Dell PowerVault ML6000 Dell PowerVault ML6000 SCSI Reference Guide - Page 65

Read Buffer – 3Ch, What the Library Does With This Command, Command Usage, Read Buffer CDB Format

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Read Buffer - 3Ch What the Library Does With This Command The library will return a requested buffer of data. The Controller logical unit is the primary device to use with this command, since the Media Changer logical units only support the echo buffer mode. Command Usage This command can be used primarily for enhanced domain validation. The initiator can use Descriptor mode first to determine the size of the data available to read, followed by Data mode to then read it. Depending on the size of the requested buffer, it can also be retrieved in blocks, utilizing offsets into the buffer. Read Buffer CDB Format The READ BUFFER CDB format is shown in the following table. Table 52 READ BUFFER CDB format Byte Bit 7 6 5 4 3 2 1 0 0 Op Code (3Ch) 1 Logical Unit Number Rsvd Mode 2 Buffer ID 3 Buffer Offset 5 6 Allocation Length 8 9 Control Mode - The supported modes are: • 2h - Data • 3h - Descriptor • Ah - Echo Buffer • Bh - Echo Buffer Descriptor In Data Mode, data is transferred from the buffer specified by the Buffer ID field. Buffer IDs are assigned beginning with zero, and are assigned contiguously. Buffer ID code assignments for the READ BUFFER command are the same as for the WRITE BUFFER command. In Descriptor Mode, a maximum of four bytes of READ BUFFER descriptor information is returned. The library returns the descriptor information for the buffer specified by the buffer ID. If there is no buffer associated with the specified Buffer ID, all zeros are returned in the READ BUFFER descriptor. The Buffer Offset field is reserved in this mode. The allocation length should be set to at least four for this mode. See Table 53 on page 56 for a definition of the READ BUFFER descriptor. PowerVault ML6000 SCSI Reference Guide 55

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PowerVault ML6000 SCSI Reference Guide
55
Read Buffer – 3Ch
What the Library Does With This Command
The library will return a requested buffer of data. The Controller logical unit is the primary device to use with
this command, since the Media Changer logical units only support the echo buffer mode.
Command Usage
This command can be used primarily for enhanced domain validation. The initiator can use Descriptor mode
first to determine the size of the data available to read, followed by Data mode to then read it. Depending
on the size of the requested buffer, it can also be retrieved in blocks, utilizing offsets into the buffer.
Read Buffer CDB Format
The READ BUFFER CDB format is shown in the following table.
Mode
– The supported modes are:
•
2h – Data
•
3h – Descriptor
•
Ah – Echo Buffer
•
Bh – Echo Buffer Descriptor
In
Data Mode
, data is transferred from the buffer specified by the Buffer ID field. Buffer IDs are assigned
beginning with zero, and are assigned contiguously. Buffer ID code assignments for the READ BUFFER
command are the same as for the WRITE BUFFER command.
In
Descriptor Mode
, a maximum of four bytes of READ BUFFER descriptor information is returned. The
library returns the descriptor information for the buffer specified by the buffer ID. If there is no buffer
associated with the specified Buffer ID, all zeros are returned in the READ BUFFER descriptor. The Buffer
Offset field is reserved in this mode. The allocation length should be set to at least four for this mode. See
Table 53
on page 56 for a definition of the READ BUFFER descriptor.
Table 52
READ BUFFER CDB format
Bit
Byte
7
6
5
4
3
2
1
0
0
Op Code (3Ch)
1
Logical Unit Number
Rsvd
Mode
2
Buffer ID
3
Buffer Offset
5
6
Allocation Length
8
9
Control