> - if during the streaming session, the sstable that was about to stream out > but was being compacted, would we see error in the log? No. Remember we never modify files on disk. And the "truth" contained in one file generated by compaction is the same as the"truth" contained in the files before compaction.
> - could this lead to data not found? No. > - is it safe to let a node serving read/write requests while repair is > running? Yes. All maintenance operations are online operations. > Data created before the last flush was still missing, according to the client > that talked to DC1 (the disaster DC). Losing data from before the flush sounds very strange. That sort of thing would get noticed, specially if you can pretty much reproduce it. I would wind the test back to a single node see if it works as expected. If it does walk the test scenario forward to multiple nodes then multiple DC's until it fails. If the single node test fails check your test and then grab someone on IRC (I'm aaron_morton and there are plenty of other smart helpful people there). Sorry it's sometimes hard to diagnose problems over email. Cheers ----------------- Aaron Morton Freelance Developer @aaronmorton http://www.thelastpickle.com On 28/08/2012, at 11:28 PM, rubbish me <rubbish...@googlemail.com> wrote: > Thanks again Aaron. > >> I think case I would not expect to see data lose. If you are still in a test >> scenario can you try to reproduce the problem ? If possible can you >> reproduce it with a single node ? > > We will try that later this week. > ---- > > We did the same exercise this week, this time we did a flush and snapshot > before the DR actually happened - as an attempt to identify if the commit > logs fsync was the problem. > > We can clearly see stables were created for the flush command. > And those sstables were loaded in when the nodes started up again after the > DR exercise. > > At this point we believed all nodes had all the data, so we let them serving > client requests while we run repair on the nodes. > > Data created before the last flush was still missing, according to the client > that talked to DC1 (the disaster DC). > > We had a look at the log of one of the DC1 nodes. The suspicious thing was > that latest sstable was being compacted during streaming sessions of the > repair. But no error was reported. > > Here comes my questions: > - if during the streaming session, the sstable that was about to stream out > but was being compacted, would we see error in the log? > - could this lead to data not found? > - is it safe to let a node serving read/write requests while repair is > running? > > Many thanks again. > > - A > > > > > aaron morton <aa...@thelastpickle.com> 於 27 Aug 2012 09:08 寫道： > >>> Brutally. kill -9. >> that's fine. I was thinking about reboot -f -n >> >>> We are wondering if the fsync of the commit log was working. >> I would say yes only because there other reported problems. >> >> I think case I would not expect to see data lose. If you are still in a test >> scenario can you try to reproduce the problem ? If possible can you >> reproduce it with a single node ? >> >> Cheers >> >> >> ----------------- >> Aaron Morton >> Freelance Developer >> @aaronmorton >> http://www.thelastpickle.com >> >> On 25/08/2012, at 11:00 AM, rubbish me <rubbish...@googlemail.com> wrote: >> >>> Thanks, Aaron, for your reply - please see the inline. >>> >>> >>> On 24 Aug 2012, at 11:04, aaron morton wrote: >>> >>>>> - we are running on production linux VMs (not ideal but this is out of >>>>> our hands) >>>> Is the VM doing anything wacky with the IO ? >>> >>> Could be. But I thought we would ask here first. This is a bit difficult >>> to prove cos we dont have the control over these VMs. >>> >>>> >>>> >>>>> As part of a DR exercise, we killed all 6 nodes in DC1, >>>> Nice disaster. Out of interest, what was the shutdown process ? >>> >>> Brutally. kill -9. >>> >>> >>>> >>>>> We noticed that data that was written an hour before the exercise, around >>>>> the last memtables being flushed,was not found in DC1. >>>> To confirm, data was written to DC 1 at CL LOCAL_QUORUM before the DR >>>> exercise. >>>> >>>> Was the missing data written before or after the memtable flush ? I'm >>>> trying to understand if the data should have been in the commit log or the >>>> memtables. >>> >>> Missing data was those written after the last flush. These data was >>> retrievable before the DR exercise. >>> >>>> >>>> Can you provide some more info on how you are detecting it is not found in >>>> DC 1? >>>> >>> >>> We tried hector, consistencylevel=local quorum. We had missing column or >>> the whole row. >>> >>> We tried cassandra-cli on DC1 nodes, same. >>> >>> However once we run the same query on DC2, C* must have then done a >>> read-repair. That particular piece of result data would appear in DC1 again. >>> >>> >>>>> If we understand correctly, commit logs are being written first and then >>>>> to disk every 10s. >>>> Writes are put into a bounded queue and processed as fast as the IO can >>>> keep up. Every 10s a sync messages is added to the queue. Not that the >>>> commit log segment may rotate at any time which requires a sync. >>>> >>>> A loss of data across all nodes in a DC seems odd. If you can provide some >>>> more information we may be able to help. >>> >>> >>> We are wondering if the fsync of the commit log was working. But we saw no >>> errors / warning in logs. Wondering if there is way to verify.... >>> >>> >>>> >>>> Cheers >>>> >>>> ----------------- >>>> Aaron Morton >>>> Freelance Developer >>>> @aaronmorton >>>> http://www.thelastpickle.com >>>> >>>> On 24/08/2012, at 6:01 AM, rubbish me <rubbish...@googlemail.com> wrote: >>>> >>>>> Hi all >>>>> >>>>> First off, let's introduce the setup. >>>>> >>>>> - 6 x C* 1.1.2 in active DC (DC1), another 6 in another (DC2) >>>>> - keyspace's RF=3 in each DC >>>>> - Hector as client. >>>>> - client talks only to DC1 unless DC1 can't serve the request. In which >>>>> case talks only to DC2 >>>>> - commit log was periodically sync with the default setting of 10s. >>>>> - consistency policy = LOCAL QUORUM for both read and write. >>>>> - we are running on production linux VMs (not ideal but this is out of >>>>> our hands) >>>>> ----- >>>>> As part of a DR exercise, we killed all 6 nodes in DC1, hector starts >>>>> talking to DC2, all the data was still there, everything continued to >>>>> work perfectly. >>>>> >>>>> Then we brought all nodes, one by one, in DC1 up. We saw a message saying >>>>> all the commit logs were replayed. No errors reported. We didn't run >>>>> repair at this time. >>>>> >>>>> We noticed that data that was written an hour before the exercise, around >>>>> the last memtables being flushed,was not found in DC1. >>>>> >>>>> If we understand correctly, commit logs are being written first and then >>>>> to disk every 10s. At worst we lost the last 10s of data. What could be >>>>> the cause of this behaviour? >>>>> >>>>> With the blessing of C* we could recovered all these data from DC2. But >>>>> we would like to understand why. >>>>> >>>>> Many thanks in advanced. >>>>> >>>>> Amy >>>>> >>>>> >>>> >>> >>
