On Thu, Sep 24 2020 at 2:56pm -0400, John Dorminy <jdorminy@xxxxxxxxxx> wrote: > On Thu, Sep 24, 2020 at 1:24 PM John Dorminy <jdorminy@xxxxxxxxxx> wrote: > > > > I am impressed at how much I read wrong... > > > > On Thu, Sep 24, 2020 at 1:00 PM Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > > > > > > On Thu, Sep 24 2020 at 12:45pm -0400, > > > John Dorminy <jdorminy@xxxxxxxxxx> wrote: > > > > > > > I don't understand how this works... > > > > > > > > Can chunk_size_bytes be 0? If not, how is discard_granularity being set to 0? > > > > > > Yeah, I had same question.. see the reply I just sent in this thread: > > > https://www.redhat.com/archives/dm-devel/2020-September/msg00568.html > > > > > > > I think also limits is local to the ti in question here, initialized > > > > by blk_set_stacking_limits() via dm-table.c, and therefore has only > > > > default values and not anything to do with the underlying queue. So > > > > setting discard_granularity=max(discard_granularity, chunk_size_bytes) > > > > doesn't seem like it should be working, unless I'm not understanding > > > > what it's there for... > > > > > > You're reading the dm-table.c limits stacking wrong. Of course DM stack > > > up the underlying device(s) limits ;) > > > > Yep, I failed to read iterate_devices... > > > > > > > > > > > > > And shouldn't melding in the target's desired io_hints into the > > > > existing queue limits be happening in blk_stack_limits() instead? > > > > (Also, it does lcm_not_zero() for stacking granularity, instead of > > > > max()...) > > > > > > > > > > DM core does do that, the .io_hints hook in the DM target is reserved > > > for when the target has additional constraints that blk_stack_limits() > > > didn't/couldn't factor in. > > Yes, I had erroneously thought the limit-stacking was after getting > > the targets' individual limits, not before. > > > > > > > > And blk_stack_limts() does use max() for discard_granularity. > > ... I'm just terrible at reading this morning. > > > > Thanks for pointing out all the things I misread! > > Actually, though, I don't understand why it should be max instead of > lcm_not_zero(). If the raid's chunk size is 1024 sectors, say, and > you're constructing it on something that has discard_granularity 812 > sectors, say, blkdev_issue_discard will be generating 1024 sector IOs > which will work poorly when passed down to the 812-sector-granularity > underlying device. While, if lcm(812,1024) were used, lcm(812,1024) > sector IOs would be compatible with both the chunk size and underlying > device's granularity, perhaps? Maybe I'm missing something, but I read > the doc and code an extra time around this time ;) Martin may correct me if I'm wrong but I _think_ it is because discard_granularity is unintuitive. The larger the discard_granularity the more constraining it is (on other devices with more relaxed, or smaller, discard_granularity). So you need to impose the most constrained limit for all when stacking. Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
