Hi Kristian,
thanks, makes sense.
On a related note: from SDC presentations and from memory, for SMB3
Persistent Handles the server must ensure that IO is persisted on stable
storage before sending a response to the client, iow force
SMB2_WRITEFLAG_WRITE_THROUGH for all IO requests on Persistent Handles.
<https://www.snia.org/educational-library/smb-22-bigger-faster-scalier-parts-1-and-2-2011>
Page 42:
"All modifications made via the handle are persisted to stable
storage before the I/O is completed."
However, I fail to find any protocol documentation corresponding to this
requirement. Can you please point me at the relevant section(s)?
Thanks!
-slow
On 3/13/25 10:50 PM, Kristian Smith wrote:
Hi Slow,
Thanks again for your patience while I researched this. I have confirmed that
the per-request write-through flag is only supported for unbuffered writes.
There is a discrepancy, however, in the wording of the doc such that the AND in
the following quote:
"If Connection.Dialect is "3.0.2" or "3.1.1", SMB2_WRITEFLAG_WRITE_THROUGH is set in
the Flags field of the request, SMB2_WRITEFLAG_WRITE_UNBUFFERED is not set in the Flags field of the request,
and Open.CreateOptions doesn't include the FILE_NO_INTERMEDIATE_BUFFERING bit, the server MUST fail the
request with STATUS_INVALID_PARAMETER."
Should be an OR as seen below:
" If Connection.Dialect is "3.0.2" or "3.1.1", SMB2_WRITEFLAG_WRITE_THROUGH is set
in the Flags field of the request, SMB2_WRITEFLAG_WRITE_UNBUFFERED is not set in the Flags field of the
request, OR Open.CreateOptions doesn't include the FILE_NO_INTERMEDIATE_BUFFERING bit, the server MUST fail
the request with STATUS_INVALID_PARAMETER."
You should see this change in a future release of the document. Please let me
know if you have any other questions or concerns.
Regards,
Kristian Smith
Support Escalation Engineer | Microsoft® Corporation
Email: [email protected]
-----Original Message-----
From: Kristian Smith
Sent: Tuesday, March 11, 2025 8:43 AM
To: Ralph Boehme <[email protected]>
Cc: [email protected]; Microsoft Support <[email protected]>
Subject: RE: MS-SMB2: write-through and unbuffered IO -
TrackingID#2502270040011252
Hi Slow,
Apologies for the delayed response on this question. I've been researching the
code on this issue and should have findings soon.
Thank you for your patience.
Regards,
Kristian Smith
Support Escalation Engineer | Microsoft® Corporation
Email: [email protected]
-----Original Message-----
From: Obaid Farooqi <[email protected]>
Sent: Thursday, February 27, 2025 9:47 AM
To: Ralph Boehme <[email protected]>
Cc: [email protected]; Microsoft Support <[email protected]>
Subject: MS-SMB2: write-through and unbuffered IO - TrackingID#2502270040011252
Hi Ralph:
Thanks for contacting Microsoft. I have created a case to track this issue. A
member of the open specifications team will be in touch soon.
Regards,
Obaid Farooqi
Escalation Engineer | Microsoft
-----Original Message-----
From: Ralph Boehme <[email protected]>
Sent: Thursday, February 27, 2025 8:42 AM
To: Interoperability Documentation Help <[email protected]>
Cc: [email protected]
Subject: [EXTERNAL] MS-SMB2: write-through and unbuffered IO
Hello dochelp,
I'm doing some research on the expected server behaviour wrt to write-through
and unbuffered IO, driven by working on the Persistent Handles implementation
in Samba.
As per MS-SMB2 the protocol offers knobs to request these
1) at open time, and
2) per write
For 1) there are
FILE_WRITE_THROUGH
FILE_NO_INTERMEDIATE_BUFFERING
and for 2) there are
SMB2_WRITEFLAG_WRITE_THROUGH
SMB2_WRITEFLAG_WRITE_UNBUFFERED
The protocol documentation (also when read together with MS-FSA and
MS-CIFS) seems to be a bit vague wrt to dependencies between per-open behaviour
and per-write behaviour.
---8<---
3.3.5.13 "Receiving an SMB2 WRITE Request"
If Connection.Dialect is "3.0.2" or "3.1.1", SMB2_WRITEFLAG_WRITE_THROUGH is
set in the Flags field of the request, SMB2_WRITEFLAG_WRITE_UNBUFFERED is not set in the Flags
field of the request, and Open.CreateOptions doesn't include the FILE_NO_INTERMEDIATE_BUFFERING
bit, the server MUST fail the request with STATUS_INVALID_PARAMETER.
If Connection.Dialect is "2.1" or "3.0", SMB2_WRITEFLAG_WRITE_THROUGH is set in
the Flags field of the request, and Open.CreateOptions doesn't include the
FILE_NO_INTERMEDIATE_BUFFERING bit, the server MUST fail the request with STATUS_INVALID_PARAMETER.
---8<---
If my reading of the above paragraph is correct, it is not possible to to
request per-write write-through IO without requesting unbuffered IO on the
handle.
Is this true? Am I missing something? Can you please clarify?
Thanks!
-slow
_______________________________________________
cifs-protocol mailing list
[email protected]
https://lists.samba.org/mailman/listinfo/cifs-protocol
