Seagate ST3600002FC, ST3300602FC, ST3450802FC manual Caching write data, Prefetch operation

Page 20

4.5.1Caching 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 18 and 19 show the mode default settings for these drives.

4.5.2Prefetch 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.

14

Cheetah NS 10K.2 FC Product Manual, Rev. B

Page 19 Page 21
Image 20
Page 19 Page 21
Contents Cheetah NS 10K.2 FC ST3600002FC ST3450802FC ST3300602FCPage Contents Installation Seagate Technology support services Cheetah NS 10K.2 FC Product Manual, Rev. B Scope Cheetah NS 10K.2 FC Product Manual, Rev. B Standards, compliance and reference documents StandardsElectromagnetic compatibility Electromagnetic susceptibilityCompliance Reference documents Ncits TR-19Cheetah NS 10K.2 FC Product Manual, Rev. B General description Standard features Media descriptionFormatted capacities PerformanceReliability Programmable drive capacity Factory-installed optionsUser-installed accessories Performance characteristics Internal drive characteristicsAccess time Seek performanceFormat command execution time minutes General performance characteristicsStart/stop time ST3600002FC ST3450802FC ST3300602FCPrefetch/multi-segmented cache control Cache operationCaching write data Prefetch operationReliability specifications Error ratesRecoverable Errors Unrecoverable ErrorsReliability and service Seek errorsInterface errors Preventive maintenanceA.R.T. delay times 4 S.M.A.R.TControlling S.M.A.R.T Performance impactReporting control Determining ratePredictive failures Thermal monitorState of the drive prior to testing Temperature Log Page 0DhDrive Self Test DST DST failure definitionInvoking DST Short and extended testsShort test Function Code 001b Extended test Function Code 010bProduct warranty AbortCheetah NS 10K.2 FC Product Manual, Rev. B Physical/electrical specifications AC power requirementsDC power requirements DC power requirements for ST3600002FC model Gbit AmpsDC power requirements for ST3450802FC model Gbit Amps General DC power requirement notes DC power requirements for ST3300602FC model Gbit AmpsCurrent profiles Power sequencingConducted noise immunity Cheetah NS 10K.2 FC Product Manual, Rev. B Cheetah NS 10K.2 FC Product Manual, Rev. B Power dissipation ST3600002FCST3450802FC ST3300602FC Relative humidity Temperature a. OperatingEnvironmental limits Shock Effective altitude sea level a. OperatingShock and vibration Package size Packaged/product weight Drop height Recommended mountingAir cleanliness Vibration a. Operating-normalAcoustics Corrosive environmentChina Restriction of Hazardous Substances RoHS Directive Mechanical specifications Mounting configuration dimensionsCheetah NS 10K.2 FC Product Manual, Rev. B Drive error recovery procedures Defect and error managementDrive internal defects/errors 77.81 Media Pre-Scan FC-AL system errorsBackground Media Scan Deferred Auto-Reallocation Idle Read After WriteInstallation Drive ID/option selectionDrive orientation CoolingDrive mounting Air flowGrounding Cheetah NS 10K.2 FC Product Manual, Rev. B FC-AL features Fibre Channel link service framesLink services supported Type of frame Interface requirementsFC Scsi FCP response codes Function name Response code Fibre Channel task management functionsFibre Channel task management responses Fibre Channel port login NPort login Plogi payload BytesFibre Channel port login accept Fibre Channel Process LoginNPort Login Accept ACC payload Bytes Process Login Plri payloadFibre Channel Process Login Accept Fibre Channel fabric loginProcess Login Accept ACC payload Bytes Fabric Login Flogi payload BytesFibre Channel fabric accept login Fabric Login Accept ACC payload BytesDual port support Fibre Channel Arbitrated Loop optionsFC-AL options supported Option SupportedScsi commands supported Supported commands Command code Command nameVerify error recovery page 07h Cheetah NS 10K.2 FC Product Manual, Rev. B Temperature page 0Dh Cheetah NS 10K.2 FC inquiry data Bytes Data hex Mode Sense dataInquiry data Page Mode Data Header Mode Pages 00 00 05 12 8B 00 50 Miscellaneous operating features and conditions Supported Feature or conditionPhysical description Physical characteristicsFC-AL physical interface Pin descriptions Connector requirementsElectrical description FC-SCA pin descriptions Pin Signal name Signal type FC-AL transmitters and receiversPower Fault LED OutMotor start controls Enable port bypass signalsActive LED Out Parallel Enclosure Services Interface ESI 11 SEL6 through SEL0 ID linesAlpa SEL ID Device control codes Signal characteristicsTTL input characteristics FC Differential input LED driver signalsFC Differential output Receive eye diagram Eye diagram data values Link rate GHz Cheetah NS 10K.2 FC Product Manual, Rev. B Technical Support Seagate Technology support servicesPresales Support Data Recovery Services Call Center Toll-free Direct dial Warranty ServiceData Recovery Services Authorized Service Centers Index FCP Flogi Plogi Page See also cooling Page Seagate Technology LLC
Top Page Image Contents