Seagate ST318438LW, ST318418N, ST336938LW, ST336918N manual Prefetch operation

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Barracuda 36ES2 Product Manual, Rev. B

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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 8 and 9 show Mode default settings for the drives.

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 can be 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. Prefetch is enabled using Mode Select page 08h, byte 12, bit 5 (Disable Read Ahead - DRA bit). DRA bit = 0 enables prefetch. Since data that is prefetched replaces data already in some buffer segment(s), the host can limit the amount of prefetch data to optimize system per- formance. The max prefetch field (bytes 8 and 9) limits the amount of prefetch. The drive does not use the Prefetch Ceiling field (bytes 10 and 11).

During a prefetch operation, the drive crosses a cylinder boundary to fetch more data only if the Discontinuity (DISC) bit is set to one in bit 4 of byte 2 of Mode parameters page 08h.

Whenever prefetch (read look-ahead) is enabled (enabled by DRA = 0), it operates under the control of ARLA (Adaptive Read Look-Ahead). If the host uses software interleave, ARLA enables prefetch of contiguous blocks from the disc when it senses that a prefetch hit will likely occur, even if two consecutive read operations were not for physically contiguous blocks of data (e.g., “software interleave”). ARLA disables prefetch when it decides that a prefetch hit will not likely occur. If the host is not using software interleave, and if two sequential read operations are not for contiguous blocks of data, ARLA disables prefetch, but as long as sequential read operations request contiguous blocks of data, ARLA keeps prefetch enabled.

Image 21 Seagate ST318438LW, ST318418N, ST336938LW, ST336918N manual Prefetch operation
Contents Product Manual, Volume Page Product Manual, Volume Page Revision Date Writer/Engineer Sheets Affected Revision status summary sheetPage Contents Installation Defect and error managementSeagate Technology support services Interface requirementsBarracuda 36ES2 Product Manual, Rev. B Vii List of FiguresPage Barracuda 36ES2 family drive ST336938LW shown ScopeBarracuda 36ES2 Product Manual, Rev. B Applicable standards and reference documentation Australian C-Tick Korean MICTaiwanese Bsmi General description Media characteristics Standard featuresPerformance ReliabilityFactory installed accessories Options factory installedProgrammable drive capacity Performance characteristics Ultra2 Scsi Ultra160 Scsi Start/stop timePrefetch/multi-segmented cache control Caching write data Cache operationPrefetch operation Barracuda 36ES2 Product Manual, Rev. B Reliability specifications Preventive maintenance Reliability and serviceService life Service philosophyControlling S.M.A.R.T 7 S.M.A.R.TReporting control Performance impactDetermining rate Predictive failuresDST Failure Definition State of the drive prior to testingImplementation Invoking DSTShort test Function Code 001b Product warrantyExtended test Function Code 010b Log page entriesProduct repair and return information Barracuda 36ES2 Product Manual, Rev. B Physical/electrical specifications AC power requirementsDC power requirements Power sequencing Conducted noise immunity3 12 V Current profile +5 Volt Current during spindle start Typical Amper es Amps Power dissipation ST336938/ST318438 Barracuda 36ES2 Product Manual, Rev. B ST336918/ST318418 Environmental limits TemperatureEffective altitude sea level Barracuda 36ES2 Product Manual, Rev. B Relative humidityShock and vibration ShockRecommended mounting Air cleanliness Barracuda 36ES2 Product Manual, Rev. B VibrationAcoustics Dimension Table Mechanical specificationsMounting configuration dimensions Barracuda 36ES2 Product Manual, Rev. B Defect and error management Drive error recovery proceduresDrive internal defects Scsi systems errors Configure drive options InstallationFormatting Drive ID/option select headerDo not install jumpers retain cover J6 jumper header for N model J5 jumper header on LW models only J2 option select header for LW models Barracuda 36ES2 Product Manual, Rev. B Barracuda 36ES2 Product Manual, Rev. B Function description Drive orientation CoolingAir flow Grounding Barracuda 36ES2 Product Manual, Rev. B Drive mountingBarracuda 36ES2 Product Manual, Rev. B Scsi interface messages supported Interface requirementsScsi interface commands supported Commands supported by Barracuda 36ES2 family drives Barracuda 36ES2 Product Manual, Rev. B Vendor ID Unit serial number page 80h Inquiry Vital Product dataImplemented operating definition page 81h Firmware numbers page C0hJumper settings page C2h Mode Sense data DEF CHG Barracuda 36ES2 Product Manual, Rev. B Condition/features supported Supported Status supported DC cable and connector 2 REQ/ACK offsetPhysical interface Model drive physical interface 50-pin Scsi I/O connector Pin Power Scsi interface cable requirements Scsi interface physical descriptionMating connectors for N model drives Barracuda 36ES2 Product Manual, Rev. B Mating connectorsScsi daisy chain interface cabling for N drives Nonshielded 68 pin Scsi device connector used on LW drives Scsi daisy chain interface cabling for LW drives GND GND DBP1 +DBP1 Multimode signals Electrical descriptionMultimode-SE and LVD alternatives LVD output signals Input characteristicsReceiver characteristics Transmitter characteristicsSingle-ended drivers/receivers for N models Single-ended drivers/receivers for LW modelsLow voltage differential I/O circuits General cable characteristicsTerminator power Drives Terminator requirementsLW drives Page Following table provides drive activity LED status Drive activity LEDAutomated Services Seagate Technology support services Online ServicesPresales Support Technical SupportCustomer Service CSO USA/Canada/Latin America Support ServicesEuropean Support Services Africa/Middle East Support Services Asia/Pacific Support ServicesCall Center Toll-free Direct dial Barracuda 36ES2 Product Manual, Rev. B Index Diffsens LED PCB System chassis Barracuda 36ES2 Product Manual, Rev. B