RE: [HACKERS][PATCH] Applying PMDK to WAL operations for persistent memory

From: Robert Haas [mailto:robertmhaas(at)gmail(dot)com]
> On Wed, Jan 24, 2018 at 10:31 PM, Tsunakawa, Takayuki
> <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com> wrote:
> > As you said, open_datasync was 20% faster than fdatasync on RHEL7.2, on
> a LVM volume with ext4 (mounted with options noatime, nobarrier) on a PCIe
> flash memory.
>
> So does that mean it was faster than your PMDK implementation?

The PMDK patch is not mine, but is from people in NTT Lab. I'm very curious about the comparison of open_datasync and PMDK, too.

> > What do you think about changing the default value of wal_sync_method
> on Linux in PG 11? I can understand the concern that users might hit
> performance degredation if they are using PostgreSQL on older systems. But
> it's also mottainai that many users don't notice the benefits of
> wal_sync_method = open_datasync on new systems.
>
> Well, some day persistent memory may be a common enough storage technology
> that such a change makes sense, but these days most people have either SSD
> or spinning disks, where the change would probably be a net negative. It
> seems more like something we might think about changing in PG 20 or PG 30.

No, I'm not saying we should make the persistent memory mode the default. I'm simply asking whether it's time to make open_datasync the default setting. We can write a notice in the release note for users who still use ext3 etc. on old systems. If there's no objection, I'll submit a patch for the next CF.

Regards
Takayuki Tsunakawa