Seagate ST9146753SS, ST9300653SS 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 9.

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 KB are available as a data cache. The remaining buffer space is reserved for internal drive use.

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.

Savvio 15K.3 SAS Product Manual, Rev. A

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Contents ST9300453SS ST9146653SS ST9300653SSST9300553SS ST9146853SSST9146753SSStandard Models Self-Encrypting Drive Models SED FIPS140-2 ModelsPage Contents Installation Defect and error managementAbout Fips About self-encrypting drives Interface requirementsPage Savvio 15K.3 SAS Product Manual, Rev. a List of Figures Page Seagate Online Support and Services Seagate Technology support servicesScope Standards Applicable standards and reference documentationElectromagnetic compatibility Electromagnetic susceptibilityElectromagnetic compliance for the European Union Electromagnetic complianceAustralian C-Tick Korean KCCEuropean Union Restriction of Hazardous Substances RoHS Reference documents Scsi Commands Reference Manual SAS Interface ManualGeneral description Media description Standard featuresPerformance ReliabilityFormatted capacities Programmable drive capacity Factory installed optionsInternal drive characteristics Performance characteristicsSeek performance characteristics Access timeFormat command execution time General performance characteristicsStart/stop time Cache operation Prefetch/multi-segmented cache controlPrefetch operation Caching write dataError rates Reliability specificationsRecoverable Errors Unrecoverable ErrorsSeek errors Reliability and serviceInterface errors Preventive maintenance4 S.M.A.R.T Maximum processing delayControlling S.M.A.R.T Performance impactTemperature Log Page 0Dh Parameter Code Description Predictive failuresThermal monitor Drive Self Test DST State of the drive prior to testingDST failure definition ImplementationShort test Function Code 001b Short and extended testsExtended test Function Code 010b Log page entriesShipping Product warrantyProduct repair and return information StoragePhysical/electrical specifications PowerChoiceTM power managementPowerChoice modes DC power requirements AC power requirementsRegulation 300GB models DC power requirements146GB models DC power requirements General DC power requirement notes Power sequencingConducted noise immunity Current profile for 300GB models Current profilesCurrent profile for 146GB models Power dissipation 300GB models in 6Gb operation300GB models in 3Gb operation 146GB models in 3Gb operation 146GB models in 6Gb operationTemperature a. Operating Environmental limitsRelative humidity Effective altitude sea level a. Operating Shock and vibrationShock Recommended mounting Vibration a. Operating-normalAir cleanliness Corrosive environmentAcoustics Dimensions Mechanical specificationsAbout Fips PurposeLevel 2 security Admin SP Controlled accessAbout self-encrypting drives Data encryptionRandom number generator RNG Default passwordDrive locking Data bandsPower requirements Authenticated firmware downloadSupported commands Cryptographic eraseDefect and error management Drive internal defects/errorsDrive error recovery procedures Background Media Scan SAS system errorsMedia Pre-Scan Deferred Auto-ReallocationIdle Read After Write Protection Information PI Setting and determining the current Type LevelLevels of PI Identifying a Protection Information driveDrive orientation InstallationAir flow CoolingGrounding Drive mountingInterface requirements SAS featuresScsi commands supported Supported commandsDual port support Supported commands Supported commands Supported commands Mode Sense data Inquiry dataSavvio inquiry data Page Mode Data Header Mode Data Header Miscellaneous operating features and conditions Miscellaneous featuresMiscellaneous status SAS physical interface Datum B Section C C Section a a Connector requirements Physical characteristicsElectrical description Pin descriptionsPower Signal characteristicsSAS transmitters and receivers Ready LED OutLED drive signal SAS-2 Specification complianceDifferential signals General interface characteristicsNumerics IndexPage Msid Mtbf See also cooling Page Savvio 15K.3 SAS Product Manual, Rev. a
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