Seagate ST3300656SS, ST3146356SS manual Prefetch/multi-segmented cache control, Cache operation

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The START STOP UNIT command may be used to command the drive to stop the spindle. Stop time is 30 sec- onds (maximum) from removal of DC power. There is no power control switch on the drive.

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.

Cheetah 15K.6 SAS Product Manual, Rev. A

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Contents ST3450856SS ST3300656SS ST3146356SS Cheetah 15K.6 SASPage Contents Interface requirements Defect and error managementInstallation Seagate Technology support servicesCheetah 15K.6 SAS disk drive List of FiguresCheetah 15K.6 SAS Product Manual, Rev. a Cheetah 15K.6 SAS disk drive ScopeCheetah 15K.6 SAS Product Manual, Rev. a Electromagnetic susceptibility Standards, compliance and reference documentsStandards Electromagnetic compatibilityEuropean Union Restriction of Hazardous Substances RoHS ComplianceReference documents Cheetah 15K.6 SAS Product Manual, Rev. a General description Media description Standard featuresPerformance Formatted capacities Factory-installed accessoriesOptions factory installed ReliabilityCheetah 15K.6 SAS Product Manual, Rev. a Seek time Performance characteristicsInternal drive characteristics Access timeGeneral performance characteristics Format command execution time minutesStart/stop time Cache operation Prefetch/multi-segmented cache controlPrefetch operation Caching write dataUnrecoverable 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 failuresParameter Code Description Temperature Log Page 0DhPrimary Temperature Reference TemperatureShort test Function Code 001b State of the drive prior to testingInvoking DST Short and extended testsProduct repair and return information Product warrantyAbort ShippingCheetah 15K.6 SAS Product Manual, Rev. a ST3450856SS DC power requirements Physical/electrical specificationsAC power requirements DC power requirementsST3146356SS Gbit mode Amps ST3300656SS DC power requirementsST3146356SS DC power requirements ST3300656SS Gbit mode AmpsCurrent profiles General DC power requirement notesPower sequencing Conducted noise immunityTypical ST3450856SS current profiles Typical ST3300656SS current profiles Typical ST3146356SS current profiles ST3450856SS in 3 Gbit operation Power dissipationST3300656SS in 3 Gbit operation ST3300656SS CURRENT/POWER vs Throughput SAS 3.0GBST3146356SS in 3 Gbit operation ST3146356SS CURRENT/POWER vs Throughput SAS 3.0GBRandom 8 Block Reads Environmental limits TemperatureShock Relative humidityEffective altitude sea level Shock and vibrationRecommended mounting Package size Packaged/product weight Drop heightCorrosive environment Air cleanlinessVibration AcousticsMechanical specifications Physical dimensions Cheetah 15K.6 SAS Product Manual, Rev. a Drive internal defects/errors Defect and error managementDrive error recovery procedures 51.87 Background Media Scan SAS system errorsMedia Pre-Scan Idle Read After Write Deferred Auto-ReallocationDrive orientation InstallationAir flow CoolingGrounding Drive mountingCheetah 15K.6 SAS Product Manual, Rev. a Interface requirements SAS featuresDual port support Commands supported by Cheetah 15K.6 SAS family drives Scsi commands supportedCommand name Command code Supported Commands supported by Cheetah 15K.6 SAS family drives Commands supported by Cheetah 15K.6 SAS family drives Commands supported by Cheetah 15K.6 SAS family drives Inquiry data Mode Sense dataBytes Data hex Page Mode Pages 2.1 ST3450856SS Mode Sense data2.2 ST3300656SS Mode Sense data 2.3 ST3146356SS 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 Cheetah 15K.6 SAS Product Manual, Rev. a Technical Support Seagate Technology support servicesInternet Presales SupportData Recovery Services Authorized Service Centers Warranty ServiceData Recovery Services Call Center Toll-free Direct dial Numerics IndexPage SAS See also cooling
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