Seagate ST936751FC Prefetch/multi-segmented cache control, Cache operation, Caching write data

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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.

4.5Cache operation

Note. Refer to the Fibre Channel 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 can be 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 Fibre Channel 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 (still segmented with the required number of segments) 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. On a cache miss, all data transfers to the host are in accordance with buffer-full ratio rules. On a cache hit, the drive ignores the buffer-full ratio rules. See the explanation provided with the information about Mode Page 02h (dis- connect/reconnect control) in the Fibre Channel Interface Manual.

The following is a simplified description of the prefetch/cache operation:

Case A—read command is received and the first logical block is 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.

Savvio 15K.1 FC Product Manual, Rev. B

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Contents ST973451FC ST936751FC Savvio 15K.1 FCPage Contents Defect and error management Physical/electrical specificationsInstallation Interface requirementsSeagate Technology support services Savvio 15K.1 FC Product Manual, Rev. B List of Figures Page Savvio 15K.1 family disc drive ScopeSavvio 15K.1 FC Product Manual, Rev. B Standards Applicable standards and reference documentationElectromagnetic compatibility Electromagnetic susceptibilityEuropean Union Restriction of Hazardous Substances RoHS Ncits TR-19 Reference documentsSavvio 15K.1 FC Product Manual, Rev. B General description Standard features Performance Media descriptionReliability Formatted capacitiesFactory-installed accessories OptionsProgrammable drive capacity Internal drive characteristics Performance characteristicsSeek performance characteristics Access timeFormat command execution time General performance characteristicsStart/stop time Prefetch/multi-segmented cache control Cache operationCaching write data Prefetch operation Reliability specifications Error ratesRecoverable Errors Unrecoverable Errors Reliability and serviceSeek errors Interface errorsHot plugging the drive 4 S.M.A.R.TControlling S.M.A.R.T Reporting control Performance impactDetermining rate Predictive failuresParameter Code Description Temperature Log Page 0DhDrive Self Test DST DST failure definitionImplementation State of the drive prior to testingInvoking DST Short and extended testsLog page entries Product warrantyAbort ShippingProduct repair and return information Physical/electrical specifications AC power requirementsDC power requirements General DC power requirement notes Conducted noise immunity Power sequencingCurrent profiles +5V +12VTypical ST936751FC drive +12V and +5V current profiles Power dissipation CURRENT/POWER vs Throughput FC 1GB Random 8 Block ReadsST973451FC CURRENT/POWER vs Throughput FC 4GB Random 8 Block Reads ST936751FC FC CURRENT/POWER vs Throughput FC 1GB Random 8 Block ReadsTemperature FC CURRENT/POWER vs Throughput FC 4GB Random 8 Block ReadsEnvironmental limits Effective altitude sea level Relative humidityShock and vibration ShockPackage size Packaged/product weight Drop height Recommended mounting Air cleanliness VibrationCorrosive environment Acoustics Drive dimensions inches Mechanical specificationsDefect and error management Drive internal defects/errorsDrive error recovery procedures FC-AL system errors Background Media Scan Media Pre-ScanDeferred Auto-Reallocation Idle Read After Write Installation Drive orientationCooling Grounding Drive mountingFibre Channel link service frames FC-AL featuresLink services supported Type of frame Interface requirementsFibre Channel task management responses Fibre Channel task management functionsFibre Channel Scsi FCP task management functions Task name SupportedNPort login Plogi payload Bytes Fibre Channel port loginFibre Channel Process Login Fibre Channel port login acceptNPort Login Accept ACC payload Bytes Process Login Plri payloadFibre Channel fabric login Fibre Channel Process Login AcceptProcess Login Accept ACC payload Bytes Fabric Login Flogi payload BytesFabric Login Accept ACC payload Bytes Fibre Channel fabric accept loginFibre Channel Arbitrated Loop options Dual port supportFC-AL options supported Option SupportedSupported commands Command code Command name Scsi commands supportedDisconnect/reconnect control page 02h Savvio 15K.1 FC Product Manual, Rev. B LBdata Supported commands Command code Supported 4 Command name Savvio inquiry data Bytes Data hex Inquiry dataMiscellaneous features Miscellaneous operating features and conditionsSupported Feature or condition Miscellaneous statusPhysical characteristics FC-AL physical interfacePhysical description Connector requirements Part description Positions Part number FeaturesElectrical description FC-SCA pin descriptions Pin Signal name Signal type Pin Signal name Signal typePin descriptions Power Fault LED OutFC-AL transmitters and receivers Enable port bypass signals Active LED OutMotor start controls 11 SEL6 through SEL0 ID lines Parallel Enclosure Services Interface ESISetting Arbitrated loop physical address Alpa valuesHex Dec Signal characteristics TTL input characteristicsDevice control codes Differential Pecl input Differential Pecl outputSavvio 15K.1 FC Product Manual, Rev. B Internet Seagate Technology support servicesPresales Support Technical SupportWarranty Service Data Recovery Services Authorized Service CentersData Recovery Services Call Center Toll-free Direct dial Numerics IndexFAN Page Plogi SEL ID See also cooling
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