You mean value ‘2’ wouldn’t be used at the 3rd step? 3, Then m2 goes down before send anything to others, then m1, m3 recovered and commit value ‘2’ with the quorum m1, m3, m4 m1: 1 2 m2: 1 3 down m3: 1 2 m4: 1 2 m5: 1 but as I assume that m2 goes down before it could send MMonPaxos::OP_BEGIN message to others, so the new leader m1 has no chance to know there exists a newer uncommited value ‘3' Thanks WANG KANG > On 27 Nov 2017, at 10:27 PM, Sage Weil <sage@xxxxxxxxxxxx> wrote: > > On Mon, 27 Nov 2017, Kang Wang wrote: >> hi >> >> I read the code of ceph paxos recently, and have a question about it, which, in my opinion, may violate the consistency. >> >> Assume we have five monitor node m1, m2, m3, m4, m5, the prior one has larger rank than the back one. >> >> Consider the situation as below: >> >> 1, m1 as the leader, and all node have the same last_commited at begin, then m1 propose a new value ‘2', which then be accept by m1 and m3: >> m1: 1 2 >> m2: 1 >> m3: 1 2 >> m4: 1 >> m5: 1 >> >> 2, Unfortunatly, both m1 and m3 go down, and m2 become leader without knowledge about the propse, and it propose a new value ‘3' >> m1: 1 2 down >> m2: 1 3 >> m3: 1 2 down >> m4: 1 >> m5: 1 >> >> 3, Then m2 goes down before send anything to others, then m1, m3 recovered and commit value ‘2’ with the quorum m1, m3, m4 >> m1: 1 2 >> m2: 1 3 down >> m3: 1 2 >> m4: 1 2 >> m5: 1 >> >> 4, Before the commit message sent to others, m1 and m3 go down again. So value ‘3’ only commit on m1. Then m2 become leader once more. >> m1: 1 2 down >> m2: 1 3 >> m3: 1 2 down >> m4: 1 2 >> m5: 1 >> >> 5, Leader m2 see the uncommited value ‘2’, but discard it by compare uncommitted_pn in function handle_last, so it commit value ‘3’ with the quorum m2, m4, m5 >> m1: 1 2 down >> m2: 1 3 >> m3: 1 2 down >> m4: 1 3 >> m5: 1 3 > > This is what the last->uncommitted_pn value is for. I believe this > prevents us from using 2's pn (and uncommitted value) because 3's pn is > larger. Can you verify? > > Thanks! > sage > > >> >> Now we see the value ‘2’ has been commited, but lost soon. Am I right on it? >> >> >> Thanks >> WANG KANG >> >> -- >> To unsubscribe from this list: send the line "unsubscribe ceph-devel" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html >> -- To unsubscribe from this list: send the line "unsubscribe ceph-devel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
