Seagate ST9146752SS, ST973452SS, ST973352SS, ST973252SS manual 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 ST9146652SS ST973252SS ST9146852SSST9146752SS ST973452SSST973352SSStandard Models Self-Encrypting Drive Models SED Fips 140-2 ModelsPage Contents Installation Defect and error managementAbout Fips About self-encrypting drives Interface requirementsPage Savvio 15K.2 SAS Product Manual, Rev. E List of Figures Page Seagate Online Support and Services Seagate Technology support servicesScope Standards Applicable standards and reference documentationElectromagnetic compatibility Electromagnetic susceptibilityElectromagnetic compliance for the European Union Electromagnetic complianceAustralian C-Tick Korean MICEuropean Union Restriction of Hazardous Substances RoHS Reference documentsGeneral description Media description Standard featuresPerformance ReliabilityFormatted capacities Options Programmable drive capacityInternal drive characteristics Performance characteristicsSeek performance characteristics Access timeFormat command execution time General performance characteristicsStart/stop time Cache operation Prefetch/multi-segmented cache controlPrefetch operation Caching write dataError rates Reliability specificationsRecoverable Errors Unrecoverable ErrorsSeek errors Reliability and serviceInterface errors Preventive maintenanceControlling S.M.A.R.T 4 S.M.A.R.TPerformance impact Reporting controlTemperature Log Page 0Dh Parameter Code Description Predictive failuresThermal monitor Drive Self Test DST State of the drive prior to testingDST failure definition ImplementationShort test Function Code 001b Short and extended testsExtended test Function Code 010b Log page entriesProduct warranty ShippingProduct repair and return information Physical/electrical specifications AC power requirementsDC power requirements Gbit mode Amps 146GB models DC power requirements73GB models DC power requirements General DC power requirement notes Power sequencingConducted noise immunity Current profile for 146GB models Current profilesCurrent profile for 73GB models Power dissipation 146GB models in 6.0 Gbit operation146GB models in 3.0 Gbit operation 73GB models in 3.0 Gbit operation 73GB models in 6.0 Gbit operationTemperature a. Operating Environmental limitsRelative humidity Effective altitude sea level a. Operating Shock and vibrationShock Recommended mounting Air cleanliness Vibration a. Operating-normalCorrosive environment Acoustics Dimensions Mechanical specificationsAbout Fips PurposeLevel 2 security Admin SP Controlled accessAbout self-encrypting drives Data encryptionRandom number generator RNG Default passwordDrive locking Data bandsPower requirements Authenticated firmware downloadSupported commands Cryptographic eraseDefect and error management Drive internal defects/errorsDrive error recovery procedures SAS system errors Background Media Scan Media Pre-ScanDeferred Auto-Reallocation Idle Read After Write Drive orientation InstallationAir flow CoolingGrounding Drive mountingInterface requirements SAS featuresDual port support Supported commands Scsi commands supportedSupported commands Supported commands Supported commands Mode Sense data Inquiry dataSavvio inquiry data Page Mode Data Header 00 00 05 12 8B 00 10 Miscellaneous operating features and conditions Miscellaneous featuresMiscellaneous status SAS physical interface Datum B Section C C Section a a Connector requirements Physical characteristicsElectrical description Pin descriptionsSAS transmitters and receivers SAS pin descriptionsSignal characteristics SAS-2 Specification compliance General interface characteristicsAdditional information Savvio 15K.2 SAS Product Manual, Rev. E Numerics IndexIraw Msid Mtbf See also cooling
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