Seagate ST3600857SS, ST3300457SS manual Prefetch/multi-segmented cache control, Cache operation

Page 17

If the drive receives a START STOP UNIT command with the START bit and IMMED bit equal to 1 and does not receive a NOTIFY (ENABLE SPINUP) primitive within 5 seconds, the drive fails the START STOP UNIT command.

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.

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 Section 12.3.2.1.

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 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.

Cheetah 15K.7 SAS Product Manual, Rev. D

11

Image 17
Contents ST3450657SS ST3600857SSST3300457SS Standard models Self-Encrypting Drive modelsPage Contents Installation Defect and error managementAbout Fips About self-encrypting drives Interface requirementsPower Cheetah 15K.7 SAS Product Manual, Rev. D Seagate Online Support and Services Seagate Technology support servicesScope Standards Standards, compliance and reference documentsElectromagnetic compatibility Electromagnetic susceptibilityEuropean Union Restriction of Hazardous Substances RoHS ComplianceReference documents General description Performance Standard featuresMedia description Reliability Factory-installed optionsFormatted capacities Programmable drive capacityInternal drive characteristics Performance characteristicsAccess time Seek timeStart/stop time Format command execution time minutesGeneral performance characteristics Cache operation Prefetch/multi-segmented cache controlPrefetch operation Caching write dataError rates Reliability specificationsRecoverable Errors Unrecoverable ErrorsSeek errors Reliability and serviceInterface errors Preventive maintenanceControlling S.M.A.R.T 4 S.M.A.R.TPerformance impact MillisecondsDetermining rate Reporting controlPredictive failures Thermal monitorState of the drive prior to testing Temperature Log Page 0Dh Parameter Code DescriptionDrive Self Test DST DST failure definitionShort and extended tests Invoking DSTShort test Function Code 001b Extended test Function Code 010bAbort Product warrantyShipping Product repair and return informationDC power requirements Physical/electrical specificationsAC power requirements Gbit mode Amps 600 GB DC power requirements450 GB DC power requirements 300 GB DC power requirements Current profiles Power sequencingConducted noise immunity Typical 600GB current profiles Typical 300GB current profiles Power dissipation 450GB model Environmental limits Temperature a. OperatingEffective altitude sea level a. Operating Relative humidityShock and vibration ShockPackage size Packaged/product weight Drop height Recommended mounting Vibration a. Operating-normalAcoustics Air cleanlinessCorrosive environment RoHS compliance statementUnits of Measure mm inches Mechanical specificationsLevel 2 security About FipsPurpose Admin SP Controlled accessAbout self-encrypting drives Data encryptionRandom number generator RNG Authenticated firmware downloadDrive locking Data bandsSupported commands Power requirementsDrive error recovery procedures Defect and error managementDrive internal defects/errors Page Background Media Scan SAS system errorsIdle Read After Write Media Pre-ScanDeferred Auto-Reallocation Cooling InstallationDrive orientation Grounding Drive mountingInterface requirements SAS featuresDual port support Scsi commands supportedCommands supported by Cheetah 15K.6 SAS family drives Commands supported by Cheetah 15K.6 SAS family drives Commands supported by Cheetah 15K.6 SAS family drives Commands supported by Cheetah 15K.6 SAS family drives Cheetah 15K.7 inquiry data Bytes Data hex Inquiry dataMode Sense data Mode Pages 12.3.2.1 600GBmodel Mode Sense dataDEF CHG Mode Data Header Block Descriptor Mode Pages 12.3.2.2 450GB model Mode Sense dataDEF CHG Read Capacity Data Buffer C D E F Mode Pages 12.3.2.3 300GB model Mode Sense dataMiscellaneous features Miscellaneous operating features and conditionsSAS physical interface Miscellaneous statusDatum B Connector requirements Physical characteristicsSAS pin descriptions Electrical descriptionPin descriptions Power Signal characteristicsSAS transmitters and receivers Ready LED OutLED drive signal SAS-2 Specification complianceDifferential signals General interface characteristicsCheetah 15K.7 SAS Product Manual, Rev. D Numerics IndexPage SAS Smart
Related manuals
Manual 62 pages 52 Kb