Hitachi HUS151473VL3800 Specifications - Page 262

20.8 Segmented Caching 20.8.1 Overview Segmented Caching divides the data buffer into several smaller buffers. Each buffer is used as Read/ Write/Read-Ahead buffer. 20.8.2 Read Ahead The Read Ahead function consists of reading data that the Initiator has not yet requested to the drive buffer. This function is intended to improve performance for an initiator that frequently accesses sequential data with successive SCSI read commands. The Read Ahead function works when RCD (the read cache disable) bit of read cache page (page 08h) is set to zero. The drive initiates the Read ahead function when the following conditions exist: • RCD is 0 • Read(6), Read (10), Read Verify, and Write and Verify is received • The consecutive LBA of the requested LBA is not available in the buffer If SCSI reset or target reset message is received, all contents of segmented buffer is flushed. Even if an error occurs during the Read ahead, the error will not be reported to the Initiator. The data read before the error occurred will be stored as valid data by the Read Ahead function. 20.9 Multiple Initiator Systems This section describes how the target behaves in a multiple initiator system. Up to 32 initiators may be supported at any one time. 20.9.1 Sense Data A separate sense data area is reserved for each initiator. Each area is maintained independently. This allows a command from one initiator to complete with a CHECK CONDITION status and generate sense data without being affected by a subsequent command from a different initiator. There is no requirement for the first initiator to send a REQUEST SENSE command to retrieve the Sense Data prior to the execution of a command from a different initiator. 20.9.2 Mode Pages A single set of Mode pages is maintained. This includes both current and saved parameters. If a MODE SELECT command is executed that updates the current parameters, a unit attention condition is generated for all initiators except the one that issued the command. See 20.1.5, "Unit Attention Condition" on page 230 for more information. 20.10 Reselection Time-out If reselection fails, it will be retried one or more times depending on the drive model. Please see the individual drive specifications for the number of retries allowed. 20.11 Single Initiator Selection For single initiator systems it is not an error to have only the target ID bit present during selection. Disconnection is not allowed for Single Initiator Selection with only one ID bit present during selection. The initiator must not send an Identify message with the disconnect permission bit(6) on. 20.12 Non-arbitrating systems The Target cannot detect whether other SCSI devices on the SCSI bus use arbitration prior to selection. As a consequence the Hitachi Ultrastar 15K147 SCSI Hard Disk Drive Specification 244