Horton

Re: Problem while setting the fpw with SIGHUP

Robert Haas <robertmhaas@gmail.com>

View thread View on postgr.es
From: Robert Haas <robertmhaas@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Michael Paquier <michael@paquier.xyz>, Heikki Linnakangas <hlinnaka@iki.fi>, Kyotaro HORIGUCHI <horiguchi.kyotaro@lab.ntt.co.jp>, Dilip Kumar <dilipbalaut@gmail.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2018-04-18T14:52:51Z
Lists: pgsql-hackers 
On Wed, Apr 18, 2018 at 10:37 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> On Fri, Apr 13, 2018 at 10:36 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> On Thu, Apr 12, 2018 at 9:29 PM, Michael Paquier <michael@paquier.xyz> wrote:
>>> Still does it matter when the change is effective?
>>
>> I don't really care deeply about when the change takes effect, but I
>> do care about whether the time when the system *says* the change took
>> effect is the same as when it *actually* took effect.  If those aren't
>> the same, it's confusing.
>>
>
> So, what in your opinion is the way to deal with this?  If we make it
> a PGC_POSTMASTER parameter, it will have a very clear behavior and
> users don't need to bother whether they have a risk of torn page
> problem or not and as a side-impact the code will be simplified as
> well.  However, as Michael said the people who get the benefit of this
> option by disabling/enabling this parameter might complain.  Keeping
> it as a SIGHUP option has the drawback that even after the user has
> enabled it, there is a risk of torn pages.

I would just document the risks.  If the documentation says that you
can't rely on the value until after the next checkpoint, or whatever
the rule is, then I think we're fine.  I don't think that we really
have the infrastructure to do any better; if we try, we'll just end up
with odd warts.  Documenting the current set of warts is less churn
and less work.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. Fix assertion failure when updating full_page_writes for checkpointer.

  2. Attach FPI to the first record after full_page_writes is turned on.

  3. Revamp the WAL record format.

  4. Move the backup-block logic from XLogInsert to a new file, xloginsert.c.