Seagate ST9146853SS, ST9300653SS, ST9300553SS, ST9300453SS 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.

Table 9 shows the mode default settings for the drive.

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.

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Contents Standard Models Self-Encrypting Drive Models ST9300653SSST9300553SS ST9146853SSST9146753SSST9300453SS ST9146653SS SED FIPS140-2 ModelsPage Contents About Fips About self-encrypting drives Defect and error managementInstallation Interface requirementsPage Savvio 15K.3 SAS Product Manual, Rev. a List of Figures Page Seagate Technology support services Seagate Online Support and ServicesScope Electromagnetic compatibility Applicable standards and reference documentationStandards Electromagnetic susceptibilityAustralian C-Tick Electromagnetic complianceElectromagnetic compliance for the European Union Korean KCCEuropean Union Restriction of Hazardous Substances RoHS Scsi Commands Reference Manual SAS Interface Manual Reference documentsGeneral description Standard features Media descriptionReliability PerformanceFormatted capacities Factory installed options Programmable drive capacitySeek performance characteristics Performance characteristicsInternal drive characteristics Access timeGeneral performance characteristics Format command execution timeStart/stop time Prefetch/multi-segmented cache control Cache operationCaching write data Prefetch operationRecoverable Errors Reliability specificationsError rates Unrecoverable ErrorsInterface errors Reliability and serviceSeek errors Preventive maintenanceControlling S.M.A.R.T Maximum processing delay4 S.M.A.R.T Performance impactPredictive failures Temperature Log Page 0Dh Parameter Code DescriptionThermal monitor DST failure definition State of the drive prior to testingDrive Self Test DST ImplementationExtended test Function Code 010b Short and extended testsShort test Function Code 001b Log page entriesProduct repair and return information Product warrantyShipping StoragePowerChoiceTM power management Physical/electrical specificationsPowerChoice modes AC power requirements DC power requirements300GB models DC power requirements Regulation146GB models DC power requirements Power sequencing General DC power requirement notesConducted noise immunity Current profiles Current profile for 300GB modelsCurrent profile for 146GB models 300GB models in 6Gb operation Power dissipation300GB models in 3Gb operation 146GB models in 6Gb operation 146GB models in 3Gb operationEnvironmental limits Temperature a. OperatingRelative humidity Shock and vibration Effective altitude sea level a. OperatingShock Vibration a. Operating-normal Recommended mountingCorrosive environment Air cleanlinessAcoustics Mechanical specifications DimensionsPurpose About FipsLevel 2 security About self-encrypting drives Controlled accessAdmin SP Data encryptionDrive locking Default passwordRandom number generator RNG Data bandsSupported commands Authenticated firmware downloadPower requirements Cryptographic eraseDrive internal defects/errors Defect and error managementDrive error recovery procedures SAS system errors Background Media ScanDeferred Auto-Reallocation Media Pre-ScanIdle Read After Write Levels of PI Setting and determining the current Type LevelProtection Information PI Identifying a Protection Information driveInstallation Drive orientationCooling Air flowDrive mounting GroundingSAS features Interface requirementsSupported commands Scsi commands supportedDual port support Supported commands Supported commands Supported commands Inquiry data Mode Sense dataSavvio inquiry data Page Mode Data Header Mode Data Header 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 transmitters and receivers Signal characteristicsPower Ready LED OutDifferential signals SAS-2 Specification complianceLED drive signal General interface characteristicsIndex NumericsPage Msid Mtbf See also cooling Page Savvio 15K.3 SAS Product Manual, Rev. a
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