Compaq 108164-003 - ProLiant - 800 White Paper page 13

Note: The File System Cache
data is stored in memory.
Accesses to this data takes place
over the Host Bus.
ECG025.0997
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HITE APER
(cont.)
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I/O Bus Transfer Rates
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The I/O Bus consists of one or more of the following: Peripheral Component Interconnect
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(PCI), Dual Peer PCI, Extended Industry Standard Architecture (EISA) or Industry
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Standard Architecture (ISA). Table 6 defines these I/O bus types and their transfer rates.
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I/O Bus Type
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. . Peripheral Component Interconnect (
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Dual Peer
PCI
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(Supported on the ProLiant 5000, 6000,
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6500 and 7000)
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Extended Industry Standard Architecture
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(EISA)
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. . Industry Standard Architecture (ISA)
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. . The theoretical threshold for the PCI Bus has a transfer rate of 133 MB/s. Because the
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PCI Bus can transfer data so quickly, it is the second least (file system cache being the
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. . first) likely of all of the disk subsystem components to be a performance bottleneck. To
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illustrate the point, it would take a minimum of three Compaq Wide-Ultra SCSI Controllers
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running at their maximum sustained transfer rate of 40 MB/s each to maintain throughput
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on the PCI Bus. Even running this configuration (3 x 40 MB/s = 120 MB/s) does not
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. . completely saturate the PCI Bus, having the capability of transferring at a rate of
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133 MB/s.
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File System and Disk Controller Caching Transfer Rates
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File system and disk controller caching plays a fundamental role in system performance.
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Accessing data in memory, also known as Random Access Memory (RAM), is extremely
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fast (refer to Table 7 for transfer rates). Accessing data on the disk is a relatively slow
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process. If, in theory, we could avoid disk access by requesting and retrieving data from
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memory or "cache", system performance would improve dramatically.
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13
Table 6:
I/O Bus Transfer Rates
Definition and Transfer Rate
A system I/O bus architecture specification that supports 32-bit
PCI)
bus-mastered data. Designed to support plug-and-play
configuration of optional peripherals. Transfers at a maximum
rate of 133 MB/s.
A system I/O bus architecture specification that supports 32-bit
bus-mastered data. Designed to support plug-and-play
configuration of optional peripherals. Each controller transfers at
a maximum rate of 133 MB/s, with a combined total throughput
of 266 MB/s.
A system I/O bus architecture specification that supports 8-, 16-
and 32-bit data throughput paths. Supports bus-mastering on 16-
and 32-bit buses. Transfers at a maximum rate of 33 MB/s.
A system I/O bus architecture specification that supports 8- and
16-bit data throughput paths. Supports bus-mastering on 16-bit
buses. Transfers at a maximum rate of 8 MB/s.