On Thu, 2016-09-29 at 11:01 -0400, Mike Snitzer wrote: > On Tue, Sep 27 2016 at 2:50pm -0400, > James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > > > On Tue, 2016-09-27 at 08:34 +0200, Hannes Reinecke wrote: > > > On 09/26/2016 09:06 PM, James Bottomley wrote: > > > > On Mon, 2016-09-26 at 09:52 -0700, Christoph Hellwig wrote: > > > > > Getting back to this after Hannes recovered from his vacation > > > > > and I had a chat with him.. > > > > > > > > > > On Mon, Aug 15, 2016 at 09:40:39AM -0400, Mike Snitzer wrote: > > > > > > Seems we still need a more sophisticated approach. But I'm > > > > > > left wondering: if we didn't do it would anything notice? > > > > > > Sadly, the same big question from the original thread from > > > > > > a > > > > > > year ago: > > > > > > > > > > Yes. I have a customer looking to push the pNFS SCSI layout > > > > > into > > > > > a product, and the major show stopper right now is that we > > > > > can > > > > > trivially get into failver loops without this (or and > > > > > equivalent) > > > > > fix. > > > > > > > > > > A year ago SCSI layout was still work in progress in the > > > > > IETF, > > > > > people use the similar block layout instead that doesn't use > > > > > PRs and we also didn't have the in-kernel PR API, so you > > > > > effectively couldn't use PRs with multipathing. > > > > > > > > > > > https://patchwork.kernel.org/patch/6797111/ > > > > > > > > > > > > > So this is throw-away for now (and I'll get Hannes' patch > > > > > > > applied for 4.8-rc3, with the tweak of returning -EBADE > > > > > > > immediately): > > > > > > > > > > > > Unfortunately, I'm _not_ staging Hannes' patch until I have > > > > > > James Bottomley's Ack (given his original issues with the > > > > > > patch > > > > > > haven't been explained away AFAICT). > > > > > > > > > > I've added James to the Cc. His argument was that the old > > > > > behavior could be implemented to use some non-standard use of > > > > > reservations without a specific example. I don't really > > > > > think > > > > > his example even is practical - once we use dm-mpath it > > > > > exclusively claims the underling block devices, so any sort > > > > > of > > > > > selective reservations would have had to happen before even > > > > > starting dm-multipath. > > > > > > > > Well, now that you've made me reread the thread from 14 months > > > > ago > > > > that wasn't quite my objection. The objection hinged on the > > > > fact > > > > that anything that uses path specific reservations would now > > > > fail > > > > instead of retrying on a different path. I thought the IBM SVC > > > > did > > > > this and Hannes implied he'd be able to check this ... did > > > > anyone > > > > check? If we've checked and there's no issue with the SVC, > > > > then I > > > > don't have any other objections. > > > > > > > > > So a dynamic SAN controller would have to tear down and > > > > > rebuild > > > > > the dm-multipath setup at all the time. > > > > > > > > That was the job of the SVC: it sat in the middle of the SAN > > > > and > > > > controlled which node saw what storage. > > > > > > > > https://www.ibm.com/support/knowledgecenter/STPVGU/com.ibm.stor > > > > age. > > > > svc.console.720.doc/svc_svcovr_1bcfiq.html > > > > > > > > The SVC can issue its own reservations in those circumstances. > > > > What I'm not at all clear on is whether they'll interact badly > > > > with the dm-mp reservations. > > > > > > > In the end SVC is (for us) just another storage array. > > > If and what SVC does in the background is of no interest to us. > > > > How can that be true? It sits *on* the san and manages devices, it > > doesn't sit between the initators and the devices. It applies > > reservations to devices under management, but every node usually > > sees > > everything else, so devices under SVC management are visible to all > > initators unless you zone them off. > > > > The last SVC manual I saw included a procedure for manually > > releasing > > stuck SVC reservations from an initator, which illustrates the > > expectation. > > > > > OTOH I'd be very surprised if the SVC would be allowing us to see > > > remnants of its internal working (like persistent reservation > > > errors); in doing so third-party applications would be able to > > > see > > > and possibly modify these persistent reservations and the SVC > > > would > > > find itself in a very fragile operating scenario. > > > > Because unless you zone the fibre, that's precisely what you do > > see. > > > > > Also interactions with GPFS (which uses it's own set of > > > reservations) > > > will become very tricky. > > > > > > So I sincerely doubt we'll ever see SVC-originated persistent > > > reservations errors. > > > > > > And as a side-note, this particular patch is included in SLES > > > since > > > 2011. With no noticeable side-effect. > > > > OK, so can you actually say that someone has tested this scenario? > > If not, do you have the capacity to test it? > > I've elected to just take this change for 4.9. Please see: > https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.g > it/commit/?h=dm-4.9&id=8ff232c1a819c2e98d85974a3bff0b7b8e2970ed That's fine. I think the answer is that SVC technology is not around much so it can't be tested, so I was going to dump it on you to make the call anyway ... James -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
