Seagate ST336807FC manual Prefetch operation

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If the number of write data logical blocks exceed the size of the segment being written into, when the end of the segment is reached, the data is written into the beginning of the same cache segment, overwriting the data that was written there at the beginning of the operation; however, the drive does not overwrite data that has not yet been written to the medium.

If write caching is enabled (WCE=1), then the drive may return Good status on a write command after the data has been transferred into the cache, but before the data has been written to the medium. If an error occurs while writing the data to the medium, and Good status has already been returned, a deferred error will be gen- erated.

The Synchronize Cache command may be used to force the drive to write all cached write data to the medium. Upon completion of a Synchronize Cache command, all data received from previous write commands will have been written to the medium.

Table 25 shows the mode default settings for the drive.

4.5.2Prefetch operation

If the Prefetch feature is enabled, data in contiguous logical blocks on the disc immediately beyond that which was requested by a Read command are retrieved and stored in the buffer for immediate transfer from the buffer to the host on subsequent Read commands that request those logical blocks (this is true even if cache operation is disabled). Though the prefetch operation uses the buffer as a cache, finding the requested data in the buffer is a prefetch hit, not a cache operation hit.

To enable Prefetch, use Mode Select page 08h, byte 12, bit 5 (Disable Read Ahead—DRA bit). DRA bit = 0 enables prefetch.

Since data that is prefetched replaces data already in some buffer segments, the host can limit the amount of prefetch data to optimize system performance. The Max Prefetch field (bytes 8 and 9) limits the amount of prefetch. The drive does not use the Prefetch Ceiling field (bytes 10 and 11).

During a prefetch operation, the drive crosses a cylinder boundary to fetch more data only if Mode parameters page 08h, byte 2, bit 4 is set to 1 (Discontinuity—DISC bit).

When prefetch (read look-ahead) is enabled (enabled by DRA = 0), it operates under the control of ARLA (Adaptive Read Look-Ahead). If the host uses software interleave, ARLA enables prefetch of contiguous blocks from the disc when it senses that a prefetch hit will likely occur, even if two consecutive read operations were not for physically contiguous blocks of data (e.g. “software interleave”). ARLA disables prefetch when it decides that a prefetch hit will not likely occur. If the host is not using software interleave, and if two sequential read operations are not for contiguous blocks of data, ARLA disables prefetch, but as long as sequential read operations request contiguous blocks of data, ARLA keeps prefetch enabled.

4.5.3Optimizing cache performance for desktop and server applications

Desktop and server applications require different drive caching operations for optimal performance. This means it is difficult to provide a single configuration that meets both of these needs. In a desktop environment, you want to configure the cache to respond quickly to repetitive accesses of multiple small segments of data without taking the time to “look ahead” to the next contiguous segments of data. In a server environment, you want to configure the cache to provide large volumes of sequential data in a non-repetitive manner. In this case, the ability of the cache to “look ahead” to the next contiguous segments of sequential data is a good thing.

The Performance Mode (PM) bit controls the way the drive switches the cache buffer into different modes of segmentation. In “server mode” (PM bit = 0), the drive can dynamically change the number of cache buffer segments as needed to optimize the performance, based on the command stream from the host. In “desktop mode” (PM bit = 1), the number of segments is maintained at the value defined in Mode Page 8, Byte 13, at all times. For additional information about the PM bit, refer to the Unit Attention Parameters page (00h) of the Mode Sense command (1Ah) in the Fibre Channel Interface Product Manual, part number 77767496.

