Re: Logical decoding on standby

From: Craig Ringer <craig(at)2ndquadrant(dot)com>
To: Simon Riggs <simon(dot)riggs(at)2ndquadrant(dot)com>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Thom Brown <thom(at)linux(dot)com>, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>, Petr Jelinek <petr(dot)jelinek(at)2ndquadrant(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Logical decoding on standby
Date: 2017-03-30 08:07:05
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On 30 March 2017 at 11:34, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
> On 29 March 2017 at 23:13, Simon Riggs <simon(dot)riggs(at)2ndquadrant(dot)com> wrote:
>> On 29 March 2017 at 10:17, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
>>> On 29 March 2017 at 16:44, Craig Ringer <craig(at)2ndquadrant(dot)com> wrote:
>>>> * Split oldestCatalogXmin tracking into separate patch
>>> Regarding this, Simon raised concerns about xlog volume here.
>>> It's pretty negligible.
>>> We only write a new record when a vacuum runs after catalog_xmin
>>> advances on the slot with the currently-lowest catalog_xmin (or, if
>>> vacuum doesn't run reasonably soon, when the bgworker next looks).
>> I'd prefer to slow things down a little, not be so eager.
>> If we hold back update of the catalog_xmin until when we run
>> GetRunningTransactionData() we wouldn't need to produce any WAL
>> records at all AND we wouldn't need to have VACUUM do
>> UpdateOldestCatalogXmin(). Bgwriter wouldn't need to perform an extra
>> task.
>> That would also make this patch about half the length it is.
>> Let me know what you think.
> Good idea.
> We can always add a heuristic later to make xl_running_xacts get
> emitted more often at high transaction rates if it's necessary.
> Patch coming soon.


A bit fiddlier than expected, but I like the result more.

In the process I identified an issue with both the prior patch and
this one where we don't check slot validity for slots that existed on
standby prior to promotion of standby to master. We were just assuming
that being the master was good enough, since it controls
replication_slot_catalog_xmin, but that's not true for pre-existing

Fixed by forcing update of the persistent safe catalog xmin after the
first slot is created on the master - which is now done by doing an
immediate LogStandbySnapshot() after assigning the slot's

Craig Ringer
PostgreSQL Development, 24x7 Support, Training & Services

Attachment Content-Type Size
log-catalog-xmin-advances-v3.patch text/x-patch 27.6 KB

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