Seagate ST3300056FC (FDE) manual Prefetch/multi-segmented cache control, Cache operation

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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.

All default cache and prefetch mode parameter values (Mode Page 08h) for standard OEM versions of this drive family are given in Table 21.

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

Cheetah 15K.6 FC Product Manual, Rev. C

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Contents ST3300656FC ST3450856FCST3146356FC Page Contents Interface requirements Physical/electrical specificationsDefect and error management InstallationSeagate Technology support services Cheetah 15K.6 FC Product Manual, Rev. C Model Number Full Disk Encryption FDE ScopeCheetah 15K.6 FC Product Manual, Rev. C Electromagnetic susceptibility Applicable standards and reference documentationStandards Electromagnetic compatibilityEuropean Union Restriction of Hazardous Substances RoHS Ncits TR-20 Reference documentsCheetah 15K.6 FC Product Manual, Rev. C General description Standard features Formatted capacities Media descriptionPerformance Reliability User-installed accessories Factory-installed optionsAccess time Performance characteristicsInternal drive characteristics Seek performance characteristics176 144 100 Format command execution time minutesGeneral performance characteristics Start/stop timeCache operation Prefetch/multi-segmented cache controlCaching write data Prefetch operation Unrecoverable Errors Reliability specificationsError rates Recoverable ErrorsPreventive maintenance Reliability and serviceSeek errors Interface errorsMilliseconds 4 S.M.A.R.TControlling S.M.A.R.T Performance impactThermal monitor Temperature Log Page 0DhDetermining rate Predictive failuresImplementation State of the drive prior to testingDrive Self Test DST DST failure definitionLog page entries Short and extended testsShort test Function Code 001b Extended test Function Code 010bShipping Product warrantyProduct repair and return information Cheetah 15K.6 FC Product Manual, Rev. C AC power requirements Physical/electrical specificationsDC power requirements ST3450056FC DC power requirements Gbit Amps ST3450856FC DC power requirements Gbit AmpsST3300056FC DC power requirements Gbit Amps ST3300656FC DC power requirements Gbit AmpsST3146756FC DC power requirements Gbit Amps ST3146356FC DC power requirements Gbit AmpsCurrent profiles General DC power requirement notesPower sequencing Conducted noise immunityTypical ST3450856FC current profiles Typical ST3450056FC current profiles Typical ST3300656FC current profiles Typical ST3300056FC current profiles Typical ST3146356FC current profiles Typical ST3146756FC current profiles 450GB models Power dissipation300GB models Environmental limits Temperature146GB models Effective altitude sea level Relative humidityShock and vibration Package size Packaged/product weight Drop heightShock Recommended mounting Corrosive environment Air cleanlinessVibration AcousticsRoHS compliance statement Mounting configuration dimensions Mechanical specificationsDrive internal defects/errors Defect and error managementDrive error recovery procedures Page Background Media Scan FC-AL system errorsDeferred Auto-Reallocation Media Pre-ScanIdle Read After Write Cooling InstallationDrive ID/option selection Drive orientationGrounding Drive mountingPage Cheetah 15K.6 FC Product Manual, Rev. C Interface requirements FC-AL featuresFibre Channel link service frames Link services supported Type of frameFibre Channel task management responses Fibre Channel task management functionsFC Scsi FCP response codes Function name Response code NPort login Plogi payload Bytes Fibre Channel port loginProcess Login Plri payload Fibre Channel port login acceptFibre Channel Process Login NPort Login Accept ACC payload BytesFabric Login Flogi payload Bytes Fibre Channel Process Login AcceptFibre Channel fabric login Process Login Accept ACC payload BytesFabric Login Accept ACC payload Bytes Fibre Channel fabric accept loginFC-AL options supported Scsi commands supportedDual port support Fibre Channel Arbitrated Loop optionsSupported commands Cheetah 15K.6 FC Product Manual, Rev. C Cheetah 15K.6 FC Product Manual, Rev. C Supported commands Inquiry data Mode Sense dataCheetah 15K.6 FC inquiry data Bytes Data hex Page Mode Data Header 000000 Cheetah 15K.6 FC Product Manual, Rev. C Miscellaneous status Miscellaneous operating features and conditionsMiscellaneous features Supported Feature or conditionFC-AL physical interface Physical characteristicsPhysical description Electrical description Connector requirementsPin descriptions Pin Signal name Signal type FC-SCA pin descriptions Pin Signal name Signal typeFC-AL transmitters and receivers Fault LED Out PowerActive LED Out Enable port bypass signalsMotor start controls 11 SEL6 through SEL0 ID lines Parallel Enclosure Services Interface ESIArbitrated loop physical address Alpa values SettingHex Dec TTL input characteristics Signal characteristicsDevice control codes FC Differential output LED driver signalsFC Differential input Receive eye diagram Eye diagram data values Link rate GHz Cheetah 15K.6 FC Product Manual, Rev. C Technical Support Seagate Technology support servicesInternet Presales SupportData Recovery Services Authorized Service Centers Warranty ServiceData Recovery Services Call CenterToll-freeDirect dialFAX Numerics IndexFC-AL Flogi Plogi Page See also cooling
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