Seagate ST373453LC manual Cache operation, Caching write data

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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 18.

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 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 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.

If read caching is enabled (RCD=0), then data written to the medium is retained in the cache to be made avail- able for future read cache hits. The same buffer space and segmentation is used as set up for read functions. The buffer segmentation scheme is set up or changed independently, having nothing to do with the state of RCD. When a write command is issued, if RCD=0, the cache is first checked to see if any logical blocks that are to be written are already stored in the cache from a previous read or write command. If there are, the respective cache segments are cleared. The new data is cached for subsequent Read commands.

Cheetah 15K.5 FC Product Manual, Rev. F

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Contents ST3300655FC ST3146855FC ST373455FC Cheetah 15K.5 FCPage Contents Defect and error management InstallationInterface requirements Seagate Technology support services Cheetah 15K.5 FC Product Manual, Rev. F List of Figures Page Scope Cheetah 15K.5 FC Product Manual, Rev. F Standards Applicable standards and reference documentationElectromagnetic compatibility Electromagnetic susceptibilityEuropean Union Restriction of Hazardous Substances RoHS Ncits TR-20 Reference documentsCheetah 15K.5 FC Product Manual, Rev. F General description Media description Standard featuresReliability PerformanceFormatted capacities Programmable drive capacityFactory-installed accessories Factory-installed optionsUser-installed accessories Internal drive characteristics Performance characteristicsSeek performance characteristics Access timeGeneral performance characteristics Format command execution time minutesStart/stop time Prefetch/multi-segmented cache controlCaching write data Cache operationPrefetch operation Error 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 monitorParameter Code Description State of the drive prior to testingDrive 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 informationCheetah 15K.5 FC Product Manual, Rev. F AC power requirements Physical/electrical specificationsDC power requirements DC power requirements for ST3300655FC model Gbit AmpsDC power requirements for ST373455FC model Gbit Amps DC power requirements for ST3146855FC model Gbit AmpsPower sequencing General DC power requirement notesConducted noise immunity Current profilesTypical ST3300655FC drive, 4 Gbit, +12V current profile Typical ST3146855FC drive, 4 Gbit, +12V current profile Typical ST373455FC drive, 4 Gbit, +12V current profile ST3300655FC Power dissipationST3146855FC ST3146855FC CURRENT/POWER vs Throughput FC 4GBST373455FC CURRENT/POWER vs Throughput FC 4GB TemperatureEnvironmental limits ST373455FCEffective altitude sea level Relative humidityShock and vibration ShockPackage size Packaged/product weight Drop height Recommended mounting Vibration Air cleanlinessAcoustics Corrosive environmentRoHS compliance statement Mounting configuration dimensions Mechanical specificationsCheetah 15K.5 FC Product Manual, Rev. F Defect and error management Drive internal defects/errorsDrive error recovery procedures Page Background Media Scan FC-AL system errorsMedia Pre-Scan Deferred Auto-ReallocationIdle Read After Write Drive ID/option selection InstallationDrive orientation CoolingAir flow Drive mountingGrounding Cheetah 15K.5 FC Product Manual, Rev. F Fibre Channel link service frames FC-AL featuresLink services supported Type of frame Interface requirementsFibre Channel task management functions Fibre Channel task management responsesFC Scsi FCP response codes Function name Response code NPort 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 supportedVerify error recovery page 07h Cheetah 15K.5 FC Product Manual, Rev. F Temperature page 0Dh Mode Sense data Inquiry dataCheetah 15K.5 FC inquiry data Bytes Data hex Page Mmode Sense 10 Byte Header Data Mode Sense 10 Byte Header Data 00 ae 00 10 00 00 00 08 08 8b b9 98 00 00 02 Miscellaneous features Miscellaneous operating features and conditionsSupported Feature or condition 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 Fault LED Out PowerEnable 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 LED driver signals FC Differential outputFC Differential input Receive eye diagram Eye diagram data values Link rate GHz Cheetah 15K.5 FC Product Manual, Rev. F 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 Flogi Plogi Page See also cooling
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