Hi Kristian, ok, thanks for checking!
-slow On 3/18/25 10:19 PM, Kristian Smith wrote:
Hi Slow, After some code research, I was able to first verify that Windows SMB Server does enable write-though by default for Continuously-Available (CA) shares. To your point, I was also unable to locate this information in the spec, so this should indeed be added. I will work on updating MS-SMB2 with this info and you can expect to see the change in a forthcoming version of the document. Thanks for helping us improve the Open Specs! Regards, Kristian Smith Support Escalation Engineer | Microsoft(r) Corporation Email: kristian.sm...@microsoft.com -----Original Message----- From: Kristian Smith Sent: Friday, March 14, 2025 5:49 PM To: Ralph Boehme <s...@samba.org> Cc: cifs-protocol@lists.samba.org; Microsoft Support <supportm...@microsoft.com> Subject: RE: [EXTERNAL] Re: MS-SMB2: write-through and unbuffered IO - TrackingID#2502270040011252 Hi Slow, I'll look into this and get back to you with what I find. Also, upon further review of the previous note I sent you, it actually appears that the document is accurate in that if *both* SMB2_WRITEFLAG_WRITE_UNBUFFERED and FILE_NO_INTERMEDIATE_BUFFERING flags are not set, it will not be considered an unbuffered write. This would then throw the error " STATUS_INVALID_PARAMETER" as the per-request write through flag is only supported for unbuffered writes. Apologies for the confusion. I will let you know what I find regarding persisting modifications to stable storage prior to completion of I/O. Regards, Kristian Smith Support Escalation Engineer | Microsoft(r) Corporation Email: kristian.sm...@microsoft.com -----Original Message----- From: Ralph Boehme <s...@samba.org> Sent: Friday, March 14, 2025 5:23 AM To: Kristian Smith <kristian.sm...@microsoft.com> Cc: cifs-protocol@lists.samba.org; Microsoft Support <supportm...@microsoft.com> Subject: [EXTERNAL] Re: MS-SMB2: write-through and unbuffered IO - TrackingID#2502270040011252 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(r) Corporation Email: kristian.sm...@microsoft.com -----Original Message----- From: Kristian Smith Sent: Tuesday, March 11, 2025 8:43 AM To: Ralph Boehme <s...@samba.org> Cc: cifs-protocol@lists.samba.org; Microsoft Support <supportm...@microsoft.com> 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(r) Corporation Email: kristian.sm...@microsoft.com -----Original Message----- From: Obaid Farooqi <oba...@microsoft.com> Sent: Thursday, February 27, 2025 9:47 AM To: Ralph Boehme <s...@samba.org> Cc: cifs-protocol@lists.samba.org; Microsoft Support <supportm...@microsoft.com> 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 <s...@samba.org> Sent: Thursday, February 27, 2025 8:42 AM To: Interoperability Documentation Help <doch...@microsoft.com> Cc: cifs-protocol@lists.samba.org 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
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