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Re: Disabled features on Hot Standby

From: Noah Misch <noah(at)leadboat(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Disabled features on Hot Standby
Date: 2012-01-14 06:10:14
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Lists: pgsql-hackers
On Sat, Jan 14, 2012 at 12:42:02AM -0500, Robert Haas wrote:
> On Fri, Jan 13, 2012 at 8:02 PM, Noah Misch <noah(at)leadboat(dot)com> wrote:
> > Simon spoke to the FPI side of the question. ?For heap pages, the
> > 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.)
> Is the big about CLUSTER/VACUUM FULL a preexisting bug?  If not, why not?

I suspect it goes back to 9.0, yes.  I'm on the fence regarding whether to call
it a bug or an unimplemented feature.  In any case, +1 for improving it.

> Other than that, it seems like we might be converging on a workable
> solution: if hot_standby_feedback=off, disable index-only scans for
> snapshots taken during recovery; if hot_standby_feedback=on, generate
> recovery conflicts when a snapshot's xmin precedes the youngest xmin
> on a page marked all-visible, but allow the use of index-only scans,
> and allow sequential scans to trust PD_ALL_VISIBLE.  Off the top of my
> head, I don't see a hole in that logic...

I wouldn't check hot_standby_feedback.  Rather, mirror what we do for
XLOG_HEAP2_CLEAN.  Unconditionally add an xid to xl_heap_visible bearing the
youngest xmin on the page (alternately, some convenient upper bound thereof).
Have heap_xlog_visible() call ResolveRecoveryConflictWithSnapshot() on that
xid.  Now, unconditionally trust PD_ALL_VISIBLE and permit index-only scans.

The user's settings of hot_standby_feedback, vacuum_defer_cleanup_age,
max_standby_streaming_delay and max_standby_archive_delay will drive the
consequential trade-off:  nothing, query cancellation, or recovery delay.


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