I sent this to Bruce but forgot to cc pgsql-hackers, The patches areActually the most interesting part of the thread was the initial post from Peter Zaitsev on a fcntl(fd, F_FULLSYNC, NULL): He wrote that this is necessary for Mac OS X to force a flush of the write caches in the disks. Unfortunately I can't find anything about this flag with google.
likely to go into 2.6.6. People interested in extremely safe fsync
writes should also follow the IDE barrier thread and the true fsync() in
Linux on IDE thread
Another interesting point is that right now, ide write caches must be disabled for reliable fsync operations with Linux. Recent suse kernels contain partial support. If the existing patches are completed and merged, it will be safe to enable write caching.
Perhaps Bruce's cache flush test could be modified slightly to check that the OS isn't lying about fsync: if fsync is faster than the rotational delay of the disks, then the setup is not suitable for postgres. This could be recommended as a setup test in the install document.
-- Manfred
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