Messages in this thread | | | Date | Mon, 10 Sep 2012 16:31:59 -0700 | From | Kent Overstreet <> | Subject | Re: [Drbd-dev] FLUSH/FUA documentation & code discrepancy |
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cc'ing Neil
On Mon, Sep 10, 2012 at 04:06:54PM -0700, Tejun Heo wrote: > Hello, again. > > cc'ing Kent and Vivek. The original thread is at > > http://thread.gmane.org/gmane.linux.network.drbd.devel/2130 > > On Mon, Sep 10, 2012 at 03:54:42PM -0700, Tejun Heo wrote: > > > We can possibly work around that by introducing an additional submitter thread, > > > or at least our own list where we queue assembled bios until the lower > > > level device queue drains. > > > > > > But we'd rather have the elevator see the FLUSH/FUA, > > > and treat them as at least a soft barrier/reorder boundary. > > > > > > I may be wrong here, but all the necessary bits for this seem to be in > > > place already, if the information would even reach the elevator in one > > > way or other, and not be completely stripped away early. > > > > > > What would you rather see, the elevator recognizing reorder boundaries? > > > Or additional higher level queueing and extra thread/work queue/whatever? > > > > > > Both are fine with me, I'm just asking for an opinion. > > > > First of all, using FLUSH/FUA for such purpose is an error-prone > > abuse. You're trying to exploit an implementation detail which may > > change at any time. I think what you want is to be able to specify > > REQ_SOFTBARRIER on bio submission, which shouldn't be too hard but I'm > > still lost why this is necessary. Can you please explain it a bit > > more? > > The problem with exposing REQ_SOFTBARRIER at bio submission is that it > would require block layer not to reorder bios while passing through > stacked adrivers until it reaches a rq-based driver. I *suspect* this > has been true until now but Kent's pending patch to fix possible > deadlock issue breaks that.
Yeah, you might be right about that. I think Neil Brown would know better than I if this ordering was ever explicitly broken.
But I don't think anything else is relying on that kind of ordering any more.
> http://thread.gmane.org/gmane.linux.kernel.bcache.devel/1017/focus=1356250 > > As for what the resolution should be, urgh... I don't know. :(
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