Re: wal_buffers

On Sun, Feb 19, 2012 at 6:33 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Sun, Feb 19, 2012 at 9:46 AM, Euler Taveira de Oliveira
>> <euler(at)timbira(dot)com> wrote:
>>> Isn't it useful to print some messages on the log when we have "wrap around"?
>>> In this case, we have an idea that wal_buffers needs to be increased.
>
>> I was thinking about that.  I think that what might be more useful
>> than a log message is a counter somewhere in shared memory.  Logging
>> imposes a lot of overhead, which is exactly what we don't want here,
>> and the volume might be quite high on a system that is bumping up
>> against this problem.  Of course then the question is... how would we
>> expose the counter value?
>
> Why do you need a counter, other than the current LSN?  Surely the
> number of WAL buffer ring cycles can be deduced directly from that.

The problem isn't how many times its cycled, the issue is whether
there was a wait induced by needing to flush wal buffers because of
too many writes. You can't count those waits in the way you suggest,
though you can calculate an upper limit on them, but that's not very
useful.

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