Seagate ST3600057FC manual Cache operation, Caching write data

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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.

If the number of write data logical blocks exceed the size of the segment being written into, when the end of the segment is reached, the data is written into the beginning of the same cache segment, overwriting the data that was written there at the beginning of the operation; however, the drive does not overwrite data that has not yet been written to the medium.

If write caching is enabled (WCE=1), then the drive may return Good status on a write command after the data has been transferred into the cache, but before the data has been written to the medium. If an error occurs while writing the data to the medium, and Good status has already been returned, a deferred error will be gen- erated.

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Contents Standard models Self-Encrypting Drive models Cheetah 15K.7 FCPage Contents Installation Defect and error managementAbout self-encrypting drives Interface requirementsSeagate Technology support services Cheetah 15K.7 FC Product Manual, Rev. a Model Number Capacity Self-Encrypting Drive SED ScopeStandards Applicable standards and reference documentationElectromagnetic compatibility Electromagnetic susceptibilityEuropean Union Restriction of Hazardous Substances RoHS Ncits TR-20 Reference documentsGeneral description Standard features Performance Media descriptionReliability Formatted capacitiesProgrammable drive capacity Factory-installed optionsUser-installed accessories Internal drive characteristics Performance characteristicsSeek performance characteristics Access timePrefetch/multi-segmented cache control General performance characteristicsStart/stop time Caching 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 Temperature Log Page 0DhPredictive failures Thermal monitorDrive 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 entriesProduct repair and return information Product warrantyShipping DC power requirements Physical/electrical specificationsAC power requirements ST3450857FC DC power requirements Gbit Amps ST3600057FC DC power requirements Gbit AmpsST3300657FC DC power requirements Gbit Amps General DC power requirement notesCurrent profiles Power sequencingConducted noise immunity Typical ST3450857FC current profiles Typical ST3300657FC current profiles 600GB model Power dissipation450GB models 300GB models Temperature a. OperatingEnvironmental limits Effective altitude sea level a. Operating Relative humidityShock Package size Packaged/product weight Drop heightShock and vibration Recommended mounting Vibration a. Operating-normal Air cleanlinessAcoustics Corrosive environmentElectromagnetic susceptibility See Section RoHS compliance statementMounting configuration dimensions Mechanical specificationsAdmin SP Controlled accessAbout self-encrypting drives Data encryptionRandom number generator RNG Default passwordDrive locking Data bandsPower requirements Authenticated firmware downloadSupported commands Cryptographic eraseDrive error recovery procedures Defect and error managementDrive internal defects/errors Page These values are subject to change FC-AL system errorsDeferred Auto-Reallocation Background Media ScanMedia Pre-Scan Idle Read After Write Drive orientation InstallationDrive ID/option selection Air flow CoolingGrounding Drive mountingFibre Channel link service frames FC-AL featuresLink services supported Type of frame Interface requirementsFC Scsi FCP response codes Function name Response code Fibre Channel task management functionsFibre Channel task management responses NPort login Plogi payload Bytes Fibre Channel port loginProcess Login Plri payload Fibre Channel port login acceptNPort Login Accept ACC payload Bytes Fibre 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 loginDual port support Scsi commands supportedFibre Channel Arbitrated Loop options FC-AL options supportedSupported commands Cheetah 15K.7 FC Product Manual, Rev. a Cheetah 15K.7 FC Product Manual, Rev. a Background Medium Scan 15h Cheetah 15K.7 FC inquiry data Bytes Data hex Mode Sense dataInquiry data Page Mode Data Header Block Descriptor Ec b2 5c 00 00 02 Miscellaneous features Miscellaneous operating features and conditionsSupported Feature or condition Miscellaneous statusPhysical description Physical characteristicsFC-AL physical interface Pin descriptions Connector requirementsElectrical description FC-AL transmitters and receivers FC-SCA pin descriptions Pin Signal name Signal typePin Signal name Signal type Fault LED Out PowerMotor start controls Enable port bypass signalsActive LED Out 10.5.11 SEL6 through SEL0 ID lines Parallel Enclosure Services Interface ESIHex Dec SettingArbitrated loop physical address Alpa values Device control codes Signal characteristicsTTL input characteristics provides the TTL characteristics FC Differential input LED driver signalsFC Differential output Receive eye diagram Eye diagram data values Link rate GHz Technical Support Seagate Technology support servicesPresales Support Data Recovery Services Call Center Toll-free Direct dial Warranty ServiceData Recovery Services Authorized Service Centers Numerics IndexDevctrlcode FCP Mtbf Page See also cooling Page Cheetah 15K.7 FC Product Manual, Rev. a Page Seagate Technology LLC
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