechanisms used by file systems to control access to data. Usually the intent is to prevent reading or modifying files by a user or group of users.
Another reason is to ensure data is modified in a controlled way so access may be restricted to a specific program. Examples include passwords stored in the metadata of the file or elsewhere and file permissions in the form of permission bits, access control lists , or capabilities.
The need for file system utilities to be able to access the data at the media level to reorganize the structures and provide efficient backup usually means that these are only effective for polite users but are not effective against intruders.
Methods for encrypting file data are sometimes included in the file system. This is very effective since there is no need for file system utilities to know the encryption seed to effectively manage the data.
The risks of relying on encryption include the fact that an attacker can copy the data and use brute force to decrypt the data. Losing the seed means losing the data.
One significant responsibility of a file system is to ensure that, regardless of the actions by programs accessing the data, the structure remains consistent.
This includes actions taken if a program modifying data terminates abnormally or neglects to inform the file system that it has completed its activities.
This may include updating the metadata, the directory entry and handling any data that was buffered but not yet updated on the physical storage media.
Other failures which the file system must deal with include media failures or loss of connection to remote systems. In the event of an operating system failure or "soft" power failure, special routines in the file system must be invoked similar to when an individual program fails.
The file system must also be able to correct damaged structures. These may occur as a result of an operating system failure for which the OS was unable to notify the file system, power failure or reset.
The file system must also record events to allow analysis of systemic issues as well as problems with specific files or directories.
The most important purpose of a file system is to manage user data. This includes storing, retrieving and updating data. Some file systems accept data for storage as a stream of bytes which are collected and stored in a manner efficient for the media.
When a program retrieves the data, it specifies the size of a memory buffer and the file system transfers data from the media to the buffer.
A runtime library routine may sometimes allow the user program to define a record based on a library call specifying a length. When the user program reads the data, the library retrieves data via the file system and returns a record.
Some file systems allow the specification of a fixed record length which is used for all writes and reads. This facilitates locating the n th record as well as updating records.
An identification for each record, also known as a key, makes for a more sophisticated file system. The user program can read, write and update records without regard to their location.
This requires complicated management of blocks of media usually separating key blocks and data blocks. Very efficient algorithms can be developed with pyramid structure for locating records.
Utilities, language specific run-time libraries and user programs use file system APIs to make requests of the file system.
These include data transfer, positioning, updating metadata, managing directories, managing access specifications, and removal. Frequently, retail systems are configured with a single file system occupying the entire storage device.
Another approach is to partition the disk so that several file systems with different attributes can be used. One file system, for use as browser cache, might be configured with a small allocation size.
This has the additional advantage of keeping the frantic activity of creating and deleting files typical of browser activity in a narrow area of the disk and not interfering with allocations of other files.
A similar partition might be created for email. Another partition, and file system might be created for the storage of audio or video files with a relatively large allocation.
One of the file systems may normally be set read-only and only periodically be set writable. A third approach, which is mostly used in cloud systems, is to use "disk images" to house additional file systems, with the same attributes or not, within another host file system as a file.
A common example is virtualization: The ext4 file system resides in a disk image, which is treated as a file or multiple files, depending on the hypervisor and settings in the NTFS host file system.
Having multiple file systems on a single system has the additional benefit that in the event of a corruption of a single partition, the remaining file systems will frequently still be intact.
This includes virus destruction of the system partition or even a system that will not boot. File system utilities which require dedicated access can be effectively completed piecemeal.
In addition, defragmentation may be more effective. Several system maintenance utilities, such as virus scans and backups, can also be processed in segments.
For example, it is not necessary to backup the file system containing videos along with all the other files if none have been added since the last backup.
As for the image files, one can easily "spin off" differential images which contain only "new" data written to the master original image.
Differential images can be used for both safety concerns as a "disposable" system - can be quickly restored if destroyed or contaminated by a virus, as the old image can be removed and a new image can be created in matter of seconds, even without automated procedures and quick virtual machine deployment since the differential images can be quickly spawned using a script in batches.
All file systems have some functional limit that defines the maximum storable data capacity within that system [ citation needed ].
These functional limits are a best-guess effort by the designer based on how large the storage systems are right now and how large storage systems are likely to become in the future.
File system complexity typically varies proportionally with the available storage capacity. Likewise, modern file systems would not be a reasonable choice for these early systems, since the complexity of modern file system structures would quickly consume or even exceed the very limited capacity of the early storage systems.
A disk file system takes advantages of the ability of disk storage media to randomly address data in a short amount of time. Additional considerations include the speed of accessing data following that initially requested and the anticipation that the following data may also be requested.
This permits multiple users or processes access to various data on the disk without regard to the sequential location of the data.
Some disk file systems are journaling file systems or versioning file systems. Mount Rainier is an extension to UDF supported since 2. A flash file system considers the special abilities, performance and restrictions of flash memory devices.
Frequently a disk file system can use a flash memory device as the underlying storage media but it is much better to use a file system specifically designed for a flash device.
A tape file system is a file system and tape format designed to store files on tape in a self-describing form [ clarification needed ]. Magnetic tapes are sequential storage media with significantly longer random data access times than disks, posing challenges to the creation and efficient management of a general-purpose file system.
