On Wednesday, June 13, 2012 06:53:17 PM Jeff Davis wrote: > On Wed, 2012-06-13 at 13:53 +0300, Peter Eisentraut wrote: > > The --help output for the -N option was copy-and-pasted wrongly. > > > > The message issued when using -N is also a bit content-free. Maybe > > something like > > > > "Running in nosync mode. The data directory might become corrupt if the > > operating system crashes.\n" > > Thank you, fixed. > > > Which leads to the question, how does one get out of this state? Is > > running sync(1) enough? Is starting the postgres server enough? > > sync(1) calls sync(2), and the man page says: > > "According to the standard specification (e.g., POSIX.1-2001), sync() > schedules the writes, but may return before the actual writing is done. > However, since version 1.3.20 Linux does actually wait. (This still > does not guarantee data integrity: modern disks have large caches.)" > > So it looks like sync is enough if you are on linux *and* you have any > unprotected write cache disabled. Protection can include write barries, it doesn't need to be a BBU....
> I don't think starting the postgres server is enough. Agreed. > Before, I think we were safe because we could assume that the OS would > flush the buffers before you had time to store any important data. But > now, that window can be much larger. I think to a large degree we didn't see any problem because of the old ext3 "sync the whole world" type of behaviour which was common for a very long time. So on ext3 (with data=ordered, the default) any fsync (checkpoints, commit, ...) would lead to all the other files being synced as well which means starting the server once would have been enough on such a system... Quick review: - defaulting to initdb -N in the regression suite is not a good imo, because that way the buildfarm won't catch problems in that area... - could the copydir.c and initdb.c versions of walkdir/sync_fname et al be unified? - I personally would find it way nicer to put USE_PRE_SYNC into pre_sync_fname instead of cluttering the main function with it Looks good otherwise! Thanks, Andres -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
