[nvmewin] Handling IO when Format NVM op is in progress

[Judy Brock-SSI]

   >>  I believe the original concern that drove us to the implementation was that w/NVMe the block size can be changed with a format whereas that can't happen with a SCSI format

The block size on a SCSI device can change with a SCSI mode select command via a mode parameter block descriptor. That would result in sense code =  UNIT ATTENTION and additional sense code = MODE PARAMETERS CHANGED. We could return this combo at the end of a Format NVM op that did in fact change the block size attribute of one or more namespaces.

But once the format NVM op finishes, wouldn't indicating that a bus change has occurred / re-enumeration is required more than cover the base of getting the upper layers to discover the potentially changed block size since that notifies Storport that it needs to do discovery all over again?

Just curious why this was thought to be insufficient.

Thanks,
Judy


From: Luse, Paul E [mailto:paul.e.luse at intel.com]
Sent: Tuesday, January 07, 2014 8:44 AM
To: Neal Galbo (ngalbo); Mayes, Barrett N; Judy Brock-SSI; nvmewin at lists.openfabrics.org
Subject: RE: Handling IO when Format NVM op is in progress

Wrt Judy's (a) below, So I believe the original concern that drove us to the implementation was that w/NVMe the block size can be changed with a format whereas that can't happen with a SCSI format... I could be mistaken but on a quick scan of the email threads I didn't see that point mentioned.  We felt like the only way to get the upper layers to discover the potentially changed block size was to tear it down/bring it back up

Thx
Paul

From: nvmewin-bounces at lists.openfabrics.org<mailto:nvmewin-bounces at lists.openfabrics.org> [mailto:nvmewin-bounces at lists.openfabrics.org] On Behalf Of Neal Galbo (ngalbo)
Sent: Tuesday, January 7, 2014 7:27 AM
To: Mayes, Barrett N; Judy Brock-SSI; nvmewin at lists.openfabrics.org<mailto:nvmewin at lists.openfabrics.org>
Subject: Re: [nvmewin] Handling IO when Format NVM op is in progress

The conditions you describe would never happen with a SCSI device. In general, LU's and LUN's never disappear in SCSI; they are not transient. They are not dynamic. They are static. They either exist or they don't. They don't hide, once enumerated/attached/located. Unlike namespaces.

The media, backing storage or provisioning can change, but the LU would always be available for communication (commands). Other LU's in the same device would not be affected either - they are independent entities relative to each other.

-Neal

From: nvmewin-bounces at lists.openfabrics.org<mailto:nvmewin-bounces at lists.openfabrics.org> [mailto:nvmewin-bounces at lists.openfabrics.org] On Behalf Of Mayes, Barrett N
Sent: Tuesday, January 07, 2014 12:15 AM
To: Judy Brock-SSI; nvmewin at lists.openfabrics.org<mailto:nvmewin at lists.openfabrics.org>
Subject: Re: [nvmewin] Handling IO when Format NVM op is in progress

What problem do you want to solve by keeping the block device around during a format and allowing IO through so it can be failed with check condition?

Namespace not ready can't generically be translated to SCSI check condition/not ready/Format In Progress.  The driver would need to know that a format command is outstanding so it could translate that correctly (for 1.0-based device support) since the namespace could be not ready for reasons other than a format in progress.  If the driver already has to know a format is in progress, it could just fail commands without sending it to the device (so no need for the new failure code).  But if that's the case, why _not_ hide the LUN until the format is complete.  By hiding the LUN and bringing it back when the format is complete, you don't have to worry about handling IO and you also take care of the re-enumeration that has to happen when the format is complete (in the case of changing the LBA Format).

-Barrett

From: nvmewin-bounces at lists.openfabrics.org<mailto:nvmewin-bounces at lists.openfabrics.org> [mailto:nvmewin-bounces at lists.openfabrics.org] On Behalf Of Judy Brock-SSI
Sent: Monday, January 06, 2014 8:09 PM
To: nvmewin at lists.openfabrics.org<mailto:nvmewin at lists.openfabrics.org>
Subject: [nvmewin] Handling IO when Format NVM op is in progress

All,

Many months ago, I initiated a thread  (see attached) in which I argued that there were some holes in the current implementation of Format NVM ioctl passthru and in which I advocated for, among other things, the addition of logic to make sure pseudo-SCSI bus re-enumeration had fully taken place in the driver (such that Storport was notified that no "luns" were present) before the actual Format NVM op was launched.

I intuitively understand - and up to this point have unquestioningly agreed with - the basic assumption that the reason the namespaces must be  removed/ "luns" disappeared prior to formatting is because "we cannot format a namespace while the OS is aware of its presence and could be potentially sending I/O to a stale namespace config (i.e. changing LBA/sector size)." (excerpt from attached thread).

The question has recently arisen in internal discussion however as to whether or not we really have to do this. It was pointed out that SCSI devices (real ones) are capable of receiving IO commands while SCSI format commands are in process. They will return the following error:

      SCSI status =   Check condition, sense code = NOT READY (0x2), additional sense code = LUN NOT READY (0x4), additional sense code qualifier = FORMAT IN PROGRESS (0x4)

Why then, instead of removing namespaces/luns, can our Windows driver not return the same error status a real SCSI drive would return in such a situation? One would assume that upper layers of the storage stack have plenty years of experience in knowing what to do when it sees that error.

As a point of comparison, there is no standard I am aware of which specifies that Storport miniports which support real SCSI devices, if they happen to provide a proprietary pass-thru to allow a SCSI format command to go through to a device, must cause all LUNS to appear offline prior to formatting,

One could even argue (and they have!) that these IO commands could even be allowed to go through to the NVMe device itself (as in the real SCSI case); NVMe 1.1 Technical Proposal 005 has defined a new format-in-progress status code that NVMe firmware will be able to return at some point in the future, current firmware could easily return NAMESPACE_NOT_READY and driver could translate to the above SCSI sense data, etc.

So ...  here I stand, devil's advocate hat in hand, hoping to find out:


a)       what the "back story" is on how this decision was ultimately made (the attached thread said a lot of discussion took place on the subject)

b)       whether or not the diametrically-opposed alternative I am discussing above was thoroughly considered and if so, why it was rejected

c)       whether the topic bears reconsidering at this point.

Thanks in advance for your collective consideration,

Judy

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