Seagate ST9500431SS, ST9500430SS 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 8.

5.5Cache 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 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 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 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 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 SAS Product Manual, Rev. E

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Contents ST9500430SSST9500431SS ST9500432SSStandard Model Self-Encrypting Drive Model SED Fips 140-2 ModelRevision history Contents Defect and error management InstallationAbout Fips About self-encrypting drives Interface requirementsPower Constellation SAS Product Manual, Rev. E List of Figures GB model current profilesConstellation SAS Product Manual, Rev. E Seagate Technology support services Seagate Online Support and ServicesScope Applicable standards and reference documentation StandardsElectromagnetic compatibility Electromagnetic susceptibilityElectromagnetic compliance Electromagnetic compliance for the European UnionAustralian C-Tick Korean KCCReference documents European Union Restriction of Hazardous Substances RoHSGeneral description Standard features Media descriptionPerformance ReliabilityFormatted capacities Programmable drive capacityFactory-installed options Performance 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 repair and return information Product warrantyShipping PowerChoice modes Physical/electrical specificationsPowerChoiceTM power management AC power requirements DC power requirementsPage Conducted noise immunity General DC power requirement notesPower sequencing Current profiles GB model current profilesPower dissipation ST9500430SS, ST9500431SS and ST9500432SS in 3 Gbit operationST9500430SS, ST9500431SS and ST9500432SS in 6 Gbit operation Temperature a. Operating Environmental limitsRelative humidity Effective altitude sea level a. OperatingShock and vibration ShockPage Recommended mounting Corrosive environment Air cleanlinessVibration a. Operating-normal Acoustics Mechanical specifications Mounting configuration dimensionsLevel 2 security About FipsPurpose Controlled access Admin SPAbout self-encrypting drives Data encryptionDefault password Random number generator RNGDrive locking Data bandsAuthenticated firmware download Power requirementsSupported commands Cryptographic eraseDrive error recovery procedures Defect and error managementDrive internal defects/errors SAS system errors Deferred Auto-Reallocation Background Media ScanMedia Pre-Scan Idle Read After Write Installation Drive orientationCooling Air flowDrive mounting GroundingSAS features Interface requirementsDual port support Scsi commands supported Supported commandsSupported commands Supported commands Supported commands Constellation inquiry data Mode Sense dataInquiry data Page Mode Data Header 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 SAS Product Manual, Rev. E Index NumericsKCC Msid Mtbf See also cooling Page Constellation SAS Product Manual, Rev. E Page Seagate Technology LLC
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