Seagate ST373405LW/LWV/LC/LCV manual Cheetah 73LP Product Manual, Rev. E Cache operation

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Cheetah 73LP Product Manual, Rev. E

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4.5Cache operation

In general, 3,600 kbytes (14,399 kbytes of the 16,384 kbytes in LWV and LCV models) of the 4,096 kbytes of physical buffer space in the drive can be used as storage space for cache operations. The buffer can be divided into logical segments (Mode Select Page 08h, byte 13) from which data is read and to which data is written. The drive maintains a table of logical block disc medium addresses of the data stored in each segment of the buffer. If cache operation is enabled (RCD bit = 0 in Mode Page 08h, byte 2, bit 0. See the SCSI Interface Prod- uct Manual. Data requested by the host with a Read command is retrieved from the buffer (if it is there), before any disc access is initiated. If cache operation is not enabled, the buffer (still segmented with required number of segments) is still used, but only as circular buffer segments during disc medium read operations (disregard- ing 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. On a cache miss, all data transfers to the host are in accordance with buffer-full ratio rules. On a cache hit the drive ignores the buffer-full ratio rules. See explanations associ- ated with Mode page 02h (disconnect/reconnect control) in the SCSI Interface Product Manual.

The following is a simplified description of a read operation with cache operation enabled:

Case A - A Read command is received and the first logical block (LB) is already in cache:

1.Drive transfers to the initiator the first LB requested plus all subsequent contiguous LBs that are already in the cache. This data may be in multiple segments.

2.When the requested LB is reached that is not in any cache segment, the drive fetches it and any remaining requested LBs from the disc and puts them in a segment of the cache. The drive transfers the remaining requested LBs from the cache to the host in accordance with the disconnect/reconnect specification men- tioned above.

3.If the prefetch feature is enabled, refer to Section 4.5.2 for operation from this point.

Case B - A Read command requests data, the first LB of which is not in any segment of the cache:

1.The drive fetches the requested LBs from the disc and transfers them into a segment, and from there to the host in accordance with the disconnect/reconnect specification referred to in case A.

2.If the prefetch feature is enabled, refer to Section 4.5.2 for operation from this point.

Each buffer segment is actually a self-contained circular storage (wrap-around occurs), the length of which is an integer number of disc medium sectors. The wrap-around capability of the individual segments greatly enhances the buffer’s overall performance as a cache storage, allowing a wide range of user selectable config- urations, which includes their use in the prefetch operation (if enabled), even when cache operation is disabled (see Section 4.5.2). The number of segments may be selected using the Mode Select command, but the size can not be directly selected. Size is selected only as a by-product of selecting the segment number specifica- tion. The size in Kbytes of each segment is not reported by the Mode Sense command page 08h, bytes 14 and

15.The value 0XFFFF is always reported. If a size specification is sent by the host in a Mode Select command (bytes 14 and 15) no new segment size is set up by the drive, and 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 unchangeable parameters (see SCSI Interface Product Manual).

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 in one or more segments 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 exceeds 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

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Contents ST373405LW/LWV/LC/LCV ST336605LW/LC/LCV Page ST373405LW/LWV/LC/LCV ST336605LW/LC/LCV Page Revision Date Writer/Engineer Sheets Affected Revision status summary sheetPage Contents Installation Defect and error managementSeagate Technology support services Interface requirementsCheetah 73LP Product Manual, Rev. E Vii List of FiguresPage Cheetah 73LP family drive ST373405LW shown ScopeCheetah 73LP Product Manual, Rev. E Applicable standards and reference documentation Australian C-Tick Korean MICTaiwanese Bsmi General description Cheetah 73LP family drive Media characteristics Cheetah 73LP Product Manual, Rev. E Standard featuresPerformance ReliabilityOptions factory installed Factory installed accessoriesAccessories user installed Unformatted and formatted capacitiesPerformance characteristics Ultra2 Scsi Ultra3 Scsi Start/stop timePrefetch/multi-segmented cache control Caching write data Cheetah 73LP Product Manual, Rev. E Cache operationPrefetch operation Reliability specifications Preventive maintenance Reliability and serviceService life Service philosophyControlling S.M.A.R.T 7 S.M.A.R.TMilliseconds Performance impactReporting control Determining rateDST Failure Definition State of the drive prior to testingImplementation Invoking DSTShort test Function Code 001b Short and extended testsExtended test Function Code 010b Log page entriesCheetah 73LP Product Manual, Rev. E Product warranty ShippingProduct repair and return information Cheetah 73LP Product Manual, Rev. E Physical/electrical specifications AC power requirementsDC power requirements Power sequencing Conducted noise immunity3 12 V Current profile Typical ST336605 drive +12 V current profile Typical ST336605 drive +5 V current profile Amperes 000 900 800 700 600 Cheetah 73LP Product Manual, Rev. E Environmental limits TemperatureRelative humidity Effective altitude sea levelShock Shock and vibrationRecommended mounting Air cleanliness VibrationAcoustics LW and LWV mounting configuration dimensions LC and LCV mounting configuration dimensions Defect and error management Drive error recovery proceduresDrive internal defects Scsi systems errors Configure drive options InstallationFormatting Drive ID/option select headerDo not install jumpers retain cover J5 jumper header on LW/LWV models only J2 option select header Does not apply to LC/LCV model Cheetah 73LP Product Manual, Rev. E Function descriptionCooling Drive orientationGrounding Cheetah 73LP Product Manual, Rev. E Drive mountingCheetah 73LP Product Manual, Rev. E Scsi interface messages supported Interface requirementsScsi interface commands supported Cheetah 73LP Product Manual, Rev. E Cheetah 73LP Product Manual, Rev. E Inquiry Vital Product data Mode Sense data DEF CHG Cheetah 73LP Product Manual, Rev. E Condition/feature supported by DC cable and connector 2 REQ/ACK offsetPhysical interface Pin Power Scsi interface cable requirements Scsi interface physical descriptionMating connectors for LW and LWV model drives Mating connectorsMating connectors for LC and LCV model drives ST373405LC/LCV and ST336605LC/LCVCheetah 73LP Product Manual, Rev. E Scsi ID Right-angle to Pcba connectors650±.005 155 100 346 Front View GND +DBP1 62Cheetah 73LP Product Manual, Rev. E REQ +REQ LVD output characteristics Electrical descriptionMultimode-SE and LVD alternatives Single-ended drivers/receivers General cable characteristicsTerminator requirements Cables for low voltage differential drivers/receiversLC and LCV drives Disc drive Scsi timing Drive activity LED Spindle status Command status LED statusCheetah 73LP Product Manual, Rev. E Automated Services Seagate Technology support services Online ServicesPresales Support Technical SupportCustomer Service CSO USA/Canada/Latin America Support ServicesEuropean Support Services Africa/Middle East Support Services Asia/Pacific Support ServicesCall Center Toll-free Direct dial Cheetah 73LP Product Manual, Rev. E Index Cheetah 73LP Product Manual, Rev. E Cheetah 73LP Product Manual, Rev. E Scam Scsi Scsi ID 80Cheetah 73LP Product Manual, Rev. E
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