VxFS System Administrator's Guide

Glossary

access control list (ACL)

A list of users or groups who have access privileges to a specified file. A file may have its own ACL or may share an ACL with other files. ACLs allow detailed access permissions for multiple users and groups.

allocation unit

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.

asynchronous writes

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.

buffered I/O

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.

contiguous file

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

current usage table

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

data blocks

Blocks that contain the actual data belonging to files and directories.

data synchronous writes

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.

defragmentation

Reorganizing data on disk to keep file data blocks physically adjacent so as to reduce access times.

direct extent

An extent that is referenced directly by an inode.

direct I/O

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

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.

extent

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

extent attributes

The extent allocation policies associated with a file.

external quotas file

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.

file system block

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

fileset

A collection of files within a file system.

fixed extent size

An extent attribute associated with overriding the default allocation policy of the file system.

GB

Gigabytes.

hard limit

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 clustering

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

indirect address extent

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.

indirect data extent

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

inode

A unique identifier for each file within a file system which also contains metadata associated with that file.

inode allocation unit

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

intent logging

A logging scheme that records pending changes to the file system structure. These changes are recorded in a circular intent log file.

internal quotas 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.

K

Kilobytes.

large file

A file larger than 2 gigabytes. VxFS supports files up to two terabytes in size.

large file system

A file system more than 2 gigabytes in size. VxFS supports file systems up to a terabyte in size.

latency

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

MB

Megabytes.

preallocation

The preallocation of space for a file so that disk blocks will physically be part of a file before they are needed. Enabling an application to preallocate space for a file guarantees that a specified amount of space will be available for that file, even if the file system is otherwise out of space.

primary fileset

A fileset that contains the files that are visible and accessible to users.

quotas

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

quotas file

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.

reservation

An extent attribute associated with preallocating space for a file.

s5

System V Release 4's name for a file system type derived from the System V Release 3 file system.

sfs

The name of the Secure file system type.

snapshot file system

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

snapped file system

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

soft limit

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.

structural fileset

A special fileset that stores the structural elements of the file system in the form of structural files. These files define the structure of the file system and are visible only when using utilities such as the file system debugger.

super-block

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 1024 bytes from the beginning of the file system and is 1024 bytes long.

synchronous writes

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.

SVID

System V Interface Definition.

SVR4

UNIX System V Release 4.

SVR4.2 MP

UNIX System V Release 4.2 Multiprocessor.

transaction

An update to the file system structure.

throughput

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

ufs

The UNIX file system type derived from the 4.2 Berkeley Fast File System.

unbuffered I/O

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.

volume

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

vxfs

The name of the VERITAS File System type.

VxVM

The VERITAS Volume Manager.