On Tue, 6 Jun 2017, xxhdx1985126 wrote: > I submitted an issue about three months > ago: http://tracker.ceph.com/issues/19252 Ah, right. Reads and writes may reorder by default. You can ensure a read is ordered as a write by adding the RWORDERED flag to the op. The OSD will then order it as a write and you'll get the behavior it sounds like you're after. I don't think this has any implications for rbd-mirror because writes are still strictly ordered, and that is what is mirrored. I haven't thought about it too deeply though so maybe I'm missing something? sage > > At 2017-06-06 06:50:49, "xxhdx1985126" <xxhdx1985126@xxxxxxx> wrote: > > > >Thanks for your reply:-) > > > >The requeueing is protected by PG::lock, however, when the write request is > add to the transaction queue, it's left for the journaling thread and files > tore thread, to do the actual write, the OSD's worker thread just release th > e PG::lock and try to retrieve the next req in OSD's work queue, which gives > the opportunity for later reqs to go before previous reqs. This did happene > d in our experiment. > > > >However, since this experiment was done serveral month ago, I'll upload the > log if I can find it, or I'll try to reproduce it. > > > >At 2017-06-06 06:22:36, "Sage Weil" <sweil@xxxxxxxxxx> wrote: > >>On Tue, 6 Jun 2017, xxhdx1985126 wrote: > >>> Thanks for your reply:-) > >>> > >>> The requeueing is protected by PG::lock, however, when the write request > is > >>> add to the transaction queue, it's left for the journaling thread and > >>> filestore thread, to do the actual write, the OSD's worker thread just > >>> release the PG::lock and try to retrieve the next req in OSD's work queu > e, > >>> which gives the opportunity for later reqs to go before previous reqs. T > his > >>> did happened in our experiment. > >> > >>FileStore should also strictly order the requests via the OpSequencer. > >> > >>> However, since this experiment was done serveral month ago, I'll upload > the > >>> log if I can find it, or I'll try to reproduce it. > >> > >>Okay, thanks! > >> > >>sage > >> > >> > >>> > >>> At 2017-06-06 00:21:34, "Sage Weil" <sweil@xxxxxxxxxx> wrote: > >>> >On Mon, 5 Jun 2017, xxhdx1985126 wrote: > >>> >> > >>> >> Uh, sorry, I don't quite follow you. According to my understanding of > > >>> >> the source code of OSD and our experiment previously mentioned in > >>> >> "https://www.mail-archive.com/ceph-users@xxxxxxxxxxxxxx/msg36178.html > ", > >>> >> there exists the following scenario where the actual finishing order > of > >>> >> the WRITEs that targets the same object is not the same as the order > >>> >> they arrived at OSD, which, I think, could be a hint that the order o > f > >>> >> writes from a single client connection to a single OSD is not gurante > ed: > >>> > > >>> >If so, it is a bug that should be fixed in the OSD. rbd-mirror relying > on > >>> >OSD ordering to be correct is totally fine--lots of other stuff does to > o. > >>> > > >>> >> Say three writes that targeting the same object A arrived at an > > >>> >> OSD in the order: "WRITE A.off 1", "WRITE A.off 2", "WRITE A.off 3". > The > >>> >> first write, "WRITE A.off 1", acquires the objectcontext lock of obje > ct > >>> >> A, and is put into a transaction queue to go through the "journaling > + > >>> >> file system write" procedure. Before it's finished, a thread of > >>> >> OSD::osd_op_tp retrieved the second write and attempt to process it > >>> >> during which it finds that the objectcontext lock of A is held by a > >>> >> previous WRITE and put the second write into A's rwstate::waiters que > ue. > >>> >> It's only when the first write is finished on all replica OSDs that t > he > >>> >> second write is put back into OSD::shardedop_wq to be processed again > in > >>> >> the future. If, after the second write is put into rwstate::waiters > >>> >> queue and the first write is finished on all replica OSDs, in which c > ase > >>> >> the first write release the A's objectcontext lock, but before the > >>> >> second write is put back into OSD::shardedop_wq, the third write is > >>> >> retrieved by OSD's worker thread, it would get processed as no previo > us > >>> >> operation is holding A's