Re: Postgres, fsync, and OSs (specifically linux)

From: Andres Freund <andres(at)anarazel(dot)de>
To: Dmitry Dolgov <9erthalion6(at)gmail(dot)com>
Cc: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Craig Ringer <craig(at)2ndquadrant(dot)com>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Postgres, fsync, and OSs (specifically linux)
Date: 2018-05-22 15:57:18
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On 2018-05-22 17:37:28 +0200, Dmitry Dolgov wrote:
> Thanks for the patch. Out of curiosity I tried to play with it a bit.


> `pgbench -i -s 100` actually hang on my machine, because the
> copy process ended up with waiting after `pg_uds_send_with_fd`
> had

Hm, that had worked at some point...

> errno == EWOULDBLOCK || errno == EAGAIN
> as well as the checkpointer process.

What do you mean with that latest sentence?

> Looks like with the default
> configuration and `max_wal_size=1GB` it writes more than reads to a
> socket, and a buffer eventually becomes full.

That's intended to then wake up the checkpointer immediately, so it can
absorb the requests. So something isn't right yet.

> I've increased SO_RCVBUF/SO_SNDBUF and `max_wal_size` independently to
> check it, and in both cases the problem disappeared (but I assume only
> for this particular scale). Is it something that was already
> considered?

It's considered. Tuning up those might help with performance, but
shouldn't required from a correctness POV. Hm.


Andres Freund

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