Seagate ST3600002SS Prefetch/multi-segmented cache control, Cache operation, Caching write data

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5.4Prefetch/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 5.5.1 and 5.5.2.

All default cache and prefetch mode parameter values (Mode Page 08h) for standard OEM versions of this drive family are given in Section 10.3.2.1.

5.5Cache operation

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

Of the 16 MB physical buffer space in the drive, approximately 13,000 KB 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 SAS 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 SAS 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 5.5.2 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.

5.5.1Caching 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.

Cheetah NS 10K.2 SAS Product Manual, Rev. F

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Contents ST3600002SS ST3450802SS Cheetah NS 10K.2 SASPage Contents Defect and error management InstallationInterface requirements Seagate Online Support and Services Seagate Technology support servicesScope Electromagnetic susceptibility Standards, compliance and reference documentsStandards Electromagnetic compatibilityCompliance Reference documents General description Reliability Standard featuresMedia description PerformanceFactory-installed options Formatted capacitiesProgrammable drive capacity Seek performance Performance characteristicsInternal drive characteristics Access timeFormat command execution time minutes General performance characteristicsStart/stop time Prefetch/multi-segmented cache control Cache operationCaching write data Prefetch operation Unrecoverable Errors Reliability specificationsError rates Recoverable ErrorsHot plugging the drive Reliability and serviceInterface errors Preventive maintenanceReporting control 4 S.M.A.R.TControlling S.M.A.R.T Performance impactDrive Self Test DST Temperature Log Page 0Dh Parameter Code DescriptionPredictive failures Thermal monitorImplementation DST failure definitionProduct warranty Short test Function Code 001bExtended test Function Code 010b Shipping ST3600002SS DC power requirements Physical/electrical specificationsAC power requirements DC power requirementsGeneral DC power requirement notes ST3450802SS DC power requirementsPower sequencing Conducted noise immunityCurrent profiles Typical ST3600002SS drive +5V and +12V current profiles Power dissipation ST3600002SSST3450802SS Environmental limits Temperature a. OperatingShock Relative humidityEffective altitude sea level a. Operating Shock and vibrationRecommended mounting Corrosive environment Air cleanlinessVibration a. Operating-normal AcousticsChina Restriction of Hazardous Substances RoHS Directive Physical dimensions Mechanical specificationsDefect and error management Drive internal defects/errorsDrive error recovery procedures 77.81 Deferred Auto-Reallocation SAS system errorsBackground Media Scan Media Pre-ScanIdle Read After Write Drive orientation InstallationAir flow CoolingGrounding Drive mountingInterface requirements SAS featuresDual port support Commands supported by Cheetah NS 10K.2 SAS family drives Scsi commands supportedCommands supported by Cheetah NS 10K.2 SAS family drives Commands supported by Cheetah NS 10K.2 SAS family drives Commands supported by Cheetah NS 10K.2 SAS family drives 10.3.2Mode Sense data Inquiry dataPage Mode Pages 10.3.2.1 ST3600002SS Mode Sense data10.3.2.2 ST3450802SS Mode Sense data Miscellaneous operating features and conditions Miscellaneous featuresMiscellaneous status SAS physical interface Datum B Connector requirements Physical characteristicsElectrical description Pin descriptionsSAS pin descriptions Ready LED Out Signal characteristicsPower SAS transmitters and receivers10.5.2.1 Eye masks 10.5.2.1.1 Eye masks overview LED drive signalDifferential signals General interface characteristicsReceive eye mask 2OP 0, 5 xadditional sinusoidal jitter X 1OP Transmitter signal characteristics Maximum allowable jitter Receiver signal characteristicsImpedance requirements Sheet 1 Compliant jitter test pattern CjtpatElectrical TxRx connections Impedance requirements Sheet 2= -5, 437dB Shows the zero-length test load SAS-2 Specification compliance Receiver characteristicsAdditional information Cheetah NS 10K.2 SAS Product Manual, Rev. F Index Page SAS See also cooling Page Americas Seagate Technology LLC
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