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Read, write, or update information on a storage medium.


The amount of time that a system is available during those time periods when it is expected to be available. Availability is often measured as a percentage of an elapsed year. For example, 99.95% availability equates to 4.38 hours of downtime in a year (0.0005 * 365 * 24=4.38) for a system that is expected to be available all the time.


  • Average Response Time (ms) - Average Time between initiation and completion of an I/O operation in milliseconds.
  • Average Read Response Time (ms) - Average Time between initiation and completion of a Read operation.
  • Average Write Response Time (ms) - Average Time between initiation and completion of a Write operation.
  • Average Transaction Time (ms) - Average Time between initiation of a request and completion of the corresponding reply.


The average time it takes for the read/write head to move to a specific location. To compute the average seek time, divide the time it takes to complete a large number of random seeks by the number of seeks performed.


(noun) A collection of data stored on (usually removable) non-volatile storage media for purposes of recovery in case the original copy of data is lost or becomes inaccessible. Also called backup copy. To be useful for recovery, a backup must be made by copying the source data image when it is in a consistent state. (verb) The act of creating a backup.


The period of time available for performing a backup. Backup windows are typically defined by operational necessity. For example, if data is used from 8AM until midnight, then the window between midnight and 8AM is available for making backup copies. For consistent backups, data may not be altered while the backup is occurring, so in some cases a backup window is an interval of time during which data and applications are unavailable.


Bandwidth is the total amount of data that can be transferred at one time between CPU and storage. Generally, bandwidth refers to large block data transfers and is usually measured in MB/sec. (For instance, the total bandwidth available on any given Ultra SCSI bus is 40 MB/sec.) Actual transfer rates are somewhat less than this.


A sector or group of sectors. By default, a sector of data consists of 512 bytes.


Shortened form of binary table, a byte is a group of eight bits handled as a logical unit.


Same technology as cache memory used in servers. Storage cache usually resides on RAID controllers and boosts performance because the CPU doesn't have to wait for a disk head to spin. Data can be written to and read directly from cache.


A body of software that provides common control and management for one or more disk arrays or tape arrays. Control software presents the arrays of disks or tapes it controls to its operating environment as one or more virtual disks or tapes.


Central Processing Unit.

CPU Effectiveness

Total I/Os per second divided by % CPU Utilization, giving a measure in I/Os per second per CPU. Indicates how efficient the I/O subsystem is in CPU utilization.

CPU Utilization (total)

Percentage of processor time spent executing threads other than the Idle thread (in other words, time spent doing useful work). Also know as % Processor Time.

Dedicated Hot Spare

The hot spare drive which is configured to replace failed hard drive only for designated RAID within the storage device.


Preventative measures including fault tolerant systems, redundant hardware, and specialized software to ensure that businesses can operate during certain failures, and quickly recover data, hardware and communications assets.


Any storage unit presented as a single contiguous block of storage. From a hardware perspective, a disk can be a physical local drive, external SCSI attached storage, or a LUN created by a RAID controller.


A storage device attached to a computer or storage device that reads from, writes to, and stores information on a disk or hard drive.


Distributed storage is set up so that each server has its own external storage subsystem.

DPC Time

Percentage of processor time spent in Deferred Procedure Calls.

Dynamic Disks

Contain volume manage management databases comprising information on all other dynamic disks and volumes on a system.


The automatic substitution of a functionally equivalent system component for a failed one. The term failover is most often applied to intelligent controllers connected to the same storage devices and host computers. If one of the controllers fails, failover occurs, and the survivor takes over its I/O load.


The ability to manage the file level data that resides on your storage. This encompasses direct file monitoring and the monitor and control of the space being utilized. An aspect of Storage Resource Management.


A unit of measure consisting of one billion bytes (one thousand megabytes).

Global Hot Spare

The hot spare drive which is configured to replace failed hard drive for any given RAID within the storage device.


The removal or addition of components while the system is running. For example, hard drives, power supplies, PCI cards and buses are often candidates for this term.

Hybrid Hard Drive

Hybrid drive is the next generation hard drive that combines traditional rotating magnetic hard drive with large flash memory.


Refers to data that is being sent from the CPU to any type of storage device or peripheral.

I/O Size

The transfer request size that specifies the number of bytes read or written in each I/O request. You can select any value from 1 byte to almost 1 Gigabyte (limited only by the amount of virtual memory available).

I/O Type

Specifies the percentage of read and write operations performed in each operation. You can have 100% Read, 0% Write, or vice versa or a combination of both.

Interrupt Time

Percentage of processor time spent handling hardware interrupts.

Interrupts per Second

Average number of interrupts per second, averaged over the length of the test so far. If there are multiple processors, this is the total number of interrupts for all processors.


Input/Output operations per second.Total IOPS - Average number of Input/Output operations per second.Read IOPS - Average number of read operations per second.Write IOPS - Average number of write operations per second.


Encapsulation of SCSI commands and data over TCP/IP.

iSCSI - Target

A storage device which the iSCSI volumes are created on.With iSCSI initiator utility and protocol, host server/PC is able to link to the iSCSI target and use the iSCSI volume as local hard disk.


JBOD (for "just a bunch of disks," or sometimes "just a bunch of drives") the official term is "spanning" - used to refer to a computer's hard disks that haven't been configured according to the RAID (for "redundant array of independent disks") system to increase fault tolerance and improve data access performance. The RAID system stores the same data redundantly on multiple disks that nevertheless appear to the operating system as a single disk. Although, JBOD also makes the disks appear to be a single one, it accomplishes that by combining the drives into one larger logical one. JBOD doesn't deliver any advantages over using separate disks independently and doesn't provide any of the fault tolerance or performance benefits of RAID.

Privileged Time

Percentage of processor time spent in Privileged Mode (including the MicrosoftR Windows NTR service layer, the Executive routines, the Windows NT Kernel, and device drivers for most devices other than graphics adapters and printers).


Storage equipment combined with networking device to provide faster performance, easy installation, space saving, interoperability, and lower cost.


A copy of a set of files and directories as they were at a particular point in the past, a read-only copy of the data set frozen at a point in time.

Stack Master

A storage/server device that supports capacity expansion function (stackable) by iSCSI protocol or other means.

Stack Target

A storage device that is connected to the stack master to expand capacity on the master device via iSCSI protocol or other means.

User Time

Percentage of processor time spent in User Mode (including application and subsystem code, the graphics engine, graphics device drivers, printer device drivers, and the window manager).


Wake-on-LAN is the ability to switch on the system through special network packet. The administrator can remotely boot up the system.