Is there a reason we only use O_DIRECT with open_* sync options?
xlogdefs.h says:
/** Because O_DIRECT bypasses the kernel buffers, and because we never* read those buffers except during crash
recovery,it is a win to use* it in all cases where we sync on each write(). We could allow O_DIRECT* with fsync(),
butbecause skipping the kernel buffer forces writes out* quickly, it seems best just to use it for O_SYNC. It is hard
toimagine* how fsync() could be a win for O_DIRECT compared to O_SYNC and O_DIRECT.* Also, O_DIRECT is never enough
toforce data to the drives, it merely* tries to bypass the kernel cache, so we still need O_SYNC or fsync().*/
This seems wrong because fsync() can win if there are two writes before
the sync call. Can kernels not issue fsync() if the write was O_DIRECT?
If that is the cause, we should document it.
-- Bruce Momjian <bruce@momjian.us> http://momjian.us EnterpriseDB
http://enterprisedb.com
+ It's impossible for everything to be true. +
