1. SOP: Tags, Size and scope
Kevin Marks
April 22, 2011
T10/11-206r0

2. NVM Express
An NVM Express device uses 2 items to uniquely identify the command within the device. The combination of the two are global across all namespaces within the device (total 4 bytes).
Command Identifier (CID): 2 bytes – local to the Submission Queue that the command was placed into.
SQ Identifier (SQID): 2 bytes - Indicates the Submission Queue the command was placed in, in the Completion Queue Entry.
SQ Identifier are included in the Completion Queue Entry, because of the ability to have multiple SQ’s map to a single CQ.
SR-IOV – because an SQ_ID can be the same across VF’s, the global-ness is not across VF’s (i.e., each VF has the full namespace.)

3. Command Identifier Sizes for SCSI Transports
ADT-2
FCP-4
iSCSI
SPL-2
SRP
UAS
Command Identifier Size (bytes)
3 bits 2 4 8
*Scope is within the I_T Nexus

4. SOP
SOP has several usage models where the Command Identifier can be scoped to :
Entire controller
Physical / Virtual Function
Unique to IT_NEXUS_ID
Unique to LU

5. Entire Controller Pros Cons Simpler for Driver
Can be a memory pointer if large enough
Allows creation of Compact Response IU without specifying the I_T_NEXUS_ID per Command Identifier
Cons
May have overlap with IOV
If large may be over kill for simple end devices (e.g., SSDs)

6. Unique to I_T_NEXUS_ID
Pros
Size of Command Identifier can be made smaller
Follows SAS concept of Command Identifiers per I_T
Cons
Requires including I_T_NEXUS_ID in compact response IU.

7. Unique to LU
Pros
Cons

8. Questions What should the size of Command Identifier and scope be?
Currently set at 8 bytes in SOP draft and assumes global, as Compact Response only includes Command Identifier and not I_T_Nexus_ID
Only SRP needs 8 bytes
I_T_L_Q = = 20 bytes to uniquely identify command
Will the Command Identifier also be global across VF’s?
Are the namespace for SCSI Commands, SOP commands and TMF’s shared?
Is the mapping from a SOP transport to another transport in the bridging use case vendor specific?

9. Proposal(s) Reduced Command Identifier size to 32 bits;
Make the 64 bit field a combination of the I_T_NEXUS_ID and Command Identifier; or
Allow a reduced namespace that is specified by the target device(e.g., 32 bits and report in some identifier).

10. Thanks