Seagate ST31000640SSIM 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 9.3.2.1.

4.5Cache operation

Note. Refer to the SAS 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 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 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.

Barracuda ES.2 SAS Product Manual, Rev. G

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Contents ST31000640SS ST3750630SS ST3500620SS Barracuda ES.2 SASPage General description Performance characteristicsReliability specifications Physical/electrical specificationsInstallation Defect and error managementInterface requirements Seagate Technology Support Services Barracuda ES.2 SAS Product Manual, Rev Scope Barracuda ES.2 SAS Product Manual, Rev. G Electromagnetic susceptibility Standards, compliance and reference documentsStandards Electromagnetic compatibilityCompliance Reference documents Barracuda ES.2 SAS Product Manual, Rev. G Barracuda ES.2 drive illustration General descriptionReliability Standard featuresMedia description PerformanceFormatted capacities Factory-installed optionsProgrammable drive capacity Barracuda ES.2 SAS Product Manual, Rev. G Seek performance Performance characteristicsInternal drive characteristics Access timeGeneral performance characteristics Format command execution time minutesStart/stop time Cache operation Prefetch/multi-segmented cache controlCaching write data Prefetch operation Unrecoverable Errors Reliability specificationsError rates Recoverable ErrorsPreventive maintenance Reliability and serviceSeek errors Interface errorsReporting control 4 S.M.A.R.TControlling S.M.A.R.T Performance impactPredictive failures Temperature Log Page 0DhThermal monitor Invoking DST State of the drive prior to testingDST failure definition ImplementationLog page entries Product warrantyShort test Function Code 001b Extended test Function Code 010bProduct repair and return information ShippingBarracuda ES.2 SAS Product Manual, Rev. G ST31000640SS DC power requirements Physical/electrical specificationsAC power requirements DC power requirementsST3500620SS DC power requirements ST3750630SS DC power requirementsCurrent profiles General DC power requirement notesPower sequencing Conducted noise immunityTypical ST31000640SS drive +5V and +12V current profiles Typical ST3500620SS drive +5V and +12V current profiles Power dissipationBarracuda ES.2 SAS Product Manual, Rev. G Environmental limits Temperature a. OperatingShock Relative humidityEffective altitude sea level a. Operating Shock and vibrationPackage size Packaged/product weight Drop height Corrosive environment Air cleanlinessVibration a. Operating-normal AcousticsSee Section Barracuda ES.2 SAS Product Manual, Rev. G Physical dimensions Mechanical specificationsDrive internal defects/errors Defect and error managementDrive error recovery procedures 108.29 Background Media Scan SAS system errorsMedia Pre-Scan Idle Read After Write Deferred Auto-ReallocationDrive orientation InstallationAir flow CoolingGrounding Drive mountingBarracuda ES.2 SAS Product Manual, Rev. G Interface requirements SAS featuresDual port support Commands supported by Barracuda ES.2 SAS family drives Scsi commands supportedCommand name Command code Supported Commands supported by Barracuda ES.2 SAS family drives Commands supported by Barracuda ES.2 SAS family drives Commands supported by Barracuda ES.2 SAS family drives Inquiry data Mode Sense dataBarracuda ES.2 SAS inquiry data Bytes Data hex Page Mode Pages 2.1 ST31000640SS Mode Sense data2.2 ST3750630SS Mode Sense data 2.3 ST3500620SS Mode Sense data Miscellaneous status Miscellaneous operating features and conditionsMiscellaneous features Supported Feature or conditionSAS physical interface Datum B Connector requirements Physical characteristicsSAS pin descriptions Pin Signal name Signal typeElectrical description Pin descriptionsReady LED Out Signal characteristicsPower SAS transmitters and receiversEye masks Eye masks overview LED drive signalDifferential signals General interface characteristicsNormalized time in UI Receive eye maskJitter tolerance masks Absolute amplitudeFrequency in kHz Peak-toSinusoidal JitterSignal characteristica Units Gbps Transmitter signal characteristicsGbps m, n Deterministic jitterq Receiver signal characteristicsSignal characteristic Units Gbps Maximum allowable jitterImpedance requirements Sheet 1 Impedance specificationsReceiver jitter tolerance Compliant jitter test pattern CjtpatImpedance requirements Sheet 2 Transmitter characteristicsElectrical TxRx connections = -5,437dB Shows the zero-length test load Receiver characteristics Barracuda ES.2 SAS Product Manual, Rev. G
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