On 04/06/2017 08:33 PM, David Steele wrote:
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

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

I don't see a particular reason for this, TBH. The sweet spots will be likely dependent hardware / OS configuration etc. Assuming there actually are sweet spots - no one demonstrated that yet.

Also, I don't see how supporting additional WAL sizes increases chance of incompatibility. We already allow that, so either the tools (e.g. backup solutions) assume WAL segments are always 16MB (in which case are essentially broken) or support valid file sizes (in which case they should have no issues with the new ones).

If we're going to do this, I'm in favor of deciding some reasonable upper limit (say, 1GB or 2GB sounds good), and allowing all 2^n values up to that limit.

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.

I'm willing to do some extensive performance testing on the patch. I don't see how that could happen in the next few day (before the feature freeze), particularly considering we're interested in long tests.

The question however is whether we need to do this testing when we don't actually change the default (at least the patch submitted on 3/27 does seem to keep the 16MB). I assume people specifying a custom value when calling initdb are expected to know what they are doing (and I don't see how we can prevent distros from choosing a bad value in their packages - they could already do that with configure-time option).

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

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.

Do we actually have any infrastructure for that? Or do you plan to add some new animals with different WAL segment sizes?


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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