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Glossary
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Glossary,
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Stripe Width
The number of disk drives across which the data are striped.
Striping
Segmentation of logically sequential data, such as a single file, so that segments can be written to
multiple physical devices in a round-robin fashion. This technique is useful if the processor can
read or write data faster than a single disk can supply or accept it. While data is being transferred
from the first disk, the second disk can locate the next segment. Data striping is used in some
modern databases and in certain RAID devices.
Terminator
A resistor connected to a signal wire in a bus or network for impedance matching to prevent
reflections, e.g., a 50 ohm resistor connected across the end of an Ethernet cable. SCSI chains and
some LocalTalk wiring schemes also require terminators.
Ultra 160M
A subset of Ultra3 SCSI, allows a maximum throughput of 160 MB/s, which is more than twice as
fast as Wide Ultra2 SCSI. Ultra 160M allows the attachment of up to 15 SCSI devices (one SCSI
ID is reserved for the controller), including a combination of LVD and older, single-end legacy
devices, while maintaining backward compatibility with older versions of SCSI.
Ultra-SCSI
An extension of SCSI-2 that doubles the transfer speed of Fast-SCSI, providing 20MB/s on an 8-
bit connection and 40MB/s on a 16-bit connection.
Virtual Sizing
FlexRAID Virtual Sizing is used to create a logical drive up to 80 GB. A maximum of 40 logical
drives can be configured on a RAID controller and RAID migration is possible for all logical
drives except the fortieth. Because it is not possible to do migration on the last logical drive, the
maximum space available for RAID migration is 560 GB.
Wide SCSI
A variant on the SCSI-2 interface. Wide SCSI uses a 16-bit bus, double the width of the original
SCSI-1. Wide SCSI devices cannot be connected to a SCSI-1 bus. Wide SCSI supports transfer
rates up to 20 MB/s, like Fast SCSI.
Write-Through/Write-Back
When the processor writes to main memory, the data is first written to cache memory,
assuming that the processor will probably read this data again soon. In write-through cache, data is
written to main memory at the same time it is written to cache memory. In write-back cache, data is
written only to main memory when it is forced out of cache memory. Write-through caching is
simpler than write-back because an entry to cache memory that must be replaced can be
overwritten in cache memory because it will already have been copied to main memory. Write-
back requires cache memory to initiate a main memory write of the flushed entry followed (for a
processor read) by a main memory read. However, write-back is more efficient because an entry
can be written many times to cache memory without a main memory access.