Re: PANIC caused by open_sync on Linux

Added to TODO:

* Be more aggressive about creating WAL files

http://archives.postgresql.org/pgsql-hackers/2007-10/msg01325.php

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Tom Lane wrote:
> Greg Smith <gsmith(at)gregsmith(dot)com> writes:
> > On Fri, 26 Oct 2007, ITAGAKI Takahiro wrote:
> >> Mixed usage of buffered and direct i/o is legal, but enforces complexity
> >> to kernels. If we simplify it, things would be more relaxed. For
> >> example, dropping zero-filling and only use direct i/o. Is it possible?
>
> > It's possible, but performance suffers considerably. I played around with
> > this at one point when looking into doing all database writes as sync
> > writes. Having to wait until the entire 16MB WAL segment made its way to
> > disk before more WAL could be written can cause a nasty pause in activity,
> > even with direct I/O sync writes. Even the current buffered zero-filled
> > write of that size can be a bit of a drag on performance for the clients
> > that get caught behind it, making it any sort of sync write will be far
> > worse.
>
> This ties into a loose end we didn't get to yet: being more aggressive
> about creating future WAL segments. ISTM there is no good reason for
> clients ever to have to wait for WAL segment creation --- the bgwriter,
> or possibly the walwriter, ought to handle that in the background. But
> we only check for the case once per checkpoint and we don't create a
> segment unless there's very little space left.
>
> regards, tom lane
>
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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://postgres.enterprisedb.com

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