Re: PGSQL, checkpoints, and file system syncs

On Thu, Apr 3, 2014 at 09:01:08PM +0300, Heikki Linnakangas wrote:
> >Is there something I can set in the PGSQL parameters or in the file
> >system parameters to force a steady flow of writes to disk rather
> >than waiting for a sync system call? Mounting with "commit=1" did not
> >make a difference.
>
> Try setting the vm.dirty_bytes sysctl. Something like 256MB might be a
> good starting point.
>
> This comes up fairly often, see e.g.:
> http://www.postgresql.org/message-id/flat/27C32FD4-0142-44FE-8488-9F36
> 6DC75966(at)mr-paradox(dot)net

Uh, should he set vm.dirty_bytes or vm.dirty_background_bytes? It is my
understanding that vm.dirty_background_bytes starts the I/O while still
accepting writes, while vm.dirty_bytes stops accepting writes during the
I/O, which isn't optimal. See:

https://www.kernel.org/doc/Documentation/sysctl/vm.txt

dirty_bytes

Contains the amount of dirty memory at which a process generating disk
writes will itself start writeback.

dirty_background_bytes

Contains the amount of dirty memory at which the background kernel
flusher threads will start writeback.

I think we want the flusher to be active, not necessarly the writing
process.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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