Tomas Vondra wrote: > On 17 Listopad 2011, 17:07, Jaime Casanova wrote: > > On Thu, Nov 17, 2011 at 7:52 AM, Schubert, Joerg <jschubert@xxxxxxxxxx> > > wrote: > >> Hello, > >> > >> I have two servers with battery backed power supply (USV). So it is > >> unlikely, that both will crash at the same time. > >> > >> Will synchronous replication work with fsync=off? > >> That means we will commit to system cache, but not to disk. Data will > >> not > >> survive a system crash but the second system should still be consistent. > >> > > > > you should never use fsync=off (in production at least) > > > > the appropiate parameter to use is synchronous_commit which is the one > > that controls synchronous replication: > > off = no local nor remote synchronous commit > > local = local synchronous commit but no remote > > on = both, local and remote, synchronous commit > > > > synchronous commit = flushed to disk > > While I don't recommend it, fsync=off definitely is an option, especially > with sync replication. The synchronous_commit is not a 1:1 replacement. > > Imagine for example a master with lot of I/O, and a sync standby. By > setting fsync=off on the master and fsync=on on the slave the master does > not need to wait for the fsync (so the I/O is not that stressed and can > handle more requests from clients), but the slave actually does fsync. > > So you don't force local fsync, but you're waiting for fsync from the > standby. But standby doesn't need to handle all the I/O the primary has. > > You can't do this with synchronous_commit - that basically forces you to > do local fsync on commit, or not to wait for the commit at all. > > Tomas > > Disclaimer: I haven't actually tried this, so maybe I missed something. I think you did. synchronous_commit really means fsync so that the system is alway consistent --- there is no waiting for the fsync to happen on the master (unless I am totally missing something). With fsync off, you can get into cases where the heap/index files are pushed to disk before the wal gets written to disk, causing the system to be inconsistent in case of a crash replay. I think the only use of fsync off is for performance testing so see how expensive fynsc is. -- Bruce Momjian <bruce@xxxxxxxxxx> http://momjian.us EnterpriseDB http://enterprisedb.com + It's impossible for everything to be true. + -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
