On Wed, Dec 23, 2015 at 2:16 PM, Tomas Vondra <tomas.von...@2ndquadrant.com> wrote: >> Another point (which Jan Wieck made me think of) is that the optimal >> behavior here likely depends on whether xlog and data are on the same >> disk controller. If they aren't, the FPW spike and background writes >> may not interact as much. > > I'm not sure what exactly you mean by "optimal behavior" here. Surely if you > want to minimize interference between WAL and regular I/O, you'll do that. > > But I don't see what that has to do with the writes generated by the > checkpoint? If we do much more writes at the beginning of the checkpoint > (due to getting confused by FPW), and OS starts flushing that to disk > because we exceed dirty_(background)_bytes, that surely interferes with > reads (which is a major issue for queries).
Well, it's true that the checkpointer dirty page writes could interfere with reads, but if you've also got lots of FPW-bloated WAL records being written to the same disk at the same time, I would think that'd be worse. No? -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
