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SCSI Read Commands  

2012-12-12 11:07:04|  分类: iscsi |  标签: |举报 |字号 订阅

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转自:http://en.wikipedia.org/wiki/SCSI_Read_Commands

SCSI Read Commands

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There are six different SCSI Read commands defined.

Contents



Read (6)

The READ(6) SCSI command uses a six byte command descriptor block and was first defined in the Shugart Associates System Interface specification from 1979. READ(6) is limited to a 21-bit logical block address or LBA, thus setting a 1 GiB limit, and an 8-bit transfer length. The CDB structure is:

bit→
↓byte
7 6 5 4 3 2 1 0
0 Operation code = 08h
1 LUN LBA
2 LBA
3 LBA
4 Transfer length
5 Control

When READ(6) is used on a stream device (e.g., a tape drive) LBA is zero and the transfer length is equal to the number of bytes to be transferred. Otherwise, LBA is the desired zero-based logical block address and transfer length is expressed in logical blocks—a transfer length of zero will be interpreted as 256 logical blocks. READ(6) has been declared obsolete by the T10 standardization committee but is still supported in many devices for backward compatibility reasons.[1] [2]

Read (10)

The Read(10) (also called 'read extended' in 1987) command was introduced to cope with 32-bit LBAs (2048 GiB) and uses a 16-bit Transfer Length field. It also included three performance-related enhancements. Disable Page Out (DPO) allows the initiator to warn the target that the data being read is unlikely to be requested again soon and so is not worth keeping in the target's data cache. Force Unit Access (FUA) tells the target to fetch the data from the media surface and to not use a cached copy. The RelAdr bit is used to indicate that the LBA value is relative (only used with linked commands). The '2' in the operation code indicates a "class 2" command, and the '8' is the real 'read' opcode. This command was present in a 1982 working document and as such it is likely to have been present from the first SCSI command definitions, showing remarkable foresight. The CDB structure is:

bit→
↓byte
7 6 5 4 3 2 1 0
0 Operation code = 28h
1 LUN DPO FUA Reserved RelAdr
2–5 LBA
6 Reserved
7–8 Transfer length
9 Control

[1] [2]

Read (12)

The Read(12) command is similar to the Read(10) command except that it has a larger 32-bit Transfer Length field. The CDB structure is:

bit→
↓byte
7 6 5 4 3 2 1 0
0 Operation code = A8h
1 Reserved DPO FUA Reserved RelAdr
2–5 LBA
6–9 Transfer length
10 Reserved
11 Control

Read (16)

The Read(16) command is similar to the Read(12) command except that it has a 64-bit LBA field and is thus capable of addressing 8 388 608 PiB (ATA-6 is capable of max 128 PiB). The Transfer Length field is 32 bits long. The CDB structure is:

bit→
↓byte
7 6 5 4 3 2 1 0
0 Operation code = 88h
1 RDPROTECT DPO FUA Reserved FUA_NV Reserved
2–9 LBA
10-13 Transfer length
14 Restricted for MMC-4 Reserved Group number
15 Control


Read (32)

The Read(32) command uses a 64-bit LBA fields (capable of addressing 8 388 608 PiB) and uses a 32-bit Transfer Length field. The CDB structure is:

bit→
↓byte
7 6 5 4 3 2 1 0
0 Operation code = 7Fh
1 Control
2-5 Reserved
6 Reserved Group number
7 Additional CDB length (18h)
8-9 Service Action (0009h)
10 RDPROTECT DPO FUA Reserved FUA_NV Reserved
11 Reserved
12-19 LBA
20-23 Initial Logical Block Reference Tag
24-25 Expected Logical Block Application Tag
26-27 Logical Block Application Tag Mask
28-31 Transfer length

Read Long

The Read Long command is used for Error Correcting Code (ECC) compliant data previously written by a Write Long command. The CORRCT bit defines whether or not the target is allowed to correct any errors found before returning the data to the target. The LBA field is 32 bits in length and the Transfer Length field is 16 bits. The CDB structure is:

bit→
↓byte
7 6 5 4 3 2 1 0
0 Operation code = 3Eh
1 Reserved CORRCT RelAdr
2–5 LBA
6 Reserved
7-8 Byte transfer length
9 Control


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