On Tue, Aug 27, 2013 at 4:51 PM, Sawada Masahiko <sawada.m...@gmail.com> wrote: > On Sun, Aug 25, 2013 at 3:21 PM, Amit Kapila <amit.kapil...@gmail.com> wrote: >> On Sat, Aug 24, 2013 at 2:46 PM, Sawada Masahiko <sawada.m...@gmail.com> >> wrote: >>> On Sat, Aug 24, 2013 at 3:14 AM, Josh Berkus <j...@agliodbs.com> wrote: >>>> On 08/23/2013 12:42 AM, Sawada Masahiko wrote: >>>>> in case (a), those priority is clear. So I think that re-taking over >>>>> is correct behaviour. >>>>> OHOT, in case (b), even if AAA and BBB are set same priority, AAA >>>>> server steals SYNC replication. >>>>> I think it is better that BBB server continue behaviour SYNC standby, >>>>> and AAA should become potential server. >>>> >>>> So, you're saying that: >>>> >>>> 1) synchronous_standby_names = '*' >>>> >>>> 2) replica 'BBB' is the current sync standby >>>> >>>> 3) replica 'AAA' comes online >>>> >>>> 4) replica 'AAA' grabs sync status >>>> >>>> ? >>> I'm sorry that you are confuse. >>> It means that >>> >>> 1) synchronous_standby_names = '*' >>> >>> 2) replica 'AAA' is the current sync standby >>> >>> 3) replica 'BBB' is the current async standby (potential sync standby) >>> >>> 4) replica 'AAA' fail. after that, replica 'BBB' is current sync standby. >>> >>> 5) replica 'AAA' comes online >>> >>> 6) replica 'AAA' grabs sync status >>> >>>> >>> >>> >>>> If that's the case, I'm not really sure that's undesirable behavior. >>>> One could argue fairly persuasively that if you care about the >>>> precendence order of sync replicas, you shouldn't use '*'. And the rule >>>> of "if using *, the lowest-sorted replica name has sync" is actually a >>>> predictable, easy-to-understand rule. >>>> >>>> So if you want to make this a feature request, you'll need to come up >>>> with an argument as to why the current behavior is bad. Otherwise, >>>> you're just asking us to document it better (which is a good idea). >>> It is not depend on name of standby server. That is, The standby server, >>> which was connected to the master server during initial configration >>> replication, is top priority even if priority of two server are same. >> >> What is happening here is that incase of '*' as priority of both are >> same, system will choose whichever >> comes in list of registered standby's first (list is maintained in >> structure WalSndCtl). >> Each standby is registered with WalSndCtl when a new WALSender is >> started in function InitWalSenderSlot(). >> As 'AAA' has been registered first it becomes preferred sync standby >> even if priorities of both are same. >> When 'AAA' goes down, it marks that Slot entry as free (by setting >> pid=0 in function WalSndKill), >> now when 'AAA' comes back again, it gets that free Slot entry and >> again becomes preferred sync standby. >> >> Now if we want to fix as you are suggesting which I don't think is >> necessary, we might need to change WalSndKill and some other place so >> that whenever any standby goes down, it changes slots for already >> registered standby's. >>> User must remember that which standby server connected to master server at >>> first. >>> I think that this behavior confuse user. >>> so I think that we need to modify this behaviour or if '*' is used, priority >>> of server is not same (modifying manual is also good). >> >> Here user has done the settings (setting synchronous_standby_names = >> '*'), after which he will not have any control which standby will >> become sync standby, so ideally he should not complain. >> >> It might be case that for some users current behavior is good enough >> which means that with '*' whichever standby has become sync standby >> first, it will be the sync standby always if alive.
> I'm thinking that it is not necessary to change WalSndKill. > For example, we add the value (e.g., sync_standby) which have that > which wal sender is active SYNC rep. > And if sync_standby is already set and it is active, server doesn't > looking for active standby. > Only if sync_standby is not set and it is inactive, server looking for > that which server is active SYNC rep. > If so, we also prevent to find active SYNC rep whenever > SyncRepReleaseWaiters() is called. For '*' case, it will be okay, but when the user has given proper names, in that case even if there is any active Sync Rep, it has to be changed based on priority. I think here how to provide a fix, so that behavior gets changed to what you describe is a second priority work, first is to show the value of use-case. Do you really know where people actually setup using '*' as configuration and if yes, are they annoyed with current behavior? I have thought about it, but could imagine a scenario where people will be using '*' in their production configurations, may be it will be useful in test labs. With Regards, Amit Kapila. EnterpriseDB: http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
