Yes, this is the 2nd issue I mentioned, where ES will pick basically any replica as primary without consideration to which one might be more 'up-to-date'
On Fri, Oct 24, 2014 at 3:57 AM, Evan Tahler <[email protected]> wrote: > Interesting! > > However, the write *may not* be the cause of the data loss here. Even if > there was no write while A and B are down, would the recovery process have > happened the same way? In some further tests, it still looks like C would > have overwritten all the data in A and B when they rebooted. > > This type of error is easily triggered by garbage collection with large > data sets, and a server becoming unresponsive for too long. (perhaps the > cluster kicks out the unresponsive node, or a supervisor restarts the > application) > > On Thursday, October 23, 2014 12:59:00 PM UTC-7, Shikhar Bhushan wrote: >> >> Very interesting. The default 'write consistency level' with >> Elasticsearch is QUORUM, i.e. verify a quorum of replicas for a shard are >> available before processing a write for it. In this case you were just left >> with 1 replica, C, and a write happened. So you would think that it should >> not go through since 2 replicas would be required for quorum. However: >> https://github.com/elasticsearch/elasticsearch/issues/6482. I think this >> goes to show this is a real, not a hypothetical problem! >> >> But guess what? *Even if this were fixed, and a write to C never >> happened: *it is still possible that once A & B were back, C could be >> picked as primary and clobber data. See: https://github.com/ >> elasticsearch/elasticsearch/issues/7572#issuecomment-59983759 >> >> On Thu, Oct 23, 2014 at 7:48 PM, Evan Tahler <[email protected]> wrote: >> >>> Bump? I would love to hear some thoughts on this flow, and if there are >>> any suggestions on how to mitigate it (other than replicating all data to >>> all nodes). >>> >>> Thanks! >>> >>> >>> On Tuesday, October 14, 2014 3:52:31 PM UTC-7, Evan Tahler wrote: >>>> >>>> Hi Mailing List! I'm a first-time poster, and a long time reader. >>>> >>>> We recently had a crash in our ES (1.3.1 on Ubuntu) cluster which >>>> caused us to loose a significant volume of data. I have a "theory" on what >>>> happened to cause this, and I would love to hear your opinions on this, and >>>> if you have any suggestions to mitigate it. >>>> >>>> Here is a simplified play-by-play: >>>> >>>> >>>> 1. Cluster has 3 data nodes, A, B, and C. The index has 10 >>>> shards. The index has a replica count of 1, so A is the master and B >>>> is a >>>> replica. C is doing nothing. Re-allocation of indexes/shards is >>>> enabled. >>>> 2. A crashes. B takes over as master, and then starts transferring >>>> data to C as a new replica. >>>> 3. B crashes. C is now master with an impartial dataset. >>>> 4. There is a write to the index. >>>> 5. A and B finally reboot, and they are told that they are now >>>> stale (as C had a write while they were away). Both A and B delete >>>> their >>>> local data. A is chosen to be the new replica and re-sync from C. >>>> 6. ... all the data A and B had which C never got is lost forever. >>>> >>>> >>>> Is the above situation scenario possible? If it is, it seems like the >>>> default behavior of ES might be better to not reallocate in this scenario? >>>> This would have caused the write in step #4 to fail, but in our use case, >>>> that is preferable to data loss. >>>> >>>> -- >>> You received this message because you are subscribed to the Google >>> Groups "elasticsearch" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to [email protected]. >>> To view this discussion on the web visit https://groups.google.com/d/ >>> msgid/elasticsearch/58e98223-c036-41e2-b53c-265343fa3173% >>> 40googlegroups.com >>> <https://groups.google.com/d/msgid/elasticsearch/58e98223-c036-41e2-b53c-265343fa3173%40googlegroups.com?utm_medium=email&utm_source=footer> >>> . >>> >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> -- > You received this message because you are subscribed to the Google Groups > "elasticsearch" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/elasticsearch/5b3c6605-da27-4119-8f1b-6fdcf43b404d%40googlegroups.com > <https://groups.google.com/d/msgid/elasticsearch/5b3c6605-da27-4119-8f1b-6fdcf43b404d%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "elasticsearch" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/elasticsearch/CAHWG4DPL6Amht_M7dOkZH0izkTAZegB-0awROVwDS35eH-aBaw%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
