On 28 April 2018 at 09:15, Andres Freund <andres@anarazel.de> wrote:
> Hi,
>
> On 2018-04-28 08:25:53 -0700, Simon Riggs wrote:
>> > - Use direct IO. Due to architectural performance issues in PG and the
>> > fact that it'd not be applicable for all installations I don't think
>> > this is a reasonable fix for the issue presented here. Although it's
>> > independently something we should work on. It might be worthwhile to
>> > provide a configuration that allows to force DIO to be enabled for WAL
>> > even if replication is turned on.
>>
>> "Use DirectIO" is roughly same suggestion as "don't trust Linux filesystems".
>
> I want to emphasize that this is NOT a linux only issue. It's a problem
> across a number of operating systems, including linux.
Yes, of course.
>> It would be a major admission of defeat for us to take that as our
>> main route to a solution.
>
> Well, I think we were wrong to not engineer towards DIO. There's just
> too many issues with buffered IO to not have a supported path for
> DIO. But given that it's unrealistic to do so without major work, and
> wouldn't be applicable for all installations (shared_buffer size becomes
> critical), I don't think it matters that much for the issue discussed
> here.
>
>
>> The people I've spoken to so far have encouraged us to continue
>> working with the filesystem layer, offering encouragement of our
>> decision to use filesystems.
>
> There's a lot of people disagreeing with it too.
Specific recent verbal feedback from OpenLDAP was that the project
adopted DIO and found no benefit in doing so, with regret the other
way from having tried.
The care we need to use for any technique is the same.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services