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Re: Disabled features on Hot Standby

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Noah Misch <noah(at)leadboat(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Disabled features on Hot Standby
Date: 2012-01-14 08:08:29
Message-ID: CA+U5nMJ9zkfpgAFDG8FguHPk-eap0oM+PCEr6q75hmUeZXYwnw@mail.gmail.com (view raw or flat)
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Lists: pgsql-hackers
On Sat, Jan 14, 2012 at 1:02 AM, Noah Misch <noah(at)leadboat(dot)com> wrote:

>> Also, what happens if an all-visible bit gets set on the standby
>> through some other mechanism - e.g. restored from an FPI or
>> XLOG_HEAP_NEWPAGE?  I'm not sure whether we ever do an FPI of the
>> visibility map page itself, but we certainly do it for the heap pages.
>>  So it might be that this infrastructure would (somewhat bizarrely)
>> trust the visibility map bits but not the PD_ALL_VISIBLE bits.
>
> Simon spoke to the FPI side of the question.  For heap pages, the
> XLOG_HEAP_NEWPAGE consumers are CLUSTER, VACUUM FULL and ALTER TABLE SET
> TABLESPACE.  For the last, we will have already logged any PD_ALL_VISIBLE bits
> through normal channels.  CLUSTER and VACUUM FULL never set PD_ALL_VISIBLE or
> visibility map bits.  When, someday, they do, we might emit a separate WAL
> record to force the recovery conflict.  However, CLUSTER/VACUUM FULL already
> remove tuples still-visible to standby snapshots without provoking a recovery
> conflict.  (Again only with hot_standby_feedback=off.)

If that were the case it would be a bug.

CLUSTER/VACUUM FULL emit an AccessExclusiveLock record that would
conflict with any current lock holders, so should be fine on that.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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