objectcontext lock, in which case, the actua > l > >>> >> finishing order of the three writes is "WRITE A.off 1", "WRITE A.off > 3", > >>> >> "WRITE A.off 2", which is different from the order they arrived. > >>> > > >>> >This should not happen. (If it happened in the past, it was a bug, but > I > >>> >would expect it is fixed in the latest hammer point release, and in jew > el > >>> >and master.) The requeing is done under the PG::lock so that requeuein > g > >>> >preserves ordering. A fair bit of code and a *lot* of testing goes int > o > >>> >ensuring that this is true. If you've seen this recently, then a > >>> >reproducer or log (and tracker ticket) would be welcome! When we see a > ny > >>> >ordering errors in QA we take them very seriously and fix them quickly. > >>> > > >>> >You might be interested in the osd_debug_op_order config option, which > we > >>> >enable in qa, which asserts if it sees ops from a client arrive out of > >>> >order. The ceph_test_rados workload generate that we use for much of t > he > >>> >rados qa suite also fails if it sees out of order operations. > >>> > > >>> >sage > >>> > > >>> >> > >>> >> In https://www.mail-archive.com/ceph-users@xxxxxxxxxxxxxx/msg36178.ht > ml, > >>> we showed our experiment result which is exactly as the above scenario s > hows > >>> . > >>> >> > >>> >> However, the ceph's version on which we did the experiment and the so > uce > >>> code of which we read was Hammer, 0.94.5. I don't know whether the scena > rio > >>> above may still exists in later versions. > >>> >> > >>> >> Am I right about this? Or am I missing anything? Please help me, I'm > real > >>> ly confused right now. Thank you. > >>> >> > >>> >> At 2017-06-05 20:00:06, "Jason Dillaman" <jdillama@xxxxxxxxxx> wrote: > >>> >> >The order of writes from a single client connection to a single OSD > is > >>> >> >guaranteed. The rbd-mirror journal replay process handles one event > at > >>> >> >a time and does not start processing the next event until the IO has > >>> >> >been started in-flight with librados. Therefore, even though the > >>> >> >replay process allows 50 - 100 IO requests to be in-flight, those IO > s > >>> >> >are actually well-ordered in terms of updates to a single object. > >>> >> > > >>> >> >Of course, such an IO request from the client application / VM would > >>> >> >be incorrect behavior if they didn't wait for the completion callbac > k > >>> >> >before issuing the second update. > >>> >> > > >>> >> >On Mon, Jun 5, 2017 at 12:05 AM, xxhdx1985126 <xxhdx1985126@xxxxxxx> > wro > >>> te: > >>> >> >> Hi, everyone. > >>> >> >> > >>> >> >> > >>> >> >> Recently, I've been reading the source code of rbd-mirror. I wonde > r ho > >>> w rbd-mirror preserves the order of WRITE operations that finished on th > e pr > >>> imary cluster. As far as I can understand the code, rbd-mirror fetches I > /O o > >>> perations from the journal on the primary cluster, and replay them on th > e sl > >>> ave cluster without checking whether there's already any I/O operations > targ > >>> eting the same object that has been issued to the slave cluster and not > yet > >>> finished. Since concurrent operations may finish in a different order th > an t > >>> hat in which they arrived at the OSD, the order that the WRITE operation > s fi > >>> nish on the slave cluster may be different than that on the primay clust > er. > >>> For example: on the primary cluster, there are two WRITE operation targe > ting > >>> the same object A which are, in the order they finish on the primary cl > uste > >>> r, "WRITE A.off data1" and "WRITE A.off data2"; while when they are repl > ayed > >>> on the slave cluster, the order may be "WRITE A.off data2" and "WRITE A > .off > >>> data1", wh > >>> > ich means that the result of the two operations on the primary cluster > is > >>> A.off=data2 while, on the slave cluster, the result is A.off=data1. > >>> >> >> > >>> >> >> > >>> >> >> Is this possible? > >>> >> >> > >>> >> >> > >>> >> >> > >>> >> > > >>> >> > > >>> >> > > >>> >> >-- > >>> >> >Jason > >>> >> N嫥叉靣笡y???氊b瞂???千v豝???藓{.n???壏渮榏z鳐妠ay???蕠跈???j???f＂穐 > 殝鄗 > >>> ???畐ア???⒎???:+v墾妛鑚豰稛???珣赙zZ+凒殠娸???"濟!秈 > >>> > >>> > >>> > >>> > >>> > >>> > > > > > >
