Seagate ST9450205SS, ST9900605SS, ST9900805SS, ST9900705SS 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 11, 12, 13 and 14 show the mode default settings for the drive.

5.5.2Prefetch 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 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 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 10K.5 SAS Product Manual, Rev. D

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Contents SED Fips 140-2 Models ST9900605SS ST9600005SS ST9450205SS ST9300405SSStandard Models Self-Encrypting Drive Models Revision history Revision Date Sheets affected or commentsContents Defect and error management InstallationAbout Fips About self-encrypting drives Interface requirementsPage Savvio 10K.5 SAS Product Manual, Rev. D List of Figures Page Seagate Technology support services Seagate Online Support and ServicesScope Standard models Standard SED models SED Fips 140-2 modelsApplicable standards and reference documentation StandardsElectromagnetic compatibility Electromagnetic susceptibilityElectromagnetic compliance Electromagnetic compliance for the European UnionAustralian C-Tick Korean KCCReference documents China Restriction of Hazardous Substances RoHS DirectiveGeneral description Performance Standard featuresMedia description Factory-installed options ReliabilityFormatted capacities Programmable drive capacity Performance characteristics Internal drive characteristics Seek performance characteristics Access timeGeneral performance characteristics Start/stop timePrefetch/multi-segmented cache control Cache operationCaching write data Prefetch operationReliability specifications Error ratesRecoverable Errors Unrecoverable ErrorsReliability and service Seek errorsInterface errors Preventive maintenance4 S.M.A.R.T Controlling S.M.A.R.TPerformance impact Reporting controlThermal monitor Temperature Log Page 0Dh Parameter Code DescriptionPredictive failures State of the drive prior to testing Drive Self Test DSTDST failure definition ImplementationShort and extended tests Short test Function Code 001bExtended test Function Code 010b Log page entriesProduct warranty ShippingStorage Product repair and return informationPhysical/electrical specifications PowerChoiceTM power managementPowerChoice modes Mode DescriptionAC power requirements DC power requirements900GB standard drive DC power requirements 0Gb mode Amps600GB standard drive DC power requirements 450GB standard model DC power requirements 300GB standard model DC power requirements Conducted noise immunity General DC power requirement notesPower sequencing Current profiles Current profiles for 900GB modelsCurrent profiles for 600GB models Current profiles for 450GB models Current profiles for 300GB models Power dissipation GB models in 3Gb operation900GB models in 6Gb operation GB models in 3Gb operation 600GB models in 6Gb operation 450GB models in 3Gb operation 450GB models in 6Gb operation 300GB models in 3Gb operation 300GB models in 6Gb operation Temperature a. OperatingEnvironmental limits Shock and vibration Relative humidityEffective altitude sea level a. Operating Shock Recommended mountingCorrosive environment Air cleanlinessVibration a. Operating-normal Acoustics Mechanical specifications Mounting configuration dimensionsLevel 2 security About FipsPurpose Controlled access Admin SPAbout self-encrypting drives Data encryptionRandom number generator RNG Drive lockingData bands Cryptographic eraseAuthenticated firmware download Power requirementsSupported commands RevertSPDrive error recovery procedures Defect and error managementDrive internal defects/errors SAS system errors Background Media ScanIdle Read After Write Media Pre-ScanDeferred Auto-Reallocation Setting and determining the current Type Level Protection Information PILevels of PI Identifying a Protection Information driveInstallation Physical interfaceDrive orientation CoolingDrive mounting GroundingSAS features Interface requirementsScsi commands supported Supported commandsCommand name Command code Supported Dual port supportSupported commands Supported commands Supported commands Bytes Data hex Inquiry dataSavvio inquiry data Mode Sense data Mode Sense data for 900GB drives Mode PagesMode Sense data for 600GB drives Mode Sense values for 450GB drives Mode Sense values for 300GB drives Miscellaneous operating features and conditions Miscellaneous featuresSupported Feature or condition Miscellaneous statusSAS physical interface Datum B Section C C Section a a Physical characteristics Connector requirementsElectrical description Pin descriptionsSignal characteristics PowerSAS transmitters and receivers Ready LED OutSAS-2 Specification compliance LED drive signalDifferential signals General interface characteristicsIndex NumericsPage Msid Mtbf See also cooling Page Savvio 10K.5 SAS Product Manual, Rev. D Page Seagate Technology LLC
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