Glossary

The information that identifies specific users or groups and their access privileges for a particular file or directory.

A group of consecutive blocks on a file system that contain resource summaries, free resource maps, and data blocks. Allocation units also contain copies of the super-block.

A delayed write in which the data is written to a page in the system's page cache, but is not written to disk before the write returns to the caller. This improves performance, but carries the risk of data loss if the system crashes before the data is flushed to disk.

During a read or write operation, data usually goes through an intermediate kernel buffer before being copied between the user buffer and disk. If the same data is repeatedly read or written, this kernel buffer acts as a cache, which can improve performance. See unbuffered I/O and direct I/O.

A file in which data blocks are physically adjacent on the underlying media.

A table containing fileset information, such as the number of blocks currently used by the fileset. Not used in the Version 3 or 4 disk layout.

A block that contains the actual data belonging to files and directories.

A form of synchronous I/O that writes the file data to disk before the write returns, but only marks the inode for later update. If the file size changes, the inode will be written before the write returns. In this mode, the file data is guaranteed to be on the disk before the write returns, but the inode modification times may be lost if the system crashes.

The process of reorganizing data on disk by making file data blocks physically adjacent to reduce access times.

An extent that is referenced directly by an inode.

An unbuffered form of I/O that bypasses the kernel's buffering of data. With direct I/O, the file system transfers data directly between the disk and the user-supplied buffer. See buffered I/O and unbuffered I/O.

Discovered Direct I/O behavior is similar to direct I/O and has the same alignment constraints, except writes that allocate storage or extend the file size do not require writing the inode changes before returning to the application.

A group of contiguous file system data blocks treated as a single unit. An extent is defined by the address of the starting block and a length.

A policy that determines how a file allocates extents.

A quotas file (named quotas) must exist in the root directory of a file system for quota-related commands to work. See quotas file and internal quotas file.

The fundamental minimum size of allocation in a file system. This is equivalent to the HFS fragment size.

A collection of files within a file system.

An extent attribute used to override the default allocation policy of the file system and set all allocations for a file to a specific fixed size.

Gigabyte (230 bytes or 1024 megabytes).

The hard limit is an absolute limit on system resources for individual users for file and data block usage on a file system. See quota.

I/O that bypasses the kernel cache to increase I/O performance. This is similar to direct I/O, except when a file is extended; for direct I/O, the inode is written to disk synchronously, for unbuffered I/O, the inode update is delayed. See buffered I/O and direct I/O.

The grouping of multiple I/O operations to achieve better performance.

An extent that contains references to other extents, as opposed to file data itself. A single indirect address extent references indirect data extents. A double indirect address extent references single indirect address extents.

An extent that contains file data and is referenced via an indirect address extent.

A unique identifier for each file within a file system that contains the data and metadata associated with that file.

A group of consecutive blocks containing inode allocation information for a given fileset. This information is in the form of a resource summary and a free inode map.

A method of recording pending changes to the file system structure. These changes are recorded in a circular intent log file.

VxFS maintains an internal quotas file for its internal usage. The internal quotas file maintains counts of blocks and inodes used by each user. See quotas and external quotas file.

Kilobyte (210 bytes or 1024 bytes).

A file larger than two gigabytes. VxFS supports files up to one terabyte in size.

A file system more than two gigabytes in size. VxFS supports file systems up to one terabyte in size.

For file systems, this typically refers to the amount of time it takes a given file system operation to return to the user.

Megabyte (2²⁰ bytes or 1024 kilobytes).

Structural data describing the attributes of files on a disk.

The information needed to locate important file system structural elements. The OLT is written to a fixed location on the underlying media (or disk).

A copy of the OLT in case of data corruption. The OLT replica is written to a fixed location on the underlying media (or disk).

A fixed-size block of virtual address space that can be mapped onto any of the physical addresses available on a system.

A method of allowing an application to guarantee that a specified amount of space is available for a file, even if the file system is otherwise out of space.

The files that are visible and accessible to the user.

Quota limits on system resources for individual users for file and data block usage on a file system. See hard limit and soft limit.

The quotas commands read and write the external quotas file to get or change usage limits. When quotas are turned on, the quota limits are copied from the external quotas file to the internal quotas file. See quotas, internal quotas file, and external quotas file.

An extent attribute used to preallocate space for a file.

A file system whose exact image has been used to create a snapshot file system.

An exact copy of a mounted file system at a specific point in time. Used to do online backups.

The soft limit is lower than a hard limit. The soft limit can be exceeded for a limited time. There are separate time limits for files and blocks. See hard limit and quota.

A facility that provides a consistent and stable view of a file system or database image and keeps track of modified data blocks since the last checkpoint.

The files that define the structure of the file system. These files are not visible or accessible to the user.

A block containing critical information about the file system such as the file system type, layout, and size. The VxFS super-block is always located 8192 bytes from the beginning of the file system and is 8192 bytes long.

A form of synchronous I/O that writes the file data to disk, updates the inode times, and writes the updated inode to disk. When the write returns to the caller, both the data and the inode have been written to disk.

Terabyte (240 bytes or 1024 gigabytes).

For file systems, this typically refers to the number of I/O operations in a given unit of time.

Updates to the file system structure that are grouped together to ensure they are all completed

A virtual disk which represents an addressable range of disk blocks used by applications such as file systems or databases.

The name of the VERITAS File System type