In a disk file system there is typically a master file directory, and a map of used and free data regions.
Random access to data regions is measured in milliseconds so this system works well for disks. Tape requires linear motion to wind and unwind potentially very long reels of media.
Consequently, a master file directory and usage map can be extremely slow and inefficient with tape. Writing typically involves reading the block usage map to find free blocks for writing, updating the usage map and directory to add the data, and then advancing the tape to write the data in the correct spot.
Each additional file write requires updating the map and directory and writing the data, which may take several seconds to occur for each file.
Tape file systems instead typically allow for the file directory to be spread across the tape intermixed with the data, referred to as streaming , so that time-consuming and repeated tape motions are not required to write new data.
However, a side effect of this design is that reading the file directory of a tape usually requires scanning the entire tape to read all the scattered directory entries.
Most data archiving software that works with tape storage will store a local copy of the tape catalog on a disk file system, so that adding files to a tape can be done quickly without having to rescan the tape media.
The local tape catalog copy is usually discarded if not used for a specified period of time, at which point the tape must be re-scanned if it is to be used in the future.
The Linear Tape File System uses a separate partition on the tape to record the index meta-data, thereby avoiding the problems associated with scattering directory entries across the entire tape.
Writing data to a tape, erasing, or formatting a tape is often a significantly time-consuming process and can take several hours on large tapes.
This is due to the inherently destructive nature of overwriting data on sequential media. Because of the time it can take to format a tape, typically tapes are pre-formatted so that the tape user does not need to spend time preparing each new tape for use.
All that is usually necessary is to write an identifying media label to the tape before use, and even this can be automatically written by software when a new tape is used for the first time.
Another concept for file management is the idea of a database-based file system. Instead of, or in addition to, hierarchical structured management, files are identified by their characteristics, like type of file, topic, author, or similar rich metadata.
Around to Frank G. Soltis and his team at IBM Rochester have successfully designed and applied technologies like the database file system where others like Microsoft later failed to accomplish.
Some programs need to update multiple files "all at once". For example, a software installation may write program binaries, libraries, and configuration files.
If the software installation fails, the program may be unusable. If the installation is upgrading a key system utility, such as the command shell , the entire system may be left in an unusable state.
Transaction processing introduces the isolation guarantee [ clarification needed ] , which states that operations within a transaction are hidden from other threads on the system until the transaction commits, and that interfering operations on the system will be properly serialized with the transaction.
Transactions also provide the atomicity guarantee, ensuring that operations inside of a transaction are either all committed or the transaction can be aborted and the system discards all of its partial results.
This means that if there is a crash or power failure, after recovery, the stored state will be consistent. Either the software will be completely installed or the failed installation will be completely rolled back, but an unusable partial install will not be left on the system.
Ensuring consistency across multiple file system operations is difficult, if not impossible, without file system transactions.
File locking can be used as a concurrency control mechanism for individual files, but it typically does not protect the directory structure or file metadata.
File locking also cannot automatically roll back a failed operation, such as a software upgrade; this requires atomicity. Journaling file systems are one technique used to introduce transaction-level consistency to file system structures.
Journal transactions are not exposed to programs as part of the OS API; they are only used internally to ensure consistency at the granularity of a single system call.
Data backup systems typically do not provide support for direct backup of data stored in a transactional manner, which makes recovery of reliable and consistent data sets difficult.
Most backup software simply notes what files have changed since a certain time, regardless of the transactional state shared across multiple files in the overall dataset.
As a workaround, some database systems simply produce an archived state file containing all data up to that point, and the backup software only backs that up and does not interact directly with the active transactional databases at all.
Recovery requires separate recreation of the database from the state file, after the file has been restored by the backup software.
A network file system is a file system that acts as a client for a remote file access protocol, providing access to files on a server.
Programs using local interfaces can transparently create, manage and access hierarchical directories and files in remote network-connected computers.
A shared disk file system is one in which a number of machines usually servers all have access to the same external disk subsystem usually a SAN.
The file system arbitrates access to that subsystem, preventing write collisions. A special file system presents non-file elements of an operating system as files so they can be acted on using file system APIs.
This is most commonly done in Unix-like operating systems, but devices are given file names in some non-Unix-like operating systems as well.
Examples in Unix-like systems include devfs and, in Linux 2. In non-Unix-like systems, such as TOPS and other operating systems influenced by it, where the full filename or pathname of a file can include a device prefix, devices other than those containing file systems are referred to by a device prefix specifying the device, without anything following it.
In the s disk and digital tape devices were too expensive for some early microcomputer users. An inexpensive basic data storage system was devised that used common audio cassette tape.
The system wrote a sound to provide time synchronization, then modulated sounds that encoded a prefix, the data, a checksum and a suffix.
When the system needed to read data, the user was instructed to press "PLAY" on the cassette recorder. The system would listen to the sounds on the tape waiting until a burst of sound could be recognized as the synchronization.
The system would then interpret subsequent sounds as data. When the data read was complete, the system would notify the user to press "STOP" on the cassette recorder.
It was primitive, but it worked a lot of the time.