Seagate ST3400755FC Prefetch/multi-segmented cache control, Cache operation, Caching write data

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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 disc access is initiated. If cache operation is not enabled, the buffer is still used, but only as circular buffer segments during disc 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 disc 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.

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.

Cheetah NS FC Product Manual, Rev. A

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Contents ST3400755FC ST3300955FC Cheetah NS FCPage Cheetah NS FC Product Manual, Rev. a Cheetah NS FC Product Manual, Rev. a Contents Interface requirements Defect and error managementInstallation Seagate Technology support services Cheetah NS FC Product Manual, Rev. a List of Figures Page Scope Cheetah NS FC Product Manual, Rev. a Electromagnetic susceptibility Standards, compliance and reference documentsStandards Electromagnetic compatibilityCompliance Ncits TR-19 Reference documentsCheetah NS FC Product Manual, Rev. a General description Media description Standard featuresProgrammable drive capacity PerformanceReliability Formatted capacities User-installed accessories Factory-installed optionsSeek performance Performance characteristicsInternal drive characteristics Access timeStart/stop time Format command execution time minutesGeneral performance characteristics Caching write data Prefetch/multi-segmented cache controlCache operation Prefetch operation Unrecoverable Errors Reliability specificationsError rates Recoverable ErrorsPreventive maintenance Reliability and serviceSeek errors Interface errorsMilliseconds 4 S.M.A.R.TControlling S.M.A.R.T Performance impactThermal monitor Reporting controlDetermining rate Predictive failuresDST failure definition State of the drive prior to testingParameter Code Description Drive Self Test DSTExtended test Function Code 010b Invoking DSTShort and extended tests Short test Function Code 001bProduct repair and return information Product warrantyAbort ShippingCheetah NS FC Product Manual, Rev. a DC power requirements for ST3400755FC model Gbit Amps Physical/electrical specificationsAC power requirements DC power requirementsDC power requirements for ST3300955FC model Gbit Amps General DC power requirement notesConducted noise immunity Power sequencingCurrent profiles Cheetah NS FC Product Manual, Rev. a Random 8 Block Reads Power dissipationST3400755 CURRENT/POWER vs Throughput FC 4GB ST3400755FCST3300955FC ST3300955 CURRENT/POWER vs Throughput FC 4GBRelative humidity TemperatureEnvironmental limits Shock Package size Packaged/product weight Drop heightEffective altitude sea level Shock and vibrationRecommended mounting Corrosive environment Air cleanlinessVibration AcousticsChina Restriction of Hazardous Substances RoHS Directive X表示该部件（于同类物品程度上）所含的危险和有毒物质中国RoHS MCV标准所定义的门槛值。Mounting configuration dimensions Mechanical specificationsCheetah NS FC Product Manual, Rev. a Drive error recovery procedures Defect and error managementDrive internal defects/errors 77.87 Media Pre-Scan FC-AL system errorsBackground Media Scan Idle Read After Write Deferred Auto-ReallocationCooling InstallationDrive ID/option selection Drive orientationAir flow Drive mountingGrounding Cheetah NS FC Product Manual, Rev. a Interface requirements FC-AL featuresFibre Channel link service frames Link services supported Type of frameFC Scsi FCP response codes Function name Response code Fibre Channel task management functionsFibre Channel task management responses 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 FC Product Manual, Rev. a Temperature page 0Dh Cheetah NS FC inquiry data Bytes Data hex Mode Sense dataInquiry data Page Mode Sense 10 Byte Header Data Mode Data Header Miscellaneous status Miscellaneous operating features and conditionsMiscellaneous features Supported Feature or conditionPhysical description Physical characteristicsFC-AL physical interface Pin descriptions Connector requirementsElectrical description FC-AL transmitters and receivers FC-SCA pin descriptions Pin Signal name Signal typePin Signal name Signal type Fault LED Out PowerMotor start controls Enable port bypass signalsActive LED Out 11 SEL6 through SEL0 ID lines Parallel Enclosure Services Interface ESIHex Dec SettingArbitrated loop physical address Alpa values 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 Technical Support Seagate Technology support servicesInternet Presales SupportData Recovery Services Call Center Toll-free Direct dial Warranty ServiceData Recovery Services Authorized Service Centers Numerics IndexFAN Flogi Plogi SEL ID See also cooling
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