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Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?

From: Jignesh Shah <jkshah(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Josh Berkus <josh(at)agliodbs(dot)com>, pgsql-performance(at)postgresql(dot)org, Greg Smith <greg(at)2ndquadrant(dot)com>
Subject: Re: Defaulting wal_sync_method to fdatasync on Linux for 9.1?
Date: 2010-11-19 14:52:47
Message-ID: AANLkTimJe1OtiTpmdXsV5Z--TRVMRNrfeb7-CWnxMRND@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Tue, Nov 16, 2010 at 8:22 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
>>> Well, we're not going to increase the default to gigabytes, but we could
>>> very probably increase it by a factor of 10 or so without anyone
>>> squawking.  It's been awhile since I heard of anyone trying to run PG in
>>> 4MB shmmax.  How much would a change of that size help?
>
>> Last I checked, though, this comes out of the allocation available to
>> shared_buffers.  And there definitely are several OSes (several linuxes,
>> OSX) still limited to 32MB by default.
>
> Sure, but the current default is a measly 64kB.  We could increase that
> 10x for a relatively small percentage hit in the size of shared_buffers,
> if you suppose that there's 32MB available.  The current default is set
> to still work if you've got only a couple of MB in SHMMAX.
>
> What we'd want is for initdb to adjust the setting as part of its
> probing to see what SHMMAX is set to.
>
>                        regards, tom lane
>
>

In all the performance tests that I have done, generally I get a good
bang for the buck with wal_buffers set to 512kB in low memory cases
and mostly I set it to 1MB which is probably enough for most of the
cases even with high memory.

That 1/2 MB wont make drastic change on shared_buffers anyway (except
for edge cases) but will relieve the stress quite a bit on wal
buffers.

Regards,
Jignesh

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