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software monitors the addresses of subsequent reads
to see if the required data is already stored in cache
memory. If it is already in cache memory (a cache hit),
it is read from cache memory immediately and the
main memory read is aborted (or not started). If the
data is not cached (a cache miss), it is fetched from
main memory and saved in cache memory.
Caching
The process of utilizing a high speed memory buffer,
referred to as a "cache," in order to speed up the
overall read or write performance. This cache can be
accessed at a higher speed than a disk subsystem. To
improve read performance, the cache usually contains
the most recently accessed data, as well as data from
adjacent disk sectors. To improve write performance,
the cache may temporarily store data in accordance
with its write back policies. See the definition of
Write-Back for more information.
Consistency Check
An operation to verify that all stripes in a virtual disk
with a redundant RAID level are consistent and
automatically fix any errors. For RAID 5, 10, and 50
arrays, consistency check verifies correct parity data
for each stripe. For RAID 1 arrays, this operation
verifies correct mirror data for each stripe.
Controller
A chip that controls the transfer of data between the
microprocessor and memory or between the
microprocessor and a peripheral device such as a
physical disk or the keyboard. In Storage
Management, the hardware or logic that interacts
with storage devices to write and retrieve data and
perform storage management. RAID controllers
perform RAID functions such as striping and
mirroring to provide data protection.
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Glossary
D
DDR SDRAM
Acronym for Double Data Rate Synchronous
Dynamic Random Access Memory. This is a type of
SDRAM that provides data throughput at double the
rate of conventional SDRAM. It uses a bursting
technique to predict the address of the next memory
location to be accessed and allows two data transfers
on each clock cycle.
Disk
A non-volatile, randomly addressable, rewriteable
mass storage device, including both rotating magnetic
and optical storage devices and solid-state storage
devices, or non-volatile electronic storage elements.
Disk Array
A set of physical disks attached to a controller or
adapter.
Disk Group
A logical grouping of disks attached to a RAID
controller on which one or more virtual disks can be
created, such that all virtual disks in the disk group
use all of the physical disks in the disk group.
Disk Migration
Moving a virtual disk or a hot spare from one
controller to another by detaching the physical disks
and re-attaching them to the new controller.
Disk Roaming
Moving disks from one slot to another on a controller.
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