Seagate ST336605LW/LC/LCV, ST373405LW/LWV/LC/LCV manual Prefetch operation

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Cheetah 73LP Product Manual, Rev. E

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 generated.

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 7 and 8 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 perfor- mance. 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.

4.5.3Optimizing cache performance for desktop and server applications

Desktop and server applications require different drive caching operations for optimal performance. This means it is difficult to provide a single configuration that meets both of these needs. In a desktop environment, you want to configure the cache to respond quickly to repetitive accesses of multiple small segments of data without taking the time to “look ahead” to the next contiguous segments of data. In a server environment, you want to configure the cache to provide large volumes of sequential data in a non-repetitive manner. In this case, the ability of the cache to “look ahead” to the next contiguous segments of sequential data is a good thing.

The Performance Mode (PM) bit controls the way the drive switches the cache buffer into different modes of segmentation. In “server mode” (PM bit = 0), the drive can dynamically change the number of cache buffer seg- ments as needed to optimize the performance, based on the command stream from the host. In “desktop mode” (PM bit = 1), the number of segments is maintained at the value defined in Mode Page 8, Byte 13, at all times. For additional information about the PM bit, refer to the Unit Attention Parameters page (00h) of the Mode Sense command (1Ah) in the SCSI Interface Product Manual, part number 75789509.

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Contents ST373405LW/LWV/LC/LCV ST336605LW/LC/LCV Page ST373405LW/LWV/LC/LCV ST336605LW/LC/LCV Page Revision status summary sheet Revision Date Writer/Engineer Sheets AffectedPage Contents Seagate Technology support services Defect and error managementInstallation Interface requirementsList of Figures Cheetah 73LP Product Manual, Rev. E ViiPage Scope Cheetah 73LP family drive ST373405LW shownCheetah 73LP Product Manual, Rev. E Applicable standards and reference documentation Korean MIC Australian C-TickTaiwanese Bsmi General description Cheetah 73LP family drive Performance Cheetah 73LP Product Manual, Rev. E Standard featuresMedia characteristics ReliabilityAccessories user installed Factory installed accessoriesOptions factory installed Unformatted and formatted capacitiesPerformance characteristics Start/stop time Ultra2 Scsi Ultra3 ScsiPrefetch/multi-segmented cache control Cheetah 73LP Product Manual, Rev. E Cache operation Caching write dataPrefetch operation Reliability specifications Service life Reliability and servicePreventive maintenance Service philosophy7 S.M.A.R.T Controlling S.M.A.R.TReporting control Performance impactMilliseconds Determining rateImplementation State of the drive prior to testingDST Failure Definition Invoking DSTExtended test Function Code 010b Short and extended testsShort test Function Code 001b Log page entriesShipping Cheetah 73LP Product Manual, Rev. E Product warrantyProduct repair and return information Cheetah 73LP Product Manual, Rev. E AC power requirements Physical/electrical specificationsDC power requirements Conducted noise immunity Power sequencing3 12 V Current profile Typical ST336605 drive +12 V current profile Typical ST336605 drive +5 V current profile Amperes 000 900 800 700 600 Relative humidity TemperatureCheetah 73LP Product Manual, Rev. E Environmental limits Effective altitude sea levelShock and vibration ShockRecommended mounting Vibration Air cleanlinessAcoustics LW and LWV mounting configuration dimensions LC and LCV mounting configuration dimensions Drive error recovery procedures Defect and error managementDrive internal defects Scsi systems errors Formatting InstallationConfigure drive options Drive ID/option select headerDo not install jumpers retain cover J5 jumper header on LW/LWV models only J2 option select header Cheetah 73LP Product Manual, Rev. E Function description Does not apply to LC/LCV modelDrive orientation CoolingCheetah 73LP Product Manual, Rev. E Drive mounting GroundingCheetah 73LP Product Manual, Rev. E Interface requirements Scsi interface messages supportedScsi interface commands supported Cheetah 73LP Product Manual, Rev. E Cheetah 73LP Product Manual, Rev. E Inquiry Vital Product data Mode Sense data DEF CHG Cheetah 73LP Product Manual, Rev. E Condition/feature supported by 2 REQ/ACK offset DC cable and connectorPhysical interface Pin Power Scsi interface physical description Scsi interface cable requirementsMating connectors for LC and LCV model drives Mating connectorsMating connectors for LW and LWV model drives ST373405LC/LCV and ST336605LC/LCVCheetah 73LP Product Manual, Rev. E Right-angle to Pcba connectors Scsi ID650±.005 155 100 346 Front View GND +DBP1 62Cheetah 73LP Product Manual, Rev. E REQ +REQ Electrical description LVD output characteristicsMultimode-SE and LVD alternatives General cable characteristics Single-ended drivers/receiversCables for low voltage differential drivers/receivers Terminator requirementsLC and LCV drives Disc drive Scsi timing Spindle status Command status LED status Drive activity LEDCheetah 73LP Product Manual, Rev. E Presales Support Seagate Technology support services Online ServicesAutomated Services Technical SupportUSA/Canada/Latin America Support Services Customer Service CSOEuropean Support Services Asia/Pacific Support Services Africa/Middle East Support ServicesCall Center Toll-free Direct dial Cheetah 73LP Product Manual, Rev. E Index Cheetah 73LP Product Manual, Rev. E Cheetah 73LP Product Manual, Rev. E Scam Scsi Scsi ID 80Cheetah 73LP Product Manual, Rev. E
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