Seagate ST1000NM0041, ST1000NM0001 manual Prefetch/multi-segmented cache control, Cache operation

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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 Table 10.

5.5Cache operation

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

Of the 64MB physical buffer space in the drive, approximately 30,000 kbytes can be used as a cache. The buf- fer 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.

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Constellation ES.1 SAS Product Manual, Rev. E

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Contents SED Fips 140-2 Models ST2000NM0041 ST1000NM0041 ST500NM0041Standard Models Self-Encrypting Drive Models Revision history Contents Defect and error management InstallationAbout Fips About self-encrypting drives Interface requirementsPage Constellation ES.1 SAS Product Manual, Rev. E List of Figures Constellation ES.1 SAS Product Manual, Rev. E Seagate Technology support services Seagate Online Support and ServicesScope Model Number Self-Encrypting Drive SED Fips 140-2 LevelApplicable standards and reference documentation StandardsElectromagnetic compatibility Electromagnetic susceptibilityElectromagnetic compliance Electromagnetic compliance for the European UnionAustralian C-Tick Korean KCCEuropean Union Restriction of Hazardous Substances RoHS China Restriction of Hazardous Substances RoHS DirectiveReference documents Self-Encrypting Drives Reference ManualGeneral description Performance Standard featuresMedia description Factory-installed options Reliability Formatted capacities Programmable drive capacityPerformance characteristics Internal drive characteristicsSeek performance characteristics Access timeGeneral performance characteristics Start/stop timePrefetch/multi-segmented cache control Cache operationCaching write data Prefetch operationReliability specifications Error ratesRecoverable Errors Unrecoverable ErrorsReliability and service Seek errorsInterface errors Preventive maintenance4 S.M.A.R.T Controlling S.M.A.R.TPerformance impact Reporting controlThermal monitor Temperature Log Page 0Dh Parameter Code DescriptionPredictive failures State of the drive prior to testing Drive Self Test DSTDST failure definition ImplementationShort and extended tests Short test Function Code 001bExtended test Function Code 010b Log page entriesProduct warranty ShippingStorage Product repair and return informationPowerChoice modes Physical/electrical specificationsPowerChoiceTM power management AC power requirements DC power requirementsRegulation ±5% Constellation ES.1 SAS Product Manual, Rev. E Constellation ES.1 SAS Product Manual, Rev. E Conducted noise immunity General DC power requirement notesPower sequencing Current profiles TB model current profilesConstellation ES.1 SAS Product Manual, Rev. E GB model current profiles Power dissipation 2TB models in 3Gb operation2TB models in 6Gb operation 1TB models in 3Gb operation 1TB models in 6Gb operation 500GB models in 3Gb operation 500GB models in 6Gb operation Relative humidity Temperature a. OperatingEnvironmental limits Shock Effective altitude sea level a. OperatingShock and vibration Vibration a. Operating-normal Recommended mountingCorrosive environment Air cleanlinessAcoustics Mechanical specifications Mounting configuration dimensionsAbout Fips PurposeValidation Program Seagate Enterprise SEDExample of Fips tamper evidence labels Controlled access Admin SPAbout self-encrypting drives Data encryptionAuthenticated firmware download Random number generator RNGDrive locking Data bandsPower requirements Supported commandsSanitize Cryptographic Erase RevertSPDrive error recovery procedures Defect and error managementDrive internal defects/errors SAS system errors Deferred Auto-Reallocation Background Media ScanMedia Pre-Scan Setting and determining the current Type Level Idle Read After WriteProtection Information PI Levels of PIIdentifying a Protection Information drive Installation Drive orientationCooling Air flowDrive mounting GroundingSAS features Interface requirementsDual port support Scsi commands supported Supported commandsSupported commands Supported commands Supported commands Constellation ES.1 inquiry data Mode Sense dataInquiry data Page Mode Sense data changeable and default values for 2TB drives Mode Sense data changeable and default values for 1TB drives 3a 38 60 30 00 00 02 Miscellaneous status Miscellaneous operating features and conditionsMiscellaneous features SAS physical interface Datum B Section C C Section a a Physical characteristics Connector requirementsElectrical description Pin descriptionsSignal characteristics PowerSAS transmitters and receivers Ready LED OutSAS-2 Specification Compliance LED drive signalDifferential signals General interface characteristicsConstellation ES.1 SAS Product Manual, Rev. E Index NumericsKCC Msid Mtbf See also cooling Page Constellation ES.1 SAS Product Manual, Rev. E Page Seagate Technology LLC Americas
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