Seagate ST39205LW/LC, ST318405LW/LC, ST336705LW/LC manual Prefetch operation

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Cheetah 36XL Product Manual, Rev. E

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 exceeds 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 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 increase the number of cache buffer segments above the value defined in Mode Page 8, Byte 13, as needed to optimize the performance, based on the com- mand 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 Cheetah 36XL Family ST336705LW/LC ST318405LW/LC ST39205LW/LC Page Cheetah 36XL Family ST336705LW/LC ST318405LW/LC ST39205LW/LC 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 36XL Product Manual, Rev. E ViiPage Scope Cheetah 36XL family drive ST336705LC shownCheetah 36XL Product Manual, Rev. E Applicable standards and reference documentation Korean MIC Australian C-TickTaiwanese Bsmi General description Cheetah 36XL family drive Performance Cheetah 36XL Product Manual, Rev. E Standard featuresMedia characteristics ReliabilityFormatted capacities Factory installed accessoriesOptions factory installed Programmable drive capacity Performance characteristics Start/stop time Ultra2 Scsi Ultra160 ScsiPrefetch/multi-segmented cache control Cache operation Caching write dataPrefetch operation Cheetah 36XL Product Manual, Rev. E Cheetah 36XL Product Manual, Rev. E Reliability specifications Service life Reliability and servicePreventive maintenance Service philosophy7 S.M.A.R.T Controlling S.M.A.R.TDetermining rate Performance impactReporting control Predictive failuresImplementation State of the drive prior to testingDST Failure Definition Invoking DSTExtended test Function Code 010b Product warrantyShort test Function Code 001b Log page entriesProduct repair and return information Cheetah 36XL Product Manual, Rev. E AC power requirements Physical/electrical specificationsDC power requirements Conducted noise immunity Power sequencing3 12 V Current profile Typical ST336705 drive +12 V current profile Typical ST39205 drive +12 V current profile TypicalST318405 drive +5 V current profile Power dissipation ST336705 Cheetah 36XL Product Manual, Rev. E ST318405 Os per SecondST39205 TemperatureEnvironmental limits Shock and vibration Relative humidityEffective altitude sea level ShockPackage size Packaged/product weight Drop height Recommended mounting Vibration Air cleanlinessAcoustics LW mounting configuration dimensions LC 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 model only J2 option select header Cheetah 36XL Product Manual, Rev. E Function description Does not apply to LC modelsDrive mounting Drive orientationCooling GroundingInterface requirements Scsi interface messages supportedScsi interface commands supported Cheetah 36XL Product Manual, Rev. E Commands supported by Cheetah 36XL family drive Mode Sense data Inquiry Vital Product dataCheetah 36XL Product Manual, Rev. E Mode Sense Data Cheetah 36XL Product Manual, Rev. E Mode sense data, ST318405 values Bytes Cheetah 36XL Product Manual, Rev. E Mode sense data, ST39205 values Bytes Cheetah 36XL Product Manual, Rev. E Condition/features supported 2 REQ/ACK offset DC cable and connectorPhysical interface Pin Power Scsi interface physical description Scsi interface cable requirementsMating connectors for LC model drives Mating connectorsMating connectors for LW model drives ST336705LC, ST318405LC and ST39205LCRight-angle to Pcba connectors Scsi IDNonshielded 68 pin Scsi device connector used on LW drives Nonshielded 80 pin Scsi SCA-2 connector, used on LC drives GND +DBP1 Charge 68Cheetah 36XL Product Manual, Rev. E Electrical description Multimode signalsMultimode-SE and LVD alternatives Output characteristics Input characteristicsLow voltage differential I/O circuits General cable characteristicsTerminator requirements Terminator power LW drives LC drivesDisc drive Scsi timing Drive activity remote LED signal status Cheetah 36XL 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 36XL Product Manual, Rev. E Index Symbols Cheetah 36XL Product Manual, Rev. E LED PCB System chassis Page
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