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Re: increasing the default WAL segment size

Stephen Frost <sfrost@snowman.net>

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From: Stephen Frost <sfrost@snowman.net>
To: Robert Haas <robertmhaas@gmail.com>
Cc: David Steele <david@pgmasters.net>, Beena Emerson <memissemerson@gmail.com>, tushar <tushar.ahuja@enterprisedb.com>, Prabhat Sahu <prabhat.sahu@enterprisedb.com>, Ashutosh Sharma <ashu.coek88@gmail.com>, Jim Nasby <Jim.Nasby@bluetreble.com>, Kuntal Ghosh <kuntalghosh.2007@gmail.com>, Andres Freund <andres@anarazel.de>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2017-03-22T00:10:24Z
Lists: pgsql-hackers 
Robert,

* Robert Haas (robertmhaas@gmail.com) wrote:
> On Tue, Mar 21, 2017 at 6:02 PM, David Steele <david@pgmasters.net> wrote:
> > The biggest downside I can see is that this would change the naming scheme
> > for the default of 16MB compared to previous versions of Postgres.  However,
> > for all other wal-seg-size values changes would need to be made anyway.
> 
> I think changing the naming convention for 16MB WAL segments, which is
> still going to be what 99% of people use, is an awfully large
> compatibility break for an awfully marginal benefit.

It seems extremely unlikely to me that we're going to actually see users
deviate from whatever we set the default to and so I'm not sure that
this is a real concern.  We aren't changing what 9.6 and below's naming
scheme is, just what PG10+ do, and PG10+ are going to have a different
default WAL size.

I realize the current patch still has the 16MB default even though a
rather large portion of the early discussion appeared in favor of
changing it to 64MB.  Once we've done that, I don't think it makes one
whit of difference what the naming scheme looks like when you're using
16MB sizes because essentially zero people are going to actually use
such a setting.

> We've already
> created quite a few incompatibilities in this release, and I'm not
> entirely eager to just keep cranking them out at top speed.  

That position would seem to imply that you're in favor of keeping the
current default of 16MB, but that doesn't make sense given that you
started this discussion advocating to make it larger.  Changing your
position is certainly fine, but it'd be good to be more clear if that's
what you meant here or if you were just referring to the file naming
scheme but you do still want to increase the default size.

I'll admit that we might have a few more people using non-default sizes
once we make it an initdb-option (though I'm tempted to suggest that one
might be able to count them using their digits ;), but it seems very
unlikely that they would do so to reduce it back down to 16MB, so I'm
really not seeing the naming scheme change as a serious
backwards-incompatibility change.

Thanks!

Stephen

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Make WAL segment size configurable at initdb time.

  2. Perform only one ReadControlFile() during startup.

  3. Introduce BYTES unit for GUCs.

  4. Remove useless duplicate inclusions of system header files.

  5. Refactor other replication commands to use DestRemoteSimple.

  6. Add a SHOW command to the replication command language.

  7. Add a new DestReceiver for printing tuples without catalog access.

  8. Support fls().

  9. Extend yesterday's patch making BLCKSZ and RELSEG_SIZE configurable to also

  10. Commit the reasonably uncontroversial parts of J.R. Nield's PITR patch, to

  11. XLOG (also known as WAL -:)) Bootstrap/Startup/Shutdown.

  12. Transaction log manager core code.