Seagate ST3450802FC, ST3300602FC manual Prefetch/multi-segmented cache control, Cache operation

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4.4Prefetch/multi-segmented cache control

The drive provides a prefetch (read look-ahead) and multi-segmented cache control algorithms that in many cases can enhance system performance. Cache refers to the drive buffer storage space when it is used in cache operations. To select this feature, the host sends the Mode Select command with the proper values in the applicable bytes in page 08h. Prefetch and cache operations are independent features from the standpoint that each is enabled and disabled independently using the Mode Select command; however, in actual opera- tion, the prefetch feature overlaps cache operation somewhat as described in sections 4.5.1 and 4.5.2.

All default cache and prefetch mode parameter values (Mode Page 08h) for standard OEM versions of this drive family are given in Section 9.3.2

4.5Cache operation

Note. Refer to the Fibre Channel Interface Manual for more detail concerning the cache bits.

Of the 16 Mbytes physical buffer space in the drive, approximately 13,000 kbytes can be used as a cache. The buffer is divided into logical segments from which data is read and to which data is written.

The drive keeps track of the logical block addresses of the data stored in each segment of the buffer. If the cache is enabled (see RCD bit in the FC Interface Manual), data requested by the host with a read command is retrieved from the buffer, if possible, before any disk access is initiated. If cache operation is not enabled, the buffer is still used, but only as circular buffer segments during disk medium read operations (disregarding Prefetch operation for the moment). That is, the drive does not check in the buffer segments for the requested read data, but goes directly to the medium to retrieve it. The retrieved data merely passes through some buffer segment on the way to the host. All data transfers to the host are in accordance with buffer-full ratio rules. See the explanation provided with the information about Mode Page 02h (disconnect/reconnect control) in the Fibre Channel Interface Manual.

The following is a simplified description of the prefetch/cache operation:

Case A—read command is received and all of the requested logical blocks are already in the cache: 1. Drive transfers the requested logical blocks to the initiator.

Case B—A Read command requests data, and at least one requested logical block is not in any segment of the cache:

1.The drive fetches the requested logical blocks from the disk and transfers them into a segment, and then from there to the host in accordance with the Mode Select Disconnect/Reconnect parameters, page 02h.

2.If the prefetch feature is enabled, refer to section 4.5.2 for operation from this point.

Each cache segment is actually a self-contained circular buffer whose length is an integer number of logical blocks. The drive dynamically creates and removes segments based on the workload. The wrap-around capa- bility of the individual segments greatly enhances the cache’s overall performance.

Note. The size of each segment is not reported by Mode Sense command page 08h, bytes 14 and 15. The value 0XFFFF is always reported regardless of the actual size of the segment. Sending a size specification using the Mode Select command (bytes 14 and 15) does not set up a new segment size. If the STRICT bit in Mode page 00h (byte 2, bit 1) is set to one, the drive responds as it does for any attempt to change an unchangeable parameter.

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

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Contents ST3600002FC ST3450802FC ST3300602FC Cheetah NS 10K.2 FCPage 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 Electromagnetic susceptibility Standards, compliance and reference documentsStandards Electromagnetic compatibilityCompliance Ncits TR-19 Reference documentsCheetah NS 10K.2 FC Product Manual, Rev. B General description Media description Standard featuresReliability PerformanceFormatted capacities User-installed accessories Factory-installed optionsProgrammable drive capacity Seek performance Performance characteristicsInternal drive characteristics Access timeST3600002FC ST3450802FC ST3300602FC Format command execution time minutesGeneral performance characteristics Start/stop timeCache operation Prefetch/multi-segmented cache controlPrefetch operation Caching write dataUnrecoverable Errors Reliability specificationsError rates Recoverable ErrorsPreventive maintenance Reliability and serviceSeek errors Interface errorsPerformance impact A.R.T. delay times4 S.M.A.R.T Controlling S.M.A.R.TThermal monitor Reporting controlDetermining rate Predictive failuresDST failure definition State of the drive prior to testingTemperature Log Page 0Dh Drive Self Test DSTExtended test Function Code 010b Invoking DSTShort and extended tests Short test Function Code 001bAbort Product warrantyCheetah NS 10K.2 FC Product Manual, Rev. B DC power requirements for ST3600002FC model Gbit Amps Physical/electrical specificationsAC power requirements DC power requirementsDC power requirements for ST3450802FC model Gbit Amps DC power requirements for ST3300602FC model Gbit Amps General DC power requirement notesConducted noise immunity Power sequencingCurrent profiles Cheetah NS 10K.2 FC Product Manual, Rev. B Cheetah NS 10K.2 FC Product Manual, Rev. B ST3600002FC Power dissipationST3450802FC ST3300602FC Environmental limits Temperature a. OperatingRelative humidity Shock and vibration Effective altitude sea level a. OperatingShock Recommended mounting Package size Packaged/product weight Drop heightCorrosive environment Air cleanlinessVibration a. Operating-normal AcousticsChina Restriction of Hazardous Substances RoHS Directive Mounting configuration dimensions Mechanical specificationsCheetah NS 10K.2 FC Product Manual, Rev. B Drive internal defects/errors Defect and error managementDrive error recovery procedures 77.81 Background Media Scan FC-AL system errorsMedia Pre-Scan Idle Read After Write Deferred Auto-ReallocationCooling InstallationDrive ID/option selection Drive orientationAir flow Drive mountingGrounding Cheetah NS 10K.2 FC Product Manual, Rev. B Interface requirements FC-AL featuresFibre Channel link service frames Link services supported Type of frameFibre Channel task management responses Fibre Channel task management functionsFC Scsi FCP response codes Function name Response code NPort login Plogi payload Bytes Fibre Channel port loginProcess Login Plri payload Fibre Channel port login acceptFibre Channel Process Login NPort Login Accept ACC payload BytesFabric Login Flogi payload Bytes Fibre Channel Process Login AcceptFibre Channel fabric login Process Login Accept ACC payload BytesFabric Login Accept ACC payload Bytes Fibre Channel fabric accept loginOption Supported Dual port supportFibre Channel Arbitrated Loop options FC-AL options supportedSupported commands Command code Command name Scsi commands supportedVerify error recovery page 07h Cheetah NS 10K.2 FC Product Manual, Rev. B Temperature page 0Dh Inquiry data Mode Sense dataCheetah NS 10K.2 FC inquiry data Bytes Data hex Page Mode Data Header Mode Pages 00 00 05 12 8B 00 50 Supported Feature or condition Miscellaneous operating features and conditionsFC-AL physical interface Physical characteristicsPhysical description Electrical description Connector requirementsPin descriptions FC-AL transmitters and receivers FC-SCA pin descriptions Pin Signal name Signal typeFault LED Out PowerActive LED Out Enable port bypass signalsMotor start controls 11 SEL6 through SEL0 ID lines Parallel Enclosure Services Interface ESIAlpa SEL ID TTL input characteristics Signal characteristicsDevice control codes FC Differential output LED driver signalsFC Differential input Receive eye diagram Eye diagram data values Link rate GHz Cheetah NS 10K.2 FC Product Manual, Rev. B Presales Support Seagate Technology support servicesTechnical Support Data Recovery Services Authorized Service Centers Warranty ServiceData Recovery Services Call Center Toll-free Direct dial Index FCP Flogi Plogi Page See also cooling Page Seagate Technology LLC
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