Re: [PATCH 1/1] NFSD: Fix SCSI reservation conflict causing pNFS client to revert I/O to MDS

[Dai Ngo <dai.ngo@oracle.com>]

Hi Ben,

On 9/30/25 12:15 PM, Benjamin Coddington wrote:
> Hi Dai,
>
> On 30 Sep 2025, at 12:28, Dai Ngo wrote:
>
>> When servicing the GETDEVICEINFO call from an NFS client, the NFS server
>> creates a SCSI persistent reservation on the target device using the
>> reservation type PR_EXCLUSIVE_ACCESS_REG_ONLY. This setting restricts
>> device access so that only hosts registered with a reservation key can
>> perform read or write operations. Any unregistered initiator is completely
>> blocked, including standard SCSI commands such as READCAPACITY.
> SBC-4, table 13 shows that READ CAPACITY should be allowed from any I_T
> nexus, no matter the state of the reservation on the LU.
>
> Is it possible that your SCSI implementation might be out of the spec?  Also
> possible that SBC-4 has been updated, I haven't been following the SCSI
> specification updates..
>
> Ben

I don't have access to SBC-4 spec, t10.org does not allow guest access
to their docs. Can you please share the content of table 13 here?

I found the "SCSI Commands Reference Manual" from Seagate. Below is
a description of the reservation type, apologize for the format due to
copy & paste from the doc. Based on this table, code 6h (Exclusive
Access - Registrant only) seems to state media-access commands (I assume
READ_CAPACITY is included) are only allowed for registered I_T nexuses.

I'll continue to research for the official definition of the Reservation
type.

Thanks,
-Dai



----------------------------------------------------------------------------------------------------------


Table 82 Persistent reservation type codes


Code Name Description

0h Obsolete


1h

Write

Exclusive

Access Restrictions: Some commands (e.g., media-access write commands) 
are only

allowed for the persistent reservation holder (see SPC-5).

Persistent Reservation Holder: There is only one persistent reservation 
holder.


2h Obsolete


3h

Exclusive

Access

Access Restrictions: Some commands (e.g., media-access commands) are 
only allowed for

the persistent reservation holder (see SPC-5).

Persistent Reservation Holder: There is only one persistent reservation 
holder.


4h Obsolete


5h

Write Exclusive –

Registrants

Only

Access Restrictions: Some commands (e.g., media-access write commands) 
are only

allowed for registered I_T nexuses.

Persistent Reservation Holder: There is only one persistent reservation 
holder (see SPC-5).


6h

Exclusive Access –

Registrants

Only

Access Restrictions: Some commands (e.g., media-access commands) are 
only allowed for

registered I_T nexuses.

Persistent Reservation Holder: There is only one persistent reservation 
holder (see SPC-5).


7h

Write Exclusive –

All

Registrants

Access Restrictions: Some commands (e.g., media-access write commands) 
are only

allowed for registered I_T nexuses.

Persistent Reservation Holder: Each registered I_T nexus is a persistent 
reservation holder

(see SPC-5).


8h

Exclusive Access –

All

Registrants

Access Restrictions: Some commands (e.g., media-access commands) are 
only allowed for

registered I_T nexuses.

Persistent Reservation Holder: Each registered I_T nexus is a persistent 
reservation holder

(see SPC-5).


9h - Fh Reserved