Seagate ST3300557FC, ST3300457FC 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 18.

5.5Cache 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 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 FC 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 Fibre Channel 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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Cheetah 15K.7 FC Product Manual, Rev. C

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Contents ST3600857FC ST3450657FC ST3300457FC Standard models Self-Encrypting Drive modelsSED Fips 140-2 models Page Contents About Fips About self-encrypting drives Defect and error managementInstallation Interface requirementsCheetah 15K.7 FC Product Manual, Rev. C Iii Cheetah 15K.7 FC Product Manual, Rev. C Seagate Technology support services Seagate Online Support and ServicesScope Electromagnetic compatibility Applicable standards and reference documentationStandards Electromagnetic susceptibilityEuropean Union Restriction of Hazardous Substances RoHS Reference documents Ncits TR-20General description Standard features Reliability Media descriptionPerformance Formatted capacities Factory-installed options User-installed accessories Programmable drive capacity Seek performance characteristics Performance characteristicsInternal drive characteristics Access timeStart/stop time Format command execution time minutesGeneral performance characteristics ST3600057FC ST3450857FC ST3300657FCPrefetch/multi-segmented cache control Cache operationCaching write data Prefetch operationRecoverable Errors Reliability specificationsError rates Unrecoverable ErrorsInterface errors Reliability and serviceSeek errors Preventive maintenancePerformance impact 4 S.M.A.R.TControlling S.M.A.R.T MillisecondsPredictive failures Temperature Log Page 0DhDetermining rate Thermal monitorDST failure definition State of the drive prior to testingDrive Self Test DST ImplementationExtended test Function Code 010b Short and extended testsShort test Function Code 001b Log page entriesProduct warranty ShippingProduct repair and return information Physical/electrical specifications AC power requirementsDC power requirements ST3600057FC DC power requirements Gbit Amps ST3450857FC DC power requirements Gbit AmpsGeneral DC power requirement notes ST3300657FC DC power requirements Gbit AmpsPower sequencing Conducted noise immunityCurrent profiles Typical ST3450857FC current profiles Typical ST3300657FC current profiles Power dissipation 600GB model450GB models Temperature a. Operating Environmental limits300GB models Relative humidity Effective altitude sea level a. OperatingPackage size Packaged/product weight Drop height Shock and vibrationShock Recommended mounting Air cleanliness Vibration a. Operating-normalAcoustics Corrosive environment RoHS compliance statementElectromagnetic susceptibility See Section Mechanical specifications Mounting configuration dimensionsAbout Fips PurposeLevel 2 security About self-encrypting drives Controlled accessAdmin SP Data encryptionDrive locking Default passwordRandom number generator RNG Data bandsSupported commands Authenticated firmware downloadPower requirements Cryptographic eraseDefect and error management Drive internal defects/errorsDrive error recovery procedures Page FC-AL system errors These values are subject to changeBackground Media Scan Media Pre-ScanDeferred Auto-Reallocation Idle Read After Write Installation Drive ID/option selectionDrive orientation Cooling Air flowDrive mounting GroundingLink services supported Type of frame FC-AL featuresFibre Channel link service frames Interface requirementsFibre Channel task management functions Fibre Channel task management responsesFC Scsi FCP response codes Function name Response code Fibre Channel port login NPort login Plogi payload BytesFibre Channel port login accept NPort Login Accept ACC payload BytesProcess Login Plri payload Process Login Accept ACC payload Bytes Fibre Channel Process Login AcceptFibre Channel fabric login Fabric Login Flogi payload BytesFibre Channel fabric accept login Fabric Login Accept ACC payload BytesFibre Channel Arbitrated Loop options Scsi commands supportedDual port support FC-AL options supportedSupported commands Cheetah 15K.7 FC Product Manual, Rev. C Cheetah 15K.7 FC Product Manual, Rev. C Background Medium Scan 15h Mode Sense data Inquiry dataCheetah 15K.7 FC inquiry data Bytes Data hex Page Mode Data Header Block Descriptor Ec b2 5c 00 00 02 Supported Feature or condition Miscellaneous operating features and conditionsMiscellaneous features Miscellaneous statusPhysical characteristics FC-AL physical interfacePhysical description Connector requirements Electrical descriptionPin descriptions FC-SCA pin descriptions Pin Signal name Signal type Pin Signal name Signal typeFC-AL transmitters and receivers Power Fault LED OutEnable port bypass signals Active LED OutMotor start controls Parallel Enclosure Services Interface ESI 12.5.11 SEL6 through SEL0 ID linesSetting Arbitrated loop physical address Alpa valuesHex Dec Signal characteristics TTL input characteristics provides the TTL characteristicsDevice control codes LED driver signals FC Differential outputFC Differential input Receive eye diagram Eye diagram data values Link rate GHz Index NumericsDevctrlcode FCP Mtbf Page See also cooling Page Cheetah 15K.7 FC Product Manual, Rev. C
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