Re: [PATCHES] O_DIRECT for WAL writes

Greg Stark <gsstark@mit.edu> writes:
> Tom Lane <tgl@sss.pgh.pa.us> writes:
>> Unfortunately, I cannot believe these numbers --- the near equality of
>> fsync off and fsync on means there is something very wrong with the
>> measurements.  What I suspect is that your ATA drives are doing write
>> caching and thus the "fsyncs" are not really waiting for I/O at all.

> I wonder whether it would make sense to have an automatic test for this
> problem. I suspect there are lots of installations out there whose admins
> don't realize that their hardware is doing this to them.

Not sure about "automatic", but a simple little test program to measure
the speed of rewriting/fsyncing a small test file would surely be a nice
thing to have.

The reason I question "automatic" is that you really want to test each
drive being used, if the system has more than one; but Postgres has no
idea what the actual hardware layout is, and so no good way to know what
needs to be tested.
        regards, tom lane