On 4/5/17 7:29 AM, Simon Riggs wrote:
> On 5 April 2017 at 06:04, Beena Emerson <memissemer...@gmail.com> wrote:
>> The  WALfilename - LSN mapping disruption for higher values you mean? Is
>> there anything else I have missed?
> I see various issues raised but not properly addressed
> 1. we would need to drop support for segment sizes < 16MB unless we
> adopt a new incompatible filename format.
> I think at 16MB the naming should be the same as now and that
> WALfilename -> LSN is very important.
> For this release, I think we should just allow >= 16MB and avoid the
> issue thru lack of time.


> 2. It's not clear to me the advantage of being able to pick varying
> filesizes. I see great disadvantage in having too many options, which
> greatly increases the chance of incompatibility, annoyance and
> breakage. I favour a small number of values that have been shown by
> testing to be sweet spots in performance and usability. (1GB has been
> suggested)

I'm in favor of 16,64,256,1024.

> 3. New file allocation has been a problem raised with this patch for
> some months now.

I've been playing around with this and I don't think short tests show
larger sizes off to advantage.  Larger segments will definitely perform
more poorly until Postgres starts recycling WAL.  Once that happens I
think performance differences should be negligible, though of course
this needs to be verified with longer-running tests.

If archive_timeout is set then there will also be additional time taken
to zero out potentially larger unused parts of the segment.  I don't see
this as an issue, however, because if archive_timeout is being triggered
then the system is very likely under lower than usual load.

> Lack of clarity and/or movement on these issues is very, very close to
> getting the patch rejected now. Let's not approach this with the
> viewpoint that I or others want it to be rejected, lets work forwards
> and get some solid changes that will improve the world without causing
> problems.

I would definitely like to see this go in, though I agree with Peter
that we need a lot more testing.  I'd like to see some build farm
animals testing with different sizes at the very least, even if there's
no time to add new tests.


Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to