Glossary

access control list (ACL)

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

agent

A process that manages predefined VERITAS Cluster Server (VCS) resource types. Agents bring resources online, take resources offline, and monitor resources to report any state changes to VCS. When an agent is started, it obtains configuration information from VCS and periodically monitors the resources and updates VCS with the resource status.

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.

API

Application Programming Interface.

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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.

atomic operation

An operation that either succeeds completely or fails and leaves everything as it was before the operation was started. If the operation succeeds, all aspects of the operation take effect at once and the intermediate states of change are invisible. If any aspect of the operation fails, then the operation aborts without leaving partial changes.

Block-Level Incremental Backup (BLI Backup)

A VERITAS backup capability that does not store and retrieve entire files. Instead, only the data blocks that have changed since the previous backup are backed up.

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.

CFS

VERITAS Cluster File System.

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cluster mounted file system

A shared file system that enables multiple hosts to mount and perform file operations on the same file. A cluster mount requires a shared storage device that can be accessed by other cluster mounts of the same file system. Writes to the shared device can be done concurrently from any host on which the cluster file system is mounted. To be a cluster mount, a file system must be mounted using the mount –o cluster option. See local mounted file system.

contiguous file

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

CVM

The cluster functionality of VERITAS Volume Manager.

data block

A block that contains 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.

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defragmentation

The process of reorganizing data on disk by making file data blocks physically adjacent 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.

encapsulation

A process that converts existing partitions on a specified disk to volumes. If any partitions contain file systems, /etc/fstab entries are modified so that the file systems are mounted on volumes instead. Encapsulation is not applicable on some systems.

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extent

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.

extent attribute

A policy that determines how a file allocates extents.

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 fragment size on some UNIX file systems.

fileset

A collection of files within a file system.

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fixed extent size

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.

fragmentation

The on-going process on an active file system in which the file system is spread further and further along the disk, leaving unused gaps or fragments between areas that are in use. This leads to degraded performance because the file system has fewer options when assigning a file to an extent.

GB

Gigabyte (230 bytes or 1024 megabytes).

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.

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.

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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 that contains the data and metadata associated with that file.

inode allocation unit

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.

intent logging

A method of recording 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 indices used by each user. See quotas and external quotas file.

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K

Kilobyte (210 bytes or 1024 bytes).

large file

A file larger than two terabytes. VxFS supports files up to 8 exabytes in size.

large file system

A file system larger than two terabytes. VxFS supports file systems up to 8 exabytes 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.

local mounted file system

A file system mounted on a single host. The single host mediates all file system writes to storage from other clients. To be a local mount, a file system cannot be mounted using the mount –o cluster option. See cluster mounted file system.

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metadata

Structural data describing the attributes of files on a disk.

MB

Megabyte (220 bytes or 1024 kilobytes).

mirror

A duplicate copy of a volume and the data therein (in the form of an ordered collection of subdisks). Each mirror is one copy of the volume with which the mirror is associated.

multi-volume file system

A single file system that has been created over multiple volumes, with each volume having its own properties.

MVS

Multi-volume support.

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node

One of the hosts in a cluster.

node abort

A situation where a node leaves a cluster (on an emergency basis) without attempting to stop ongoing operations.

node join

The process through which a node joins a cluster and gains access to shared disks.

object location table (OLT)

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

object location table replica

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).

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page file

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

preallocation

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.

primary fileset

The files that are visible and accessible to the user.

Quick I/O file

A regular VxFS file that is accessed using the ::cdev:vxfs: extension.

Quick I/O for Databases

Quick I/O is a VERITAS File System feature that improves database performance by minimizing read/write locking and eliminating double buffering of data. This allows online transactions to be processed at speeds equivalent to that of using raw disk devices, while keeping the administrative benefits of file systems.

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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 used to preallocate space for a file.

root disk group

A special private disk group that always exists on the system. The root disk group is named rootdg.

shared disk group

A disk group in which the disks are shared by multiple hosts (also referred to as a cluster-shareable disk group).

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shared volume

A volume that belongs to a shared disk group and is open on more than one node at the same time.

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 quotas.

Storage Checkpoint

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 Storage Checkpoint.

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structural fileset

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

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 8192 bytes from the beginning of the file system and is 8192 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.

TB

Terabyte (240 bytes or 1024 gigabytes).

transaction

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

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throughput

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

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.

volume set

A container for multiple different volumes. Each volume can have its own geometry.

vxfs

The VERITAS File System type. Used as a parameter in some commands.

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VxFS

The VERITAS File System.

VxVM

The VERITAS Volume Manager.