On Tue, 2008-09-09 at 20:12 +0900, Fujii Masao wrote: > I'd like to introduce new parameter "synchronous_replication" which specifies > whether backends waits for the response from WAL sender process. By > combining synchronous_commit and synchronous_replication, users can > choose various options.
We already discussed that on -hackers. See "Transaction Controlled Robustness". But yes, something like that. Please note the design mentions fsyncing after applying WAL. I'm sure you're aware we don't fsync after *applying* WAL now, and I hope we never do. You might want to fsync data to WAL files on the standby, but that is a slightly different thing. > > After (or during) XLogInsert backends will sleep in a proc queue, > > similar to LWlocks and protected by a spinlock. When preparing to > > write/send the WAL process should read the proc at the *tail* of the > > queue to see what the next LogwrtRqst should be. Then it performs its > > action and wakes procs up starting with the head of the queue. We would > > add LSN into PGPROC, so WAL processes can check whether the backend > > should be woken. The LSN field can be accessed without spinlocks since > > it is only ever set by the backend itself and only read while a backend > > is sleeping. So we access spinlock, find tail, drop spinlock then read > > LSN of the backend that (was) the tail. > > You mean only XLogInsert treating "commit record" or every XLogInsert? Just the commit records, when synchronous_commit = on. > Anyway, ISTM that the response time get worse :( No, because it would have had to wait in the queue for the WALWriteLock while prior writes occur. If the WALWriter sleeps on a semaphore, it too can be nudged into action at the appropriate time, so no need for a delay between backend beginning to wait and WALWriter beginning to act. (Well, IPC delay between two processes, so some, but that is balanced against efficiency of Send). -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
