Seagate ST973451SS, ST936751SS manual Caching write data, Prefetch operation

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1.The drive fetches the requested logical blocks from the disc and transfers them into a segment, and then from there to the host in accordance with the Mode Select Disconnect/Reconnect parameters, page 02h.

2.If the prefetch feature is enabled, refer to section 4.5.2 for operation from this point.

Each cache segment is actually a self-contained circular buffer whose length is an integer number of logical blocks. The drive dynamically creates and removes segments based on the workload. The wrap-around capa- bility of the individual segments greatly enhances the cache’s overall performance.

Note. The size of each segment is not reported by Mode Sense command page 08h, bytes 14 and 15. The value 0XFFFF is always reported regardless of the actual size of the segment. Sending a size specification using the Mode Select command (bytes 14 and 15) does not set up a new segment size. If the STRICT bit in Mode page 00h (byte 2, bit 1) is set to one, the drive responds as it does for any attempt to change an unchangeable parameter.

4.5.1Caching 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.

Table 9 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.

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 disc 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.

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Savvio 15K SAS Product Manual, Rev. B

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Contents Savvio 15K SAS 100407739 Rev. BPage Revision status summary sheet Savvio 15K SAS Product Manual, Rev. B Contents Seagate Technology support services Defect and error managementInstallation Interface requirementsList of Figures Page Scope Savvio 15K SAS Product Manual, Rev. B Electromagnetic compatibility Applicable standards and reference documentationStandards Electromagnetic susceptibilityAustralian C-Tick Electromagnetic complianceElectromagnetic compliance for the European Union Korean MICReference documents European Union Restriction of Hazardous Substances RoHSSavvio 15K SAS Product Manual, Rev. B General description Media description Standard featuresPerformance Formatted capacities Factory-installed accessoriesReliability Programmable drive capacityOptions Seek performance characteristics Performance characteristicsInternal drive characteristics Access time Gen 2.0 LP dataFormat command execution time General performance characteristicsPrefetch/multi-segmented cache control Start/stop timeCache operation Caching write data Prefetch operationRecoverable Errors Reliability specificationsError rates Unrecoverable ErrorsInterface errors Reliability and serviceSeek errors Preventive maintenancePerformance impact 4 S.M.A.R.TControlling S.M.A.R.T Reporting controlThermal monitor Temperature Log Page 0DhPredictive failures Parameter Code DescriptionImplementation State of the drive prior to testingDST failure definition Invoking DSTExtended test Function Code 010b Product warrantyShort test Function Code 001b Log page entriesShipping Product repair and return informationSavvio 15K SAS Product Manual, Rev. B DC power requirements Physical/electrical specificationsAC power requirements ST973451SS DC power requirementsST936751SS DC power requirements General DC power requirement notesConducted noise immunity Power sequencingCurrent profiles Typical ST936751SS current profile ST973451SS in 3 Gbit operation Power dissipationST973451SS in 1.5 Gbit operation ST936751SS in 3 Gbit operation ST936751SS in 1.5 Gbit operationEnvironmental limits TemperatureRelative humidity Shock and vibration Effective altitude sea levelShock Recommended mounting Vibration Air cleanlinessCorrosive environment Acoustics Mechanical specifications Mounting configuration dimensionsDrive internal defects/errors Defect and error managementDrive error recovery procedures SAS system errors Media Pre-Scan Background Media ScanDeferred Auto-Reallocation Idle Read After Write Installation Drive orientationCooling Air flowDrive mounting GroundingSavvio 15K SAS Product Manual, Rev. B SAS features Interface requirementsDual port support Scsi commands supported Commands supported by Savvio SAS family driveCommands supported by Savvio SAS family drive Commands supported by Savvio SAS family drive Commands supported by Savvio SAS family drive Inquiry data Mode Sense dataSavvio inquiry data Page Mode Data Header 04 45 dc cc 00 00 02 Miscellaneous features Miscellaneous operating features and conditionsMiscellaneous status SAS physical interface Datum B Section C C Section a a Electrical description Physical characteristicsConnector requirements Pin descriptionsSAS pin descriptions SAS transmitters and receiversSignal characteristics Eye masks Eye masks overview General interface characteristicsReceive eye mask Jitter tolerance masks Z1 OPSinusoidal Transmitter signal characteristicsPeak-to JitterReceiver signal characteristics Receiver jitter tolerance Maximum allowable jitterCompliant jitter test pattern Cjtpat Impedance specifications Impedance requirements Sheet 1Impedance requirements Sheet 2 Transmitter characteristicsElectrical TxRx connections = -5,437dB Shows the zero-length test load Receiver characteristics Savvio 15K SAS Product Manual, Rev. B Presales Support Seagate Technology support servicesInternet Technical SupportData Recovery Services Authorized Service Centers Warranty ServiceData Recovery Services Call Center Toll-free Direct dial Index NumericsPage SAS See also cooling
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