On mån, 2010-10-04 at 23:41 -0400, Robert Haas wrote: > > Well, it's not really useful, but that's how it works "everywhere". On > > Linux, fsync carries the stuff from the kernel's RAM to the disk > > controller's RAM, and then it depends on some hdparm magic or something > > what happens next. > > That's a bit vaguer than I'd like. TFD says "The aim of WAL is to > ensure that the log is written before database records are altered, > but this can be subverted by disk drives that falsely report a > successful write to the kernel, when in fact they have only cached the > data and not yet stored it on the disk. A power failure in such a > situation might lead to irrecoverable data corruption. Administrators > should try to ensure that disks holding PostgreSQL's WAL log files do > not make such false reports." This leaves open the question of how > they should attempt to do this; we should say what we know about that.
That is explained in section 29.1 "Reliability". > I also notice the following sentence in our documentation, which now > appears to me to be flat-out wrong: "The wal_sync_method parameter > determines how PostgreSQL will ask the kernel to force WAL updates > out to disk. All the options should be the same in terms of > reliability, but it's quite platform-specific which one will be the > fastest." Obviously, we know now (if we didn't before) that this > isn't the case, per my OP. Right. It was true before fsync_writethrough was invented. -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers