Seagate ST9450404SS Prefetch/multi-segmented cache control, Cache operation, Caching write data

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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 Table 9.

4.5Cache operation

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

Of the 16Mb physical buffer space in the drive, approximately 13,000Kb 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 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.

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Contents Standard Models Self-Encrypting Drive Models ST9600204SSST9600104SS ST9450404SSST9450304SSRev. a 12/15/09 Initial release Revision historyContents Interface requirements Defect and error managementInstallation About self-encrypting drivesSeagate Technology support services Savvio 10K.4 SAS Product Manual, Rev. a List of Figures Page Scope Electromagnetic susceptibility Applicable standards and reference documentationStandards Electromagnetic compatibilityKorean KCC Electromagnetic complianceElectromagnetic compliance for the European Union Australian C-TickEuropean Union Restriction of Hazardous Substances RoHS Reference documentsGeneral description Media description Standard featuresProgrammable drive capacity PerformanceReliability Formatted capacitiesFactory-installed options Access time Performance characteristicsInternal drive characteristics Seek performance characteristicsStart/stop time General performance characteristicsCache operation Prefetch/multi-segmented cache controlCaching write data Prefetch operation Unrecoverable Errors Reliability specificationsError rates Recoverable ErrorsPreventive maintenance Reliability and serviceSeek errors Interface errorsPerformance impact Maximum processing delay Fully-enabled delay4 S.M.A.R.T Controlling S.M.A.R.TPredictive failures Temperature Log Page 0Dh Parameter Code DescriptionThermal monitor Implementation State of the drive prior to testingDrive Self Test DST DST failure definitionLog page entries Short and extended testsShort test Function Code 001b Extended test Function Code 010bShipping Product warrantyProduct repair and return information PowerChoiceTM power management Physical/electrical specificationsPowerChoice modes DC power requirements AC power requirementsSavvio 10K.4 SAS Product Manual, Rev. a Page Page Power sequencing General DC power requirement notesConducted noise immunity Current profiles for 600GB models Current profilesCurrent profiles for 450GB models GB models in 3Gb operation Power dissipation600GB models in 6Gb operation 450GB models in 3Gb operation Environmental limits Temperature a. Operating450GB models in 6Gb operation Effective altitude sea level a. Operating Relative humidityShock and vibration Shock Recommended mounting Vibration a. Operating-normal Air cleanlinessCorrosive environment Acoustics Mounting configuration dimensions Mechanical specificationsData encryption Controlled accessAdmin SP About self-encrypting drivesData bands Default passwordRandom number generator RNG Drive lockingCryptographic erase Authenticated firmware downloadPower requirements Supported commandsDrive internal defects/errors Defect and error managementDrive error recovery procedures SAS system errors Media Pre-Scan Background Media ScanDeferred Auto-Reallocation Idle Read After Write Drive orientation InstallationAir flow CoolingGrounding Drive mountingInterface requirements SAS featuresDual port support Supported commands Scsi commands supportedSupported commands Supported commands Supported commands Inquiry data Mode Sense dataSavvio inquiry data Page Mode Pages Mode Sense data for ST9600204SS and ST9600104SS drivesMode Sense values for ST9450404SS and ST9450304SS drives Miscellaneous features Miscellaneous operating features and conditionsMiscellaneous status SAS physical interface Datum B Section C C Section a a Pin descriptions Physical characteristicsConnector requirements Electrical descriptionReady LED Out Signal characteristicsPower SAS transmitters and receiversGeneral interface characteristics SAS-2 Specification complianceLED drive signal Differential signalsTechnical Support Seagate Technology support servicesInternet Presales SupportData Recovery Services Authorized Service Centers Warranty ServiceData Recovery Services Call Center Toll-free Direct dialFAX Numerics IndexKCC Msid Mtbf See also cooling Page Savvio 10K.4 SAS Product Manual, Rev. a Page Seagate Technology LLC
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