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

From: "Tsunakawa, Takayuki" <tsunakawa(dot)takay(at)jp(dot)fujitsu(dot)com>
To: 'Robert Haas' <robertmhaas(at)gmail(dot)com>
Cc: Yoshimi Ichiyanagi <ichiyanagi(dot)yoshimi(at)lab(dot)ntt(dot)co(dot)jp>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, "menjo(dot)takashi(at)lab(dot)ntt(dot)co(dot)jp" <menjo(dot)takashi(at)lab(dot)ntt(dot)co(dot)jp>, "ishizaki(dot)teruaki(at)lab(dot)ntt(dot)co(dot)jp" <ishizaki(dot)teruaki(at)lab(dot)ntt(dot)co(dot)jp>
Subject: RE: [HACKERS][PATCH] Applying PMDK to WAL operations for persistent memory
Date: 2018-01-26 01:54:04
Message-ID: 0A3221C70F24FB45833433255569204D1F8A67AF@G01JPEXMBYT05
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From: Robert Haas [mailto:robertmhaas(at)gmail(dot)com]
> If I understand correctly, those results are all just pg_test_fsync results.
> That's not reflective of what will happen when the database is actually
> running. When you use open_sync or open_datasync, you force WAL write and
> WAL flush to happen simultaneously, instead of letting the WAL flush be
> delayed.

Yes, that's pg_test_fsync output. Isn't pg_test_fsync the tool to determine the value for wal_sync_method? Is this manual misleading?
pg_test_fsync - determine fastest wal_sync_method for PostgreSQL

pg_test_fsync is intended to give you a reasonable idea of what the fastest wal_sync_method is on your specific system, as well as supplying diagnostic information in the event of an identified I/O problem.

Anyway, I'll use pgbench, and submit a patch if open_datasync is better than fdatasync. I guess the current tweak of making fdatasync the default is a holdover from the era before ext4 and XFS became prevalent.

> I don't have the results handy at the moment. We found it to be faster
> on a database benchmark where the WAL was stored on an NVRAM device.

Oh, NVRAM. Interesting. Then I'll try open_datasync/fdatasync comparison on HDD and SSD/PCie flash with pgbench.

Takayuki Tsunakawa

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