On Tue, Jul 27, 2010 at 10:53:45PM +0900, Fujii Masao wrote: > On Tue, Jul 27, 2010 at 10:12 PM, Joshua Tolley <eggyk...@gmail.com> wrote: > > My concern is that in a quorum system, if the quorum number is less than the > > total number of replicas, there's no way to know *which* replicas composed > > the > > quorum for any given transaction, so we can't know which servers to fail to > > if > > the master dies. > > What about checking the current WAL receive location of each standby by > using pg_last_xlog_receive_location()? The standby which has the newest > location should be failed over to.
That makes sense. Thanks. > > This isn't different from Oracle, where it looks like > > essentially the "quorum" value is always 1. Your scenario shows that all > > replicas are not created equal, and that sometimes we'll be interested in > > WAL > > getting committed on a specific subset of the available servers. If I had > > two > > nearby replicas called X and Y, and one at a remote site called Z, for > > instance, I'd set quorum to 2, but really I'd want to say "wait for server X > > and Y before committing, but don't worry about Z". > > > > I have no idea how to set up our GUCs to encode a situation like that :) > > Yeah, quorum commit alone cannot cover that situation. I think that > current approach (i.e., quorum commit plus replication mode per standby) > would cover that. In your example, you can choose "recv", "fsync" or > "replay" as replication_mode in X and Y, and choose "async" in Z. Clearly I need to read through the GUCs and docs better. I'll try to keep quiet until that's finished :) -- Joshua Tolley / eggyknap End Point Corporation http://www.endpoint.com
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