Re: Disable WAL completely - Performance and Persistency research

On Thu, Jul 7, 2016 at 1:01 AM, Netanel Katzburg <netanel10k@gmail.com> wrote:
> Hi All,
>
> As part of my masters at TAU, I'm currently conducting some research
> regarding new persistent memory technology.
> I'm using PG for this research and would like to better understand some of
> the performance bottlenecks.
> For this reason I'm trying to disable the WAL completely, using some hacks
> on the source code and compiling my own version.
>
> So what I'm actually looking for, is some guidance about a simple way to:
>
> 1. Disable the WAL by not writing anything to the xlog directory. I don't
> care about recovery/fault tolerance or PITR/ replication etc at the moment.
> I'm aware that the WAL and checkpoint are bind in many ways and are crucial
> for PG core features.
> I tried changing the status of all tables to "unlogged" tables by changing
> RelationNeedsWAL MACRO, as well as "needs_wal" parameter at storage.c.
> But, got no performance benefit, so I guess this was the wrong place to
> change.
>
> 2. Cancel the locking around WAL files  - I don't care about corrupted files
> at the moment, I just want to see what is the maximum performance benefit
> that I can get without lock contention.
>
> Any guidance on how to do so would be appreciated :)

I have a very old patch which introduces a config variable (JJNOWAL)
that skips all WAL, except for the WAL of certain checkpoints (which
are needed for initdb and to restart the server after a clean
shutdown).

I have rebased it up to HEAD.  It seems to work, but I haven't tested
thoroughly that it still does the correct thing in every corner case.
(a lot of changes have been made to xlog code since last time I used
this.)

Obviously if the server goes down uncleanly while this setting is
active, it will not be usable anymore.

Cheers,

Jeff