Re: Can PostgreSQL create new WAL files instead of reusing old ones?

On Fri, Jun 22, 2018 at 11:22 AM, David Pacheco <dap(at)joyent(dot)com> wrote:
> On Wed, Jun 20, 2018 at 10:35 AM, Jerry Jelinek <jerry(dot)jelinek(at)joyent(dot)com>
> wrote:
>> I have implemented a small change to allow WAL recycling to be disabled.
>> It is visible at:
>> https://cr.joyent.us/#/c/4263/
>>
>> I'd appreciate getting any feedback on this.

>> tl;dr: We've found that under many conditions, PostgreSQL's re-use of old
>> WAL files appears to significantly degrade query latency on ZFS.

I haven't tested by it looks reasonable to me. It needs documentation
in doc/src/sgml/config.sgml. It should be listed in
src/backend/utils/misc/postgresql.conf.sample. We'd want a patch
against our master branch. Could you please register it in
commitfest.postgresql.org so we don't lose track of it?

Hey, a question about PostgreSQL on ZFS: what do you guys think about
pg_flush_data() in fd.c? It does mmap(), msync(), munmap() to try to
influence writeback? I wonder if at least on some operating systems
that schlepps a bunch of data out of ZFS ARC into OS page cache, kinda
trashing the latter?

--
Thomas Munro
http://www.enterprisedb.com