Re: patch to allow disable of WAL recycling

On Mon, Jul 16, 2018 at 10:12 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> But anyway, this means we have two nearly independent issues to
> investigate: whether recycling/renaming old files is cheaper than
> constantly creating and deleting them, and whether to use physical
> file zeroing versus some "just set the EOF please" filesystem call
> when first creating a file. The former does seem like it's purely
> a performance question, but the latter involves a tradeoff of
> performance against an ENOSPC-panic protection feature that in
> reality only works on some filesystems.

It's been a few years since I tested this, but my recollection is that
if you fill up pg_xlog, the system will PANIC and die on a vanilla
Linux install. Sure, you can set max_wal_size, but that's a soft
limit, not a hard limit, and if you generate WAL faster than the
system can checkpoint, you can overrun that value and force allocation
of additional WAL files. So I'm not sure we have any working
ENOSPC-panic protection today. Given that, I'm doubtful that we
should prioritize maintaining whatever partially-working protection we
may have today over raw performance. If we want to fix ENOSPC on
pg_wal = PANIC, and I think that would be a good thing to fix, then we
should do it either by finding a way to make the WAL insertion ERROR
out instead of panicking, or throttle WAL generation as we get close
to disk space exhaustion so that the checkpoint has time to complete,
as previously proposed by Heroku.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company