Cheetah 10K.7 36GB FC Product Manual, Rev. A

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Contents Cheetah 10K.7 FC Page Cheetah 10K.7 FC Page Revision status summary sheet Rev. a 05/04/05 All. Initial releaseRevision Date Sheets Affected Page Contents Interface requirements Physical/electrical specificationsDefect and error management InstallationSeagate Technology support services Viii Cheetah 10K.7 36GB FC Product Manual, Rev. a Cheetah 10K.7 FC family disc drive List of FiguresCheetah 10K.7 36GB FC Product Manual, Rev. a Cheetah 10K.7 FC family disc drive ScopeCheetah 10K.7 36GB FC Product Manual, Rev. a Electromagnetic susceptibility Applicable standards and reference documentationStandards Electromagnetic compatibilityEuropean Union Restriction of Hazardous Substances RoHS Electromagnetic complianceFibre Channel Interface Manual Reference documentsCheetah 10K.7 36GB FC Product Manual, Rev. a General description Media description Standard featuresProgrammable drive capacity PerformanceReliability Formatted capacitiesFactory-installed accessories Factory-installed optionsUser-installed accessories Performance characteristics Internal drive characteristicsST336807FC Generalized performance characteristics Seek performance characteristicsAccess time Format command execution time in minutesStart/stop time Prefetch/multi-segmented cache controlCache operation Caching write data Prefetch operation Cheetah 10K.7 36GB FC Product Manual, Rev. a Reliability specifications Error ratesRecoverable Errors Reliability and service Hot plugging the drive 4 S.M.A.R.TControlling S.M.A.R.T Determining rate Performance impactMilliseconds Reporting controlTemperature Log page 0Dh Parameter Code DescriptionDrive Self Test DST Invoking DST State of the drive prior to testingDST Failure Definition ImplementationLog page entries Product warrantyShort test Function Code 001b Extended test Function Code 010bProduct repair and return information ShippingST336807FC DC power requirements Physical/electrical specificationsAC power requirements DC power requirementsGeneral DC power requirement notes Power sequencingConducted noise immunity Typical ST336807FC drive, 2 Gbit, +5V current profile Typical ST336807FC drive, 2 Gbit, +12V current profileST336807FC Power dissipationEffective altitude sea level TemperatureEnvironmental limits Relative humidityPackage size Packaged/product weight Drop height Shock and vibrationShock Recommended mounting Air cleanliness VibrationCorrosive environment Acoustics Mechanical specificationsMounting configuration dimensions Defect and error management Drive internal defects/errorsDrive error recovery procedures 77.7 FC-AL system errors Background Media ScanMedia Pre-Scan Cheetah 10K.7 36GB FC Product Manual, Rev. a Drive ID/option selection InstallationPhysical interface Drive Front LED connections1 J6 connector requirements Drive orientationCooling Grounding Drive mountingType of frame Link service FC-AL featuresFibre Channel link service frames Link services supportedFC Scsi FCP response codes Fibre Channel task management functionsFibre Channel Scsi FCP task management functions Fibre Channel task management responsesFibre Channel port login NPort login Plogi payloadBytes Process Login Plri payload Fibre Channel port login acceptNPort Login Accept ACC payload Fibre Channel Process LoginFabric Login Flogi payload Fibre Channel Process Login AcceptProcess Login Accept ACC payload Fibre Channel fabric loginFabric Login Accept ACC payload Fibre Channel fabric accept loginOption Supported Dual port supportFibre Channel Arbitrated Loop options FC-AL options supportedScsi commands supported Supported commandsCommand code Supported Command name Supported commands Supported commands Supported commands Supported commands Bytes Data hex Mode Sense dataInquiry data Cheetah 10K.7 FC inquiry dataPage DEF CHG Miscellaneous status Miscellaneous operating features and conditionsMiscellaneous features Supported Feature or conditionPhysical characteristics FC-AL physical interfacePhysical description Part description Positions Part number Features Connector requirementsElectrical description J6 connector dimensionsPin Signal name Signal type Pin descriptionsFC-SCA pin descriptions Power Fault LED OutFC-AL transmitters and receivers Active LED Out conditions Enable port bypass signalsNormal command activity LED status Active LED Out11 SEL6 through SEL0 ID lines Motor start control signalsParallel Enclosure Services Interface ESI Motor start controlsSetting Arbitrated loop physical address Alpa valuesHex Dec Signal characteristics Differential Pecl input characteristics LED driver signalsLED drive signal Differential Pecl output characteristicsLink rate GHz Eye diagram data valuesCheetah 10K.7 36GB FC Product Manual, Rev. a Technical Support Seagate Technology support servicesInternet MySeagate Reseller.seagate.com Presales SupportWarranty Service Data Recovery Services Authorized Service CentersData Recovery Services FAX services-All Europe toll call Call center Presales and Technical Support Warranty ServiceToll-free support numbers Asia/Pacific support services Publication feedback survey Cheetah 10K.7 36GB FC Product Manual, Rev. a Numerics IndexFAN Flogi PBC Page See also cooling
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