Seagate ST9300605FC, ST9900805FC, ST9600205FC, ST9450405FC Caching write data, Prefetch operation

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5.5.1Caching write data

Write caching is a write operation by the drive that makes use of a drive buffer storage area where the data to be written to the medium is stored while the drive performs the Write command.

If read caching is enabled (RCD=0), then data written to the medium is retained in the cache to be made avail- able for future read cache hits. The same buffer space and segmentation is used as set up for read functions. The buffer segmentation scheme is set up or changed independently, having nothing to do with the state of RCD. When a write command is issued, if RCD=0, the cache is first checked to see if any logical blocks that are to be written are already stored in the cache from a previous read or write command. If there are, the respective cache segments are cleared. The new data is cached for subsequent Read commands.

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.

Tables 19 through 22 show the mode default settings for these drives.

5.5.2Prefetch operation

If the Prefetch feature is enabled, data in contiguous logical blocks on the disk immediately beyond that which was requested by a Read command are retrieved and stored in the buffer for immediate transfer from the buf- fer 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.

The drive does not use the Max Prefetch field (bytes 8 and 9) or the Prefetch Ceiling field (bytes 10 and 11).

When prefetch (read look-ahead) is enabled (enabled by DRA = 0), the drive enables prefetch of contiguous blocks from the disk when it senses that a prefetch hit will likely occur. The drive disables prefetch when it decides that a prefetch hit is not likely to occur.

Savvio 10K.5 FC Product Manual, Rev. D

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Contents ST9900805FC ST9600205FC ST9450405FC ST9300605FC Savvio 10K.5 FCRevision history Contents Installation Savvio 10K.5 FC Product Manual, Rev. D Iii Savvio 10K.5 FC Product Manual, Rev. D Seagate Online Support and Services Seagate Technology support servicesScope Electromagnetic susceptibility Applicable standards and reference documentationStandards Electromagnetic compatibilityEuropean Union Restriction of Hazardous Substances RoHS China Restriction of Hazardous Substances RoHS Directive Reference documentsGeneral description Media description Standard featuresReliability PerformanceFormatted capacities Programmable drive capacity Factory-installed optionsSeek performance Performance characteristicsInternal drive characteristics Access timePrefetch/multi-segmented cache control Format command execution time minutesGeneral performance characteristics Start/stop timeCache operation Prefetch operation Caching write dataUnrecoverable Errors Reliability specificationsError rates Recoverable ErrorsPreventive maintenance Reliability and serviceSeek errors Interface errorsControlling S.M.A.R.T 4 S.M.A.R.TPerformance impact Thermal monitor Reporting controlDetermining rate Predictive failuresDST failure definition State of the drive prior to testingTemperature Log Page 0Dh Drive Self Test DSTExtended test Function Code 010b Invoking DSTShort and extended tests Short test Function Code 001bAbort Product warrantyStorage PowerChoiceTM power management Physical/electrical specificationsPowerChoice modes AC power requirements DC power requirements for ST9900805FC model 2Gb 4Gb Amps DC power requirementsDC power requirements for ST9600205FC model 2Gb 4Gb Amps DC power requirements for ST9450405FC model 2Gb 4Gb Amps DC power requirements for ST9300605FC model 2Gb 4Gb Amps Power sequencing General DC power requirement notesConducted noise immunity Current profiles Savvio 10K.5 FC Product Manual, Rev. D Savvio 10K.5 FC Product Manual, Rev. D Savvio 10K.5 FC Product Manual, Rev. D ST9900805FC Power dissipationST9600205FC ST9450405FC ST9300605FC Environmental limits Temperature a. OperatingRelative humidity Shock and vibration Effective altitude sea level a. OperatingShock Recommended mounting Vibration a. Operating-normal Air cleanlinessAcoustics Corrosive environment Mounting configuration dimensions Mechanical specificationsDrive internal defects/errors Defect and error managementDrive error recovery procedures FC-AL system errors Media Pre-Scan Background Media ScanDeferred Auto-Reallocation Levels of PI Setting and determining the current Type LevelIdle Read After Write Protection Information PIIdentifying a Protection Information drive Cooling InstallationDrive ID/option selection Drive orientationAir flow Drive mountingGrounding Interface requirements FC-AL featuresFibre Channel link service frames Link services supported Type of frameFibre Channel task management responses Fibre Channel task management functionsFC Scsi FCP response codes Function name Response code NPort login Plogi payload Bytes Fibre Channel port loginProcess Login Plri payload Fibre Channel port login acceptFibre Channel Process Login NPort Login Accept ACC payload BytesFabric Login Flogi payload Bytes Fibre Channel Process Login AcceptFibre Channel fabric login Process Login Accept ACC payload BytesFabric Login Accept ACC payload Bytes Fibre Channel fabric accept loginOption Supported Dual port supportFibre Channel Arbitrated Loop options FC-AL options supportedSupported commands Command code Command name Scsi commands supportedVerify error recovery page 07h Savvio 10K.5 FC Product Manual, Rev. D Temperature page 0Dh Inquiry data Mode Sense dataSavvio 10K.5 FC inquiry data Bytes Data hex Page Mode Data Header 00 00 00 00 45 dd 2f b0 00 00 00 00 00 00 02 00 00 00 00 34 65 f8 70 00 00 00 00 00 00 02 00 00 00 00 22 ec b2 5c 00 00 00 00 00 00 02 Supported Feature or condition Miscellaneous operating features and conditionsFC-AL physical interface Physical characteristicsPhysical description Electrical description Connector requirementsPin descriptions FC-AL transmitters and receivers FC-SCA pin descriptions Pin Signal name Signal typeFault LED Out PowerActive LED Out Enable port bypass signalsMotor start controls 10.5.11 SEL6 through SEL0 ID lines Parallel Enclosure Services Interface ESIAlpa SEL ID 10.6.1 TTL input characteristics Signal characteristicsDevice control codes FC Differential output LED driver signalsFC Differential input Receive eye diagram Eye diagram data values Link rate GHz Savvio 10K.5 FC Product Manual, Rev. D Index Devctrlcode Flogi Plogi Page Page Page Savvio 10K.5 FC Product Manual, Rev. D
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