SCSI Primary Commands - 2.pdf

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T10/1236-D Revision 20 18 July 2001The state of the log parameter control bits for parameters 0000h through 0FFFh is specified in table 129.Table 129 — Parameter control bits for general usage parameters (0000h through 0FFFh)The values stored in the GENERAL USAGE PARAMETER BYTES represent data sent to the device server in a previousLOG SELECT command. If a previous LOG SELECT command has not occurred, the data is vendor specific.In the application client page, parameter codes 1000h through FFFFh are reserved.8.2.3 Buffer over-run/under-run pageBit Value DescriptionDU 1 Value provided by application clientDS 0 Device server supports saving of parameterTSD 0 Device server manages saving of parameterETC 0 No threshold comparison is made on this valueTMC xx Ignored when ETC is 0LBIN 1 The parameter is in binary formatLP 1 The parameter is a list parameterThe buffer over-run/under-run page (page code 01h) defines 24 data counters that may be used to record thenumber of buffer over-runs or under-runs for the logical unit. A target that implements this page may implementone or more of the defined data counters.A buffer over-run or under-run may occur when an initiator does not transmit data to or from the target’s buffer fastenough to keep up with reading or writing the media. The cause of this problem is protocol specific. A bufferover-run condition may occur during a read operation when a buffer full condition prevents continued transfer ofdata from the media to the buffer. A buffer under-run condition may occur during a write operation when a bufferempty condition prevents continued transfer of data to the media from the buffer. Most devices incur a delay at thispoint while the media is repositioned.Table 130 defines the PARAMETER CODE field for the buffer over-run/under-run counters.Table 130 — Parameter code field for buffer over-run/under-run countersBitByte7 6 5 4 3 2 1 00 Reserved1 COUNT BASIS CAUSE TYPEThe PARAMETER CODE field for buffer over-run/under-run counters is a 16-bit value comprised of eight reserved bits,a three-bit COUNT BASIS field (see table 131), a four-bit CAUSE field (see table 132), and a one-bit TYPE field. Theseare concatenated to determine the value of the parameter code for that log parameter. For example, a counter forparameter code value of 0023h specifies a count basis of 001b; a cause of 0001b; and a type of 1b; this counter isincremented once per command that experiences an over-run due to the SCSI bus being busy.178 dpANS SCSI Primary Commands - 2 (SPC-2)
18 July 2001 T10/1236-D Revision 20The COUNT BASIS field defines the criteria for incrementing the counter. The criteria are defined in table 131.Table 131 — Count basis definitionCount basis000b001b010b011b100b - 111bDescriptionUndefinedPer commandPer failed reconnectPer unit of timeReservedNOTE 42 The per unit of time count basis is device type specific. Direct-access devices typically use a latencyperiod (i.e., one revolution of the medium) as the unit of time.The CAUSE field indicates the reason that the over-run or under-run occurred. The following causes are defined intable 132.Table 132 — Cause field definitionCause0h1h2h3h - FhDescriptionUndefinedBus busyTransfer rate too slowReservedThe TYPE field indicates whether the counter records under-runs or over-runs. A value of zero specifies a bufferunder-run condition and a value of one specifies a buffer over-run condition.The counters contain the total number of times buffer over-run or under-run conditions have occurred since the lasttime the counter was cleared. The counter shall be incremented for each occurrence of an under-run or over-runcondition and may be incremented more than once for multiple occurrences during the execution of a singlecommand.dpANS SCSI Primary Commands - 2 (SPC-2) 179
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