On Tue, Aug 18, 2015 at 12:38 PM, Fabien COELHO <coelho@cri.ensmp.fr> wrote:
Hello Amit,
So the option is best kept as "off" for now, without further data, I'm fine with that.
One point to think here is on what basis user can decide make this option on, is it predictable in any way? I think one case could be when the data set fits in shared_buffers.
Yep.
In general, providing an option is a good idea if user can decide with ease when to use that option or we can give some clear recommendation for the same otherwise one has to recommend that test your workload with this option and if it works then great else don't use it which might also be okay in some cases, but it is better to be clear.
My opinion, which is not backed by any data (anyone can feel free to provide a FreeBSD box for testing...) is that it would mostly be an improvement if you have a significant write load to have the flush option on when running on non-Linux systems which provide posix_fadvise.
If you have a lot of reads and few writes, then postgresql currently works reasonably enough, which is why people do not complain too much about write stalls, and I expect that the situation would not be significantly degraded.
Now there are competing positive and negative effects induced by using posix_fadvise, and moreover its implementation varries from OS to OS, so without running some experiments it is hard to be definite.
Sure, I think what can help here is a testcase/'s (in form of script file
or some other form, to test this behaviour of patch) which you can write
and post here, so that others can use that to get the data and share it.
Ofcourse, that is not mandatory to proceed with this patch, but still can
help you to prove your point as you might not have